Everything About Us

Lyric One Last Breath from Creed

2009-09-09T14:36:00.000+07:00

This is lyric of One Last Breath from Creed

Please come now I think I’m falling
I’m holding to all I think is safe
It seems I found the road to nowhere
And I’m trying to escape
I yelled back when I heard thunder
But I’m down to one last breath
And with it let me say
Let me say
Hold me now
I’m six feet from the edge and I’m thinking
That maybe six feet
Ain’t so far down
I’m looking down now that it’s over
Reflecting on all of my mistakes
I thought I found the road to somewhere
Somewhere in His grace
I cried out heaven save me
But I’m down to one last breath
And with it let me say
Let me say
Hold me now
I’m six feet from the edge and I’m thinking
That maybe six feet
Ain’t so far down
Sad eyes follow me
But I still believe there’s something left for me
So please come stay with me
’Cause I still believe there’s something left for you and me
For you and me
For you and me
Hold me now
I’m six feet from the edge and I’m thinking

Someday Lyrics - MLTR (Michael Learns To Rock)

2009-09-09T14:33:00.001+07:00

this lyric is Michael Learns To Rock with Someday

In my search for freedom
and peace of mind
I’ve left the memories behind
Wanna start a new life
but it seems to be rather absurd
when I know the truth
is that I always think of you

Chorus:
Someday someway
together we will be baby
I will take and you will take your time
We’ll wait for our fate
cos’ nobody owns us baby
We can shake we can shake the rock
Try to throw the picture out of my mind
try to leave the memories behind
Here by the ocean
wave’s carry voices from you
Do you know the truth
I am thinking of you too

Chorus:
Someday someway
together we will be baby…
The love we had together
just fades away in time
And now you’ve got your own world
and I guess I’ve got mine
But the passion that you planted
in the middle of my heart
is a passion that will never stop

Hotel California Lyrics - Eagles

2009-09-09T14:30:00.001+07:00

Hi guys..if you needs many lyrics from many artits in the world. this lyrics is one of them. Eagles with Hotel California

On a dark desert highway, cool wind in my hair
Warm smell of colitas, rising up through the air
Up ahead in the distance, I saw shimmering light
My head grew heavy and my sight grew dim
I had to stop for the night
There she stood in the doorway;
I heard the mission bell
And I was thinking to myself,
’This could be Heaven or this could be Hell’
Then she lit up a candle and she showed me the way
There were voices down the corridor,
I thought I heard them say…
Welcome to the Hotel California
Such a lovely place (Such a lovely place)
Such a lovely face
Plenty of room at the Hotel California
Any time of year (Any time of year)
You can find it here
Her mind is Tiffany-twisted, she got the Mercedes Benz
She got a lot of pretty, pretty boys she calls friends
How they dance in the courtyard, sweet summer sweat.
Some dance to remember, some dance to forget
So I called up the Captain,
’Please bring me my wine’
He said, ’We haven’t had that spirit here since nineteen sixty nine’
And still those voices are calling from far away,
Wake you up in the middle of the night
Just to hear them say…
Welcome to the Hotel California
Such a lovely place (Such a lovely place)
Such a lovely face
They livin’ it up at the Hotel California
What a nice surprise (what a nice surprise)
Bring your alibis
Mirrors on the ceiling,
The pink champagne on ice
And she said ’We are all just prisoners here, of our own device’
And in the master’s chambers,
They gathered for the feast
They stab it with their steely knives,
But they just can’t kill the beast
Last thing I remember, I was
Running for the door
I had to find the passage back
To the place I was before
’Relax,’ said the night man,
’We are programmed to receive.
You can check-out any time you like,
But you can never leave!’

Varicap and Phase Modulator, and Foster-Seeley (Balanced) Discriminator

2009-09-07T08:51:00.002+07:00

Langsung aje ye..ane kasih sedikiiiiiiiiit lg bahasan tentang title diatas.(btw kok dr kemaren sdikit terus ya?huehehe)..

Merdeka!

Varicap modulator

Pada level frekuensi tinggi, adalah normal jika sebuah bentuk modulasi amplitudo menumpuk ke tampilan modulasi frekuensi. Hal ini terjadi akibat rangkaian modulator dibuat untuk menyediakan deviasi frekuensi dari keadaan relevan. Keuntungan dari penggunaan sebuah gelombang rectangular dibandingkan sinus adalah bahwa gelombang ini akan menampilkan hasil yang lebih baik pada osiloskop.

Modulator Phase

Secara normal, phase modulator yang digunakan pada praktikum dibentuk dan diatur untuk menghasilkan variasi fase. Dengan linieritas tinggi, phase modulator pada panel DL2501, akan berfungsi sama, sehingga menghasilkan deviasi yang berharga dan mudah untuk diukur.

Foster-Seeley (Balanced) Discriminator

Foster-Seeley (Balanced) Discriminator mendapatkan kembali tegangan modulasi dari modulasi frekuensi dengan menggunakan pergeseran sudut fasa antara tegangan – tegangan primer dan sekunder, dan suatu transformator yang ditala. Sudut fasa ini adalah fungsi dari frekuensi, dan dengan mengaturnya maka komponen-komponen jumlah phasor dan selisih phasor dari tegangan-tegangan primer dan sekunder dimasukkan kedua buah detektor selubung yang keluarannya kemudian digabungkan sehingga demodulasi telah diperoleh.
Karena tergantung pada variasi sudut fasa, karenanya rangkaian ini dikenal sebagai diskrominator fasa meskipun rangkaian ini tidak mendeteksi modulasi fasa secara langsung, namun rangkaian ini mengubah suatu frekuensin yang sebenarnya atau yang ekivalen menjadi suatu variasi sudut fase rangkaian yang nantinya diubah pula menjadi suatu variasi amplitudo. Lengkung karakteristik dan diskriminator dapat diperoleh dari tegangan keluaran yang dibandingkan dengan deviasi frekuensi.

Modulasi Kolektor, Half Wave dan Full Wave Demodulator

2009-09-07T08:16:00.003+07:00

Hadirin yang berbahagia. Selamat berjumpa dengan saya di acara....(we'e'e'e. kyk pertemuan apaa gt..he2)

Gni prend. ni ada sedikit bahasan tentang judul diatas. sedikiiiiiiiit...bgt. tp smoga bs bantu sampeyan semua baik lahir maupun batin (kyk lebaran aj).

Sukses Bro!

Modulasi Kolektor dan Lissajous

Frekuensi rendah (fm) selalu lebih kecil daripada frekuensi frekuensi pembawa (fc) dalam modulasi amplitudo. Dalam rangkaian yang dilakukan pada praktikum Elkom, bahwa fc merupakan masukna ke penguat tunggal emitor dengan tegangan setinggi Vc. fm merupakan komponen dari tegangan muka DC, yaitu Vm. Vm mengubah arus kolektor Ic. Tegangan masukan Vc dan Vm perlu memenuhi syarat :
•Tidak boleh berpengaruh pada penguatan tegangan
•Hanya Vm yang boleh berpengaruh pada penguatan tegangan. Oleh karena itu operasinya harus berupa operasi sinyal kecil bagian fc dan operasi sinyal besar bagian fm.

Half Wave Demodulator
Pada jenis demodulator ini, informasi pemodulasi terkandung dalam selubung dari gelombang yang dimodulasikan dapat ditampilkan kembali dengan menyearahkan gelombang dengan sistem detektor, yaitu demodulator. Karena menggunakan half wave demodulator, gelombang yang disearahkan hanaya sebagaian saja, sehingga terbentuk gelombang sinus yang terpotong di siklus negatifnya. Nilai frekuensi dari sinyal demodulasi sama dengan sinyal pemodulasi (fm).

Full Wave Demodulator

Karena yang dimodulasi adalah sinyal pada kolektor, maka informasi pemodulasi yang terkandung dalam selubung dari gelombang yang dimodulasikan adalah gelombang sinyal pada kolektor dan informasi ini di demodulasi dengan penyearah gelombang penuh. Sehingga hasilnya
diperoleh sinyal sinus sempurna tidak terpotong dengan nilai amplitudo hampir sama dengan amplitudo selubung.

Soal-Jawab Transmisi Telekomunikasi

2009-09-03T07:38:00.004+07:00

Rekan-rekan, ni kite kasih beberapa soal n jawabannye sekalian. Isinya ya about transmisi telekomunikasi. InsyaAlloh berguna bwt rekan2 temuanya.Amin..Sukses bro!(btw, plis comment ni posting ya.he2.sp tw kite bs sharing lg.jgn lp click 'Follow us' and meet my invite.ha2)

1.Perbedaan line coding dan channel coding dengan beberapa contoh
Line coding : jenis pengkodean sinyal digital yang terdiri dari 2 kategori, yaitu unipolar dan bipolar. Contoh :
NRZ : bit 1 polaritas (+), bit 0 komponen dc, perubahan pada 1 perioda clock
RZ : bit 1, ½ periode clock (+), (-) bergantian, sedangkan bit 0 nya tidak ada
sinyal
AMI : pada frek 0, komponen dc, sedangkan bit 1 bergantian polar (+) dan (-)
HDB3 : jika ada runtun bit 0 berturut-turut 3 kali
B6ZS : bit 0, enam berturut-turut baru dilakukan violasi
CMI : bit 1 direpresentasikan (+) dan (-) state.
Channel coding : merupakan error control, dimana data yang ditransmisikan ditambah algoritma sehingga menambah runtun bit yang panjang. Contoh :

Sandi blok : sederetan pesan bit ditambahkan bit check pada encoding sehingga runtun bit message menjadi lebih panjang. Bit check ini yang digunakan untuk mendeteksi dan memperbaiki galat.
Convolution code : informasi yang meninggalkan encoder tergantung pada blok bit pesan dan blok bit pesan sebelumnya. Proses ini dilakukan oleh shift register dan adder. Menggunakan algoritma : viterbi.
Concanated code : teknik gabungan sandi konvolusi dan sandi blok yang terdiri dari dua decoder dan dua encoder, sepasang berguna untuk membersihkan noise gaussian dan sepasang yang lain untuk burst noise.

2.Bandingkan kelebihan dan kekurangan antara dua teknik kendali galat
FEC (Forward Error Correction)

Kelebihan :
•Menggunakan redundancy untuk mendeteksi error dan memperbaiki data
•Digunakan untuk sistem broadcast
•Memperbaiki error secara langsung
•Tidak membutuhkan jalur balik untuk permintaan pentransmisian ulang
Kekurangan :
Mempunyai jalur yang tetap sehingga kualitas data tergantung noise kanal

ARQ (Automatic Request for Repeat)

Kelebihan :
Menggunakan jalur yang terpisah untuk transmisi ulang
Digunakan pada packet switching
Jika dikombinasikan dengan FEC, maka ARQ akan membersihkan error yang tidak terdeteksi oleh FEC.

Kekurangan :
Hanya mendeteksi error secepat mungkin tanpa memperbaikinya
Digunakan pada kanal yang umumnya bebas error

3.Jelaskan cara kerja pseudorandom generator dengan 3 flip-flop

Runtun bit pseudorandom adalah periodic karena keperiodikannya sangat besar yang biasanya lebih dari 1000 bit terjadi sebelum runtun mengulang dirinya sendiri. Keperiodikan tergantung jumlah flip=flop dlm rangkaian n.dengan panjang runtun 2^n–1. Ini dinamakan pembangkit runtun maximal length, jika n = 3, maka runtun bit akan mengulang kembali setelah 7 bit.

4.Jelaskan cara mengatasi pengaruh atmosfer dan keadaan sekitar sistem gelombang mikro

Dapat diatasi dengan cara diversity, yaitu operasi dua atau lebih sistem atau sebagaian sistem yang dimaksudkan untuk memperbaiki kehandalan sistem.
Terdiri dari tiga macam :

Space diversity
Receiver mempunyai dua antena yang ditempatkan terpisah secara vertikal panjang gelombang yang berbeda satu sama lainnya sekitar 200 lambda, dimana sinyal setiap antena diterima, kemudian dihubungkan ke penggabung diversity. Jika ditempatkan terpisah secara horisontal dinamakan Angle diversity dengan sudut elevasi yang berbeda 1 derajat.

Frekuensi diversity
Dua transmitter mengirimkan informasi secara bersamaan pada satu antena yang sama namun dengan frekuensi yan berbeda.

Polarisation diversity
Energi microwave dipancarkan pada polaritas yang berbeda yaitu secara vertikal dan horisontal dari transmitter yang sama, sehingga membutuhkan dua antena dengan polarisasi berbeda (vertikal dan horisontal) baik pada transmitter atau receiver.

5.Bandingkan kedua sumber cahaya LED dan ILD

LED :
Sumber cahaya tidak koheren
Kehandalan lebih tinggi
Rangkaian drive nya lebih sederhana
Sensitivitas terhadap cahaya lebih rendah
Input dc dan output light bersifat linear
Biaya rendah

ILD :
Sumber cahaya lebih koheren
Daya output besar
Bandwidth lebar
Spektrum sempit
Rise time nya lebih cepat

6.Hal-hal apa saja yang perlu diperhatikan dalam instalasi fiber optis
Penyambungan terdiri dari :

Fusion : kedua ujung fiber optik dimasukkan kedalam suatu piranti dimana ujung-ujung fiber optik tersebut haru bersih dan rata, kemudian elektroda diaktifkan sehingga fiber optik meleleh dan tersambung. Karakteristik fiber optik kiri dan kanan harus sama agar tidak menimbulkan atenuasi yang tinggi.

Mekanikal : tidak adanya proses peleburan dimana ujungnya dipotong dengan sudut tegak lurus serta harus bersih dan rata kemudian dimasukkan ke elestomotorik (bentuk selongsongnya kerucut) atau fastomorik (bentuk selongsongnya sama dengan corenya)

7.Struktur fisis fiber optik selengkapnya

Core atau lapisan inti, yaitu pusat yang terbuat dari gelas halus yang menjadi tempat berjalannya cahaya
Cladding atau kulit, yaitu lapisan optik luar yang membungkus core dan memantulkan kembali cahaya yang terpancar keluar.
Buffer (pelindung) atau coating (mantel), yaitu lapisan plastik pelindug serat dari kerusakan dan kelembaban.

8.Jelaskan tentang :

Bit stuffing : penyelipan bit-bit ekstra ke dalam aliran data untuk menghindari kemunculan rangkaian kontrol yang tidak diharapkan.
Bit interleaving : suatu metode yang digunakan dalam multiplexing asinkron untuk keperluan perbandingan dengan sistem sinkron.
Enkripsi : konversi data menjadi bentuk yang tidak mudah dimengerti dengan menggunakan sebuah kode tertentu sedemikian rupa sehingga merekonversikan bentuk yang asli masih bisa dikembalikan.
Low Probability of Detection : teknik pendeteksian spread spectrum yang digunakan sehingga tidak mudah dideteksi adanya lalu lintas pesan atau informasi agar pihak lain sulit menangkap pesan informasi komunikasi yang berlangsung.
Splicing : suatu teknik penyambungan pada fiber optik dengan cara fusion dan mekanik.

9.Hal-hal yang harus diperhatikan dalam teknik pemilihan pengkodean

•Tidak mengandung komponen dc
•Tidak boros BW
•Noise dapat dipulihkan
•Kode harus menyediakan multiplikasi rendah
•Mengekstrak frekuensi clock

10.Mana yang lebih baik antara FH-SS dan DS-SS

Yang lebih baik adalah DS-SS, karena pada akhir penerimaan DS-SS terdapat dua loop feedback, yang satu loop berfungsi untuk mengunci kode fase yang benar, dan satu loop lagi untuk mengikuti carrier dan kesuksesan DS-SS tergantung pesat chip yang lebih besar, sedangkan FH-SS mempunyai kelemahan tidak mampunya frekuensi synthesizer mengubah frekuensi dengan cepat tanpa membangkit sinyal noise yang tidak dikehendaki tetapi receiver tidak perlu fase yang koheren mengenai transmitter yang berada diatas BW spread spectrum yang penuh.

11.Beri penjelasan mengapa 16QAM lebih baik daripada 16PSK

Karena jarak antar simbol pada 16-QAM adalah jauh, sehingga kemungkinan terjadinya interferensi antar simbol adalah kecil, sedangkan pada 16-PSK jarak antar simbolnya dekat.

12.Mengapa PSK lebih efisien terhadap bandwidth

Karena pada PSK bit-bit yang dikirimkan lebih banyak dan PSK dapat dibuat bervariasi dengan tingkat level yang tertentu dimana semakin tinggi tingkat levelnya maka semakin tinggi efisiensi bandwdthnya.

13.Jelaskan PCM 30

Satu multiframe terdiri dari 16 frame berjarak 2 ms, satu frame terdiri dari 32 timeslot berjarak 125 mikroseconds, sedangkan satu time slot terdiri dari 8 bit. Untuk jenis frame genap digunakan mode genap pula (Y 0 0 1 1 0 0 1 1), sedangkan untuk frame ganjil menggunakan mode ganjil ( Y 1 Z X X X X X X ). Inisial Z berarti 1 adalah sukses dan 0 adalah gagal dalam pendeteksian frame, sedangkan Y adalah untuk hubungan internasional atau tertentu. Untuk time slot 0 berfungsi sebagai pensinkronisasian dalam setiap frame, sementara time slot 16 digunakan pensinkron keseluruhan dan pensinyalan.

14.Kelebihan sinyal digital dibanding analog

Mudah dalam pemeliharaan
Konsep ISDN dapat direalisasikan
Tahan terhadap noise
Efisiensi lebih tinggi
Alat yang digunakan lebih kompleks
Dimungkinkan penggabungan sinyal antara video, audio, dan data

15.Karakteristik Fiber optik :

Modal dispersion : dispersi karena mode yang digunakan
Material dispersion : dispersi karena bahan pembuatnya
Waveguide dispersion : dispersi yang disebabkan oleh bentuk dari core

16.Efek Atmosfer

a.Absorption (penyerapan)
Oksigen pada atmosfer menyerap sejumlah energi microwave
Attenuasi bertambah dengan cepat seiring dengan pertambahan kandungan air di atmosfer pada jalur microwave
Attenuasi relatif kecil untuk komunikasi microwave, sekitar 0,01 dB/km pd 2 Ghz

b.Refraksi (pembiasan)
Merupakan pembelokan gelombang radio yang seharusnya berubah pada karakteristik atmosfer
Efek refraksi juga menyebabkan beam microwave menyimpang dari jalur LOS

c.Ducting (lorong)
Biasanya disebabkan oleh ketinggian yang rendah, kepadatan lapisan atmosfer yang tinggi
Sangat sering terjadi di daerah permukaan air yang sangat luas atau pada iklim yang terjadi pembalikan temperatur

17.Efek Terrain

Perambatan energi microwave dipengaruhi oleh obstacle yang ditempatkan pada jalur tersebut. Obstacle yang dimaksud adalah : pepohonan, batu, bangunan.

Reflection (pemantulan)
Jika 2 gelombang sefase maka akan saling memperkuat sinyal dan sebaliknya
Untuk gelombang polarisasi horisontal, beda fase adalah 180 derajat
Untuk gelombang polarisasi vertikal, beda fase adalah 180 derajat

Freznel zone
1st Freznel zone adalah daerah dimana beda fase antara sinyal langsung san sinyal yang dipantulkan daerah tersebut adalah 180 derajat
Pusat lingkaran merupakan jalur LOS yang merupakan batas semua freznel zone

Difraksi
Rugi bayangan (shadow loss) merupakan daerah di belakang obstacle
Merupakan karekteristik GEM yang terjadi saat beamnya melewati obstacle dengan menyerempet

18.Fading
Disebabkan oleh iklim dan permukaan daratan

Flat fading
2 bentuk FF adalah ducting dan rain attenuation fading
Beam microwave disebabkan oleh perubahan indeks bias udara

Freq Selective Fading
Sinyalnya melemah hanya pada frekuensi tertentu

Ada 2 yaitu :
Atmospheris Multipath Fading : jika kondisi atmosfer terbagi dalam lapisan-lapisan dengan kerapatan yang berbeda2, maka terjadi ducting
Ground Reflection Multipath Fading : pantulan dapat disebabkan oleh tangkapan multi lintasan yang diamati sebagai fading jika gel diterima tidak sefase.

Lissajous

2009-08-31T21:25:00.011+07:00

Lissajous - Langsung aje ye..he3, today kita akan mengupas apa itu lissajous (emgnya mangga dikupas.:) mau dunk), kemudian dibahas bagaimana cara menggambar lissajous dan menghitung beda fasenya serta penjelasannya. jadi, jangan kemana-mana ok!

Gambar / Diagram Lissajous definisinya sederhana saja, yaitu adalah sebuah penampakan pada layar osiloskop yang mencitrakan perbedaan atau perbandingan Beda Fase, Frekuensi & Amplitudo dari 2 gelombang inputan pada probe osiloskop.

Sebelum membahas lebih jauh seperti apa pencitraan lissajous itu ada baiknya kita mantabbbkan definisi dari Beda Fase, Frekuensi & Amplitudo itu sendiri, agar dalam pemahaman lissajous nanti tidak mengalami kebingungan dan kesulitan.

Definisi Amplitudo
Adalah nilai puncak / Maksimum positif dari sebuah gelombang sinusoidal. Bila Amplitudo suatu gelombang tertuliskan " 20 " maka nilai keluaran dari gelombang tersebut akan bergerak dari 0 ke 20 ke 0 ke -20 ke 0 dan ke 20 lagi, begitu seterusnya.

Definisi Frekuensi
Adalah suatu pernyataan yang menggambarkan " Berapa banyak gelombang yang terjadi tiap detiknya" dalam satuan Hz. Bila disitu tertulis 25Hz berarti ada 25 gelombang ( 1 gelombang terdiri atas1 Bukit & 1 Lembah ) yang terjadi dalam 1 detik, ini berarti 1 buah gelombang memakan waktu 1/25 detik = 0.04 detik untuk tereksekusi sepenuhnya ( Inilah yang biasa disebut dengan Periode Gelombang = Waktu yang dibutuhkan 1 gelombang untuk tereksekusi seluruhnya ) . Untuk lebih jelasnya lihat gambar dibawah ini:

Domain Y menggambarkan Amplitudo, sedangkan domain X menggambarkan waktu. dari gambar diatas dapat kita ambil kesimpulan bahwa gelombang tersebut memilikiAmplitudo 50, Frekuensi 1 Hz dan Periode 1 Detik. Gambar ke 2:

Nah, sekarang perhatikan gambar gelombang diatas!! 1 bukit & 1 lembah dapat tereksekusi seluruhnya pada waktu 0,2 detik! Berarti apa yang dapat kita simpulkan?? Yup, Gelombang diatas memiliki Periode = 0,2 detik yang berarti, akan ada 5 gelombang yang dapat terselesaikan dalam 1 detiknya, yang berarti gelombang tersebut memilikiFrekuensi sebesar 5 Hz.

Secara singkat frekuensi merupakan kebalikan dari periode demikian pula sebaliknya, 5 Hz = 1 / 0,2 det ||| 0,2 det = 1 / 5 Hz [ Frekuensi = 1 / Periode & Periode = 1 / Frekuensi ]

Definisi Beda Fase
Adalah perbedaan sudut mulai antara 2 gelombang sinusoidal yang sedang diamati. Sederhana bukan?? agar lebih jelas perhatikan ketiga gambar dibawah ini ( Ketiga gelombang dibawah memiliki Frekuensi 1 Hz ) :

Apa perbedaan dari ketiga jenis gelombang sinus diatas?? Yup, sudut dalam memulai besaran nilainya. Jika Gelombang A memulai awalannya dari nilai sudut nol maka, Gel B memulai dari sudut 45 dan Gel. C memulainya dari sudut -90. Jika anda bingung, maka cam kan saja, bila ada gelombang digeser kekiri maka dalam persamaanya akan Di tambahkan sebesar pergeserannya [ Ex : Persamaan Gel. B ], Demikian pula sebaliknya.

Cukup untuk permulaannya, seperti apakah proses menggambar lissajous itu sebenarnya?? Perhatikan gambar dibawah ini:

Inti dari gambar diatas adalah cara menggambar lissajous secara manual, yaitu dimulai dengan:

1. Menggambar 2 gelombang yang akan diperbandingkan kedalam Domain X dan Y ( Lihat Gambar, Gel 1 diletakkan sebagai input Y [ Vertikal ] dan Gel 2 sebagai input X [ Horizontal ] ),
2. Lalu memilah milahnya menjadi bagian bagian, dan jarak antar bagian2 pada masing2 gelombang haruslah sama ( contoh dalam gambar adalah 16 bagian )
3. Dan yang terahir MemPlot masing masing titik dengan pasangannya masing masing. Dengan menggambar garis bantuan ke tengah bidang kertas dan mencari titik potongnya dengan perpanjangan garis bantu dari gelombang yang satunya lagi.
4. Hubungkan titik2 tersebut sesuai urutanya, Selesai.

Dalam kenyataannya hasil gambar lissajous sendiri sangat banyak jenisnya tergantung dari Frekuensi, Beda Fase & Amplitudo kedua gelombang yang diperbandingkan ( Dalam contoh diatas kurva lissajous yang terbentuk terjadi dari 2 gelombang yang memilikiRasio Frekuensi 1 : 2 || Rasio Amplitudo 1 : 1 || Beda Fase = 0 derajat ) . Berikut contoh-contoh dari hasil kuva lissajous yang lain:

Lalu Bagaimana kita mengetahui Beda Fase secara pasti dari lissajous - lissajous diatas??. Dalam beberapa kasus, hanya kurva2 lissajous tertentu sajalah yang dapat dengan mudah diketahui Beda Fase antara 2 gelombang pembentuknya. Lissajous yang seperti apakah itu? ialah lissajous yang 2 gelombang pembentuknya memiliki Frekuensi sama. Ciri cirinya adalah " lissajous yang hanya terdiri dari 1 lingkaran saja ". Lalu bagaimana cara menghitungnya?? mari kita simak gambar dibawah ini:

Itu adalah rumus untuk kuva yang lingkaranya serong ke kanan untuk kurva lissajous yang lingkarannya serong ke kiri, perhatikan gambar dibawah ini:

Bagaimana dengan lissajous - lissajous yang lain?? kita masih dapat menyimpulkan satuhal dari kurva2 lissajous tersebut yaitu perbandingan rasio frekuensi antara 2 gelombang pembentuknya, Caranya:

Perhatikan gambar!! Tarik garis Vertikal dan Horizontal Hitung Perpotongan Garis Merah dengan grafik dan anggap ini sebagai variabel "M". Hitung Perpotongan Garis Biru dengan grafik dan anggap ini sebagai veriabel "N"
Maka Frek X : Frek Y === M : N

Pada Gambar 1 maka Rasio Frekuensi X banding Y adalah :
5 : 4

Bagaimana dengan Gambar lissajous ke 2??
Jelas, bahwa Rasio Frek X banding Y adalah :

2 : 3

Good Luck yaa!! Sampai Jumpa Lagi..!! huehehe..

Sinyal

2009-08-31T20:55:00.004+07:00

Definisi sinyal
Oce rekan2..let’s discuss about SIGNAL (bner gak English-nya?..he3) Disini saya sedikiit memberikan teori pengantar tentang What’s signal? Langsung aj ke main topic yaa..

Sinyal, apaan tuh??! menurut Rec ITU - T G.701 , sinyal adalah suatu gejala fisika dimana satu atau lebih dari karakteristiknya melambangkan informasi.

Jenis-jenis Sinyal
Setelah kita mengetahui tentang apa itu sinyal, lalu ada berapakah jenis sinyal yang ada secara umum?. menurut hakikatnya sinyal terbagi menjadi ke dalam 2 yaitu Sinyal Analog dan Sinyal Diskrit

Sinyal Analog
Jenis sinyal pertama adalah sinyal analog. Apa sih sebenrnya sinyal analog itu?nih diriku kasih buat rekan2 smua..he3.. sinyal analog merupakan suatu sinyal dimana salah satu besaran karakteristiknya mengikuti secara kontinyu perubahan dari besaran fisik lainnya yang melambangkan informasi. Secara fisik sinyal analog berarti selalu mempunyai nilai di sepanjang waktu. Karakteristik (parameter) yang dimiliki oleh sinyal analog antara lain : amplitudo, frekuensi, dan fae.

Sinyal Diskrit
Sebelumnya kita telah tahu apa itu sinyal analog. Lalu kita diskusi tentang sinyal diskrit. Apa ya sinyal diskrit itu?sinyal diskrit merupakan sinyal yang terdiri atas sederetan elemen yang berurutan terhadap waktu, dimana salah satu atau lebih karakteristiknya membawa informasi. Karakteristik dari sinyal diskrit adalah : amplitudo, lebar dan bentuk gelombangnya.

Sinyal Digital

Sinyal digital, sebenarnya apa sih sinyal digital itu?? apa definisi dari sinyal digital?? Sinyal digital adalah sebuah sinyal diskrit dimana informasinya dilambangkan oleh sejumlah deretan sinyal diskrit yang telah ditentukan jumlahnya.

Protocol Verification

2009-08-30T16:14:00.005+07:00

Realistic protocols and the programs that implement them are often quite complicated. Consequently, much research has been done trying to find formal, mathematical techniques for specifying and verifying protocols. In the following sections we will look at some models and techniques. Although we are looking at them in the context of the data link layer, they are also applicable to other layers.

Finite State Machine Models
A key concept used in many protocol models is the finite state machine. With this technique, each protocol machine (i.e., sender or receiver) is always in a specific state at every instant of time. Its state consists of all the values of its variables, including the program counter.
In most cases, a large number of states can be grouped for purposes of analysis. For example, considering the receiver in protocol 3, we could abstract out from all the possible states two important ones: waiting for frame 0 or waiting for frame 1. All other states can be thought of as transient, just steps on the way to one of the main states. Typically, the states are chosen to be those instants that the protocol machine is waiting for the next event to happen [i.e., executing the procedure call wait(event) in our examples]. At this point the state of the protocol machine is completely determined by the states of its variables. The number of states is then 2n, where n is the number of bits needed to represent all the variables combined.
The state of the complete system is the combination of all the states of the two protocol machines and the channel. The state of the channel is determined by its contents. Using protocol 3 again as an example, the channel has four possible states: a 0 frame or a 1 frame moving from sender to receiver, an acknowledgement frame going the other way, or an empty channel. If we model the sender and receiver as each having two states, the complete system has 16 distinct states.
A word about the channel state is in order. The concept of a frame being ''on the channel'' is an abstraction, of course. What we really mean is that a frame has possibly been received, but not yet processed at the destination. A frame remains ''on the channel'' until the protocol machine executes FromPhysicalLayer and processes it.
From each state, there are zero or more possible transitions to other states. Transitions occur when some event happens. For a protocol machine, a transition might occur when a frame is sent, when a frame arrives, when a timer expires, when an interrupt occurs, etc. For the channel, typical events are insertion of a new frame onto the channel by a protocol machine, delivery of a frame to a protocol machine, or loss of a frame due to noise. Given a complete description of the protocol machines and the channel characteristics, it is possible to draw a directed graph showing all the states as nodes and all the transitions as directed arcs.
One particular state is designated as the initial state. This state corresponds to the description of the system when it starts running, or at some convenient starting place shortly thereafter. From the initial state, some, perhaps all, of the other states can be reached by a sequence of transitions. Using well-known techniques from graph theory (e.g., computing the transitive closure of a graph), it is possible to determine which states are reachable and which are not. This technique is called reachability analysis (Lin et al., 1987). This analysis can be helpful in determining whether a protocol is correct.
Formally, a finite state machine model of a protocol can be regarded as a quadruple (S, M, I, T), where:
•S is the set of states the processes and channel can be in.
•M is the set of frames that can be exchanged over the channel.
•I is the set of initial states of the processes.
•T is the set of transitions between states.
At the beginning of time, all processes are in their initial states. Then events begin to happen, such as frames becoming available for transmission or timers going off. Each event may cause one of the processes or the channel to take an action and switch to a new state. By carefully enumerating each possible successor to each state, one can build the reachability graph and analyze the protocol.
Reachability analysis can be used to detect a variety of errors in the protocol specification. For example, if it is possible for a certain frame to occur in a certain state and the finite state machine does not say what action should be taken, the specification is in error (incompleteness). If there exists a set of states from which no exit can be made and from which no progress can be made (i.e., no correct frames can be received any more), we have another error (deadlock). A less serious error is protocol specification that tells how to handle an event in a state in which the event cannot occur (extraneous transition). Other errors can also be detected.
As an example of a finite state machine model, consider Fig. 1(a). This graph corresponds to protocol 3 as described above: each protocol machine has two states and the channel has four states. A total of 16 states exist, not all of them reachable from the initial one. The unreachable ones are not shown in the figure. Checksum errors are also ignored here for simplicity.

Figure 1. (a) State diagram for protocol 3. (b) Transitions.

Each state is labeled by three characters, SRC, where S is 0 or 1, corresponding to the frame the sender is trying to send; R is also 0 or 1, corresponding to the frame the receiver expects, and C is 0, 1, A, or empty (–), corresponding to the state of the channel. In this example the initial state has been chosen as (000). In other words, the sender has just sent frame 0, the receiver expects frame 0, and frame 0 is currently on the channel.
Nine kinds of transitions are shown in Fig.1. Transition 0 consists of the channel losing its contents. Transition 1 consists of the channel correctly delivering packet 0 to the receiver, with the receiver then changing its state to expect frame 1 and emitting an acknowledgement. Transition 1 also corresponds to the receiver delivering packet 0 to the network layer. The other transitions are listed in Fig.1(b). The arrival of a frame with a checksum error has not been shown because it does not change the state (in protocol 3).
During normal operation, transitions 1, 2, 3, and 4 are repeated in order over and over. In each cycle, two packets are delivered, bringing the sender back to the initial state of trying to send a new frame with sequence number 0. If the channel loses frame 0, it makes a transition from state (000) to state (00–). Eventually, the sender times out (transition 7) and the system moves back to (000). The loss of an acknowledgement is more complicated, requiring two transitions, 7 and 5, or 8 and 6, to repair the damage.
One of the properties that a protocol with a 1-bit sequence number must have is that no matter what sequence of events happens, the receiver never delivers two odd packets without an intervening even packet, and vice versa. From the graph of Fig.1 we see that this requirement can be stated more formally as ''there must not exist any paths from the initial state on which two occurrences of transition 1 occur without an occurrence of transition 3 between them, or vice versa.'' From the figure it can be seen that the protocol is correct in this respect.
A similar requirement is that there not exist any paths on which the sender changes state twice (e.g., from 0 to 1 and back to 0) while the receiver state remains constant. Were such a path to exist, then in the corresponding sequence of events, two frames would be irretrievably lost without the receiver noticing. The packet sequence delivered would have an undetected gap of two packets in it.
Yet another important property of a protocol is the absence of deadlocks. A deadlock is a situation in which the protocol can make no more forward progress (i.e., deliver packets to the network layer) no matter what sequence of events happens. In terms of the graph model, a deadlock is characterized by the existence of a subset of states that is reachable from the initial state and that has two properties:
1.There is no transition out of the subset.
2.There are no transitions in the subset that cause forward progress.
Once in the deadlock situation, the protocol remains there forever. Again, it is easy to see from the graph that protocol 3 does not suffer from deadlocks.
Petri Net Models
The finite state machine is not the only technique for formally specifying protocols. In this section we will describe a completely different technique, the Petri net (Danthine, 1980). A Petri net has four basic elements: places, transitions, arcs, and tokens. A place represents a state which (part of) the system may be in. Figure 2 shows a Petri net with two places, A and B, both shown as circles. The system is currently in state A, indicated by the token (heavy dot) in place A. A transition is indicated by a horizontal or vertical bar. Each transition has zero or more input arcs coming from its input places, and zero or more output arcs, going to its output places.

Figure 2. A Petri net with two places and two transitions.

A transition is enabled if there is at least one input token in each of its input places. Any enabled transition may fire at will, removing one token from each input place and depositing a token in each output place. If the number of input arcs and output arcs differs, tokens will not be conserved. If two or more transitions are enabled, any one of them may fire. The choice of a transition to fire is indeterminate, which is why Petri nets are useful for modeling protocols. The Petri net of Fig.2 is deterministic and can be used to model any two-phase process (e.g., the behavior of a baby: eat, sleep, eat, sleep, and so on). As with all modeling tools, unnecessary detail is suppressed.
Figure 3 gives the Petri net model of Fig. 2. Unlike the finite state machine model, there are no composite states here; the sender's state, channel state, and receiver's state are represented separately. Transitions 1 and 2 correspond to transmission of frame 0 by the sender, normally, and on a timeout respectively. Transitions 3 and 4 are analogous for frame 1. Transitions 5, 6, and 7 correspond to the loss of frame 0, an acknowledgement, and frame 1, respectively. Transitions 8 and 9 occur when a data frame with the wrong sequence number arrives at the receiver. Transitions 10 and 11 represent the arrival at the receiver of the next frame in sequence and its delivery to the network layer.

Figure 3. A Petri net model for protocol 3.

Petri nets can be used to detect protocol failures in a way similar to the use of finite state machines. For example, if some firing sequence included transition 10 twice without transition 11 intervening, the protocol would be incorrect. The concept of a deadlock in a Petri net is similar to its finite state machine counterpart.
Petri nets can be represented in convenient algebraic form resembling a grammar. Each transition contributes one rule to the grammar. Each rule specifies the input and output places of the transition. Since Fig.3 has 11 transitions, its grammar has 11 rules, numbered 1–11, each one corresponding to the transition with the same number. The grammar for the Petri net of Fig.3 is as follows:

It is interesting to note how we have managed to reduce a complex protocol to 11 simple grammar rules that can easily be manipulated by a computer program.
The current state of the Petri net is represented as an unordered collection of places, each place represented in the collection as many times as it has tokens. Any rule, all of whose left-hand side places are present can be fired, removing those places from the current state, and adding its output places to the current state. The marking of Fig.3 is ACG, (i.e., A, C, and G each have one token). Consequently, rules 2, 5, and 10 are all enabled and any of them can be applied, leading to a new state (possibly with the same marking as the original one). In contrast, rule 3 ( AD->BE ) cannot be applied because D is not marked.

Surprizzzzeeddd..!!!!

2009-08-30T16:07:00.004+07:00

Tanggal 8: Sudah pkl 00 pagi Alit gelisah. Ih sebel banget klo gini deh! Emang mau ada apa sih ?? mana badan cape banget. tapi mata gak bisa diajak kompromi buat merem. Tapi kok tiba” badanku demam ya, didepan mata byk banget kunang” aduh kenapa nih kepala kayak mau pecah saja & niperut kok mual buanget, ??Ya ampoun BT””. Jam udah nunjukin jam 01.30 pagi. Akhirnya Alit bisa tidur deh!!

Bangoonnn, teriak suara cempreng dari tempat tidur sebelah, non mau saor gak??. Iya “ aku saur teriak Alit!! Liz sobat Alit yang satu ini emang heboh banget. Beginilah suasana asrama cewek dentist_student universitas favorit salah satu kota di Indonesia ini. Apa lg asrama ini dikenal paling hidup BGT. Saking hidupnya, namanya istilah mati kagak ada! 24 jam non-stop masih ada suara makhluk hidup ! Eit’s jangan salah ini kusus kaum hawa yang ada, adam kagak boleh la yau masuk garis batas pengaman/ alias pager besi asrama Tapi klo suster” jaga lagi patroli, kita ngibul abis pura” tidur. Aduh kok jadi nyimpang sih critanya….

Abis saur alit ngelanjutin pergi kealam mimpinya, baru aja 5 menit merem, Liz sudah teriak lagi “Alit suster patroli datang, ayo ganti bajumu kita senam pagi… spontan alit lompat dari tempat tidur& ganti baju. Sambil manyun Carmen,Liz & Lusia menunggunya di pintu kamar. Let’s go girl. We come late. Lit sudah 3 hr kau seperti tak bersemangat kenapa, critakan pada kami kali saja kami dapat memecahkan persoalanmu kata 3 sahabatnya berbarengan! I’m it’s ok friend!Aq hanya merasa bodoh tidak dapat mengerjakan soal ujianku kemarin! Oke siang nanti kau harus semangat mengerjakan. Friend’s, sesudah olahraga kita kumpul diruang belajar kita belajar bersama kata Carmen menimpali.

Sebenarnya bukan ujian itu yang dia persoalkan. Alit sendiri tidak tahu ada apa dengan dirinya!! Sejujuernya 2 hari lagi dia berulang tahun adakah sahabat” mengingat hari istimewanya?? Satu lg yang membuat dia resah selama seminggu ini, Advent cowok berdarah pilipine-manado itu yang selalu membuatnya gelisah. Alitpun tak mengerti kenapa bayangan cowok angkuh itu selalu hadir dalam sepekan ini.

Beberapa tahun lalu, saat Alit msh jd maba dia mengunjungi moeslem center . And disitu pertama kali dia melihatnya. Tak ada kesan khusus bagi Alit tentang cowok itu. Namun kejadian yang bikin keki abis, saat Alit lagi ga PD ikutan tu acara, cowok angkuh itu memperhatikannya dengan sinis, entah apa yang dipikirkannya. 2 tahun berlalu sudah tanpa terasa, tiba “ dalam suatu hari takdir mempertemukan Alit & cowok itu kembali dalam kondisi yang sama dalam sebuah organisasi mahasiswa kedokteran. Situasi jauh berbeda, tak Alit sangka mahasiswa yang menjadi ketua ikatan ini adalah dia.. sosok yang pernah dia benci. Hai Lit, suara Daimy tiba”..hai lama tak jumpa, sebenarnya aku memperhatikanmu sejak tadi Lit, kau mengenal Advent?? Advent siapa dia, Alit balik bertanya. Dia Ketua organisasi ini Lit, Ooo cowok itu bernama Advent dalam hati Alit bergumam. Sejak itu Alit mengetahui segalanya tentang advent, mulai dari dia seorang mahasiswa KU, dia sedang Co-asst & tinggal di asrama cowok KU.dll

Tanggal 10 : pukul 00.00.Byurr… Ha ha ha terdengar suara Carmen, Liz dan Lusia tertawa. Happy B-day to You alit… eits lagi ultah gak boleh marah OK!!! Uhh, sebelll… jahat banget kalian, ni liat mana kasurku basah smua lagi.. Kita minta traktiran Lit… besok buat buka& saur kita, kagak bisa klo kalian minta traktir bantuin gue jemur nih kasur sampai kering kata Alit marah!.

Jam 07 pagi alit masih ujian, hari ini dia memakai warna kesukaannya serba putih. Uhh serasa baru dilahirkan kembali suci gitu lho!! Cuma perasaan Alit saja kaliiii saking PD-nya. Entah kenapa tiba” terlintas di pikiran Alit wajah Advent. Oh my God, apa yang kupikirkan barusan, tak mungkin aq telah memikirkannya, kenapa tiba” aq ingin sekali dia mengucapkan selamat padaku. Sudahlah lit, jangan terlalu banyak bekhayal tak mungkin dia mengetahui hari istimewamu ini, kenalpun hanya sebatas tahu nama dan wajah Alit bergumam sendiri. Tp tak tau ah.. meski ragu Alit akhirnya pergi ke Base_Camp untuk melihat sosok Advent dihari istimewanya ini.

Setelah memakirkan motor di teras, alit ragu untuk masuk dalam Base_Camp! Apa alasan yang tepat ya, jika tiba” dia ditanya. Sebab tak biasanya dia datang kesitu hanya untuk sekedar main. Belum sempat menemukan alasan yang tepat dia dikejutkan suara Youngkie, Amanda, & Tidus memanggil namanya. Ya sudah kepalang basah tak bisa beralasan lain kecuali ingin mampir. Alit gelisah mencari-cari, kenapa sosok yang dia cari tak muncul” juga. Belum sempat cukp tenang. Brem brem.. suara motor berhenti didepan dan sang pengendara masuk. Oh my God hamper saja Alit salah tingkah ketika sedang mencari- cari Advent, tiba” sosok itu melaluinya tanpa perduli dengan kehadirannya. Oh, seperti tubuh yang terpelanting dari atap gedung pencakar langit dia sunguh kecewa dengan perlakuan Advent barusan. Tak sadar keadaan disekelilingnya, Alit pulang tanpa pamit dan mengakibatkan keheranan dipikiran teman”nya.

Kue tart dan macam “ pudding menjadi menu pembuka buka puasa Alit dkk kali ini. Ada Carmen, Liz, Lusia , Sandy & Bian. Tentu saja itu buatan tangan”mereka sendiri, surprise untuk sahabat tersayangnya Alit Manis. What happened Lit, you have any problem?? Please you must Story!! Timpal Carmen cewek berdarah Belanda itu. I think you falling in love. Tau apa kau, nada Alit marah karena malu. Lihat tebakanmu benar Carmen, dia memang sedang jatuh cinta kata Bian ikut andil. Apaan sih kalian!! Mereka bergurau hingga larut malam.

Tanggal 11 : Lit kamu harus mengantarkan undangan ini ke Base-Camp. Okelah, dengan sedikit malas Alit pun menuju base camp dengan motornya. Deg deg deg, belum saja mencapai pintu basecamp, advent ?? cowok itu memanggilnya, Lit apa engkau sudah membaca pesanku di white board?? Tumben sikapnya agak baik.. piker Alit. Belum jawab Alit lagi.. Oke aku pulang dulu kata advent lagi. Alit buru-buru pergi kedalam keruangan. Penasaran pesan apakah yang ditulisnya. Oh My God.. Alit terpekik sesaat ya ampun Advent memberi ucapan selamat ulang tahun padanya. Alit hamper gak prcaya dia ingat ultahku.. Hari itu alit bahagia sekali..

Tidak terasa setengah tahun berlalu, kepengurusan hamper berakhir. waktunya perpisahan, Advent sibuk dengan kegiatan organisasi lainnya dan tugas-tugas akhirnya. Sedang Alit sendiri harus pergi dari kota tempatnya kuliah menuju ke kota lain untuk melaksanakan tugas kuliah selama 4 bulan. Tidak ada kabar-kabari tentang Advent, kabar pertama yang Alit terima dari Liz dia menjadi Koordinator organisasi penting antar Mahasiswa Kedokteran se Propinsi. Kabar kedua yang diterima lagi dia sudah lulus dengan menyandang cumlaude. Yang itu dia memberi kabar sendiri pada Alit lewat ponselnya. Congratulation from him..

Signalling Systems

2009-08-30T15:44:00.006+07:00

This section discusses the nervous system of the network: the signaling system. A great deal of information needs to be passed back and forth between the network elements in the completion of a call and also in the servicing of specialized features. Four main types of signals handle this passing of information:
• Supervisory signals— Supervisory signals handle the on-hook/off-hook condition. For instance, when you lift a telephone handset (that is, go off-hook), a signal tells the local exchange that you want a dial tone, and if you exist in the database as an authenticated user, you are then delivered that service; when you hang up (that is, go back on-hook), you send a notice that says you want to remove the service. A network is always monitoring for these supervisory signals to determine when someone needs to activate or deactivate service.
• Address signals— Address signals have to do with the number dialed, which essentially consists of country codes, city codes, area codes, prefixes, and the subscriber number. This string of digits, which we refer to as the telephone number, is, in effect, a routing instruction to the network hierarchy.
• Information signals— Information signals are associated with activating and delivering various enhanced features. For instance, a call-waiting tone is an information signal, and pressing *72 on your phone might send an information signal that tells your local exchange to forward your calls.
• Alerting signals— Alerting signals are the ringing tones, the busy tones, and any specific busy alerts that are used to indicate network congestion or unavailability.
Signaling takes place in two key parts of the network: in the access network, where it's called loop signaling, and in the core, where it's called interoffice signaling (see Figure 1).

Figure 1. Customer loop and interoffice signaling

With analog loop signaling, two types of starts exist:
• Ground start— Ground start means that when you seize that line, it's immediately grounded so that no other call can potentially conflict with it. Ground start is used with a contentious system, perhaps a PBX at a corporate enterprise, to avoid collisions. For example, say you seize a trunk and place a call, and now you're in the ringing state. There are short periods of silence between ringing tones. The local exchange could mistake one of these periods of silence to mean that that trunk is available and try to send a call in over that same trunk that you're trying to place a call out over; this would cause a collision (referred to as glare). Consequently, when you're dealing with systems and contention for the resource, grounding the trunk up front is the most efficient procedure.
• Loop start— Pay telephones and residential phones use loop start, which means that the circuit is grounded when the connection is completed.
There are various start standards for digital subscriber signaling, and they are defined in accordance with the service being provided.
Interoffice signaling has been through several generations of signaling approaches. In the first generation, called per-trunk signaling, the complete path—all the way to the destination point—is set up in order to just carry the signaling information in the first place (see Figure 2). This method uses trunks very inefficiently; trunks may be put into place to carry 20 or 30 ringing tones, but if nobody is on the other end to take that call, the network trunk is being used but not generating any revenue. Also, when a call is initiated and begins to progress, you can no longer send any other signaling information over that trunk; being able to pass a call-waiting tone, for instance, would not be feasible.

Figure 2. Per-trunk signaling

We have moved away from the per-trunk signaling environment to what we use today—common-channel signaling (see Figure 3). You can think of common-channel signaling as being a separate subnetwork over which the signaling message flows between intelligent networking components that assist in the call completion and assist in the delivery of the service logic needed to deliver the requested feature. Today, we predominantly use the ITU-T standard for common-channel signaling: SS7.

Figure 3. Common-channel signaling

SS7 Architecture
SS7 is critical to the functioning and operation of the modern network. With SS7, a packet data network overlays and controls the operation of the underlying voice networks; signaling information is carried on an entirely different path than voice and data traffic. Signaling doesn't take a great deal of time, so we can multiplex many signaling messages over one channel, and that's why the signaling system is a packet network. The signaling system takes advantage of the efficiencies of statistical multiplexing for what is essentially bursty data. The SS7 signaling data link is a full-duplex digital transmission channel that operates at either 56Kbps or 64Kbps, depending on the standards under which the network is operating (for example, T-carrier and J-carrier operate at 56Kbps, E-carrier operates at 64Kbps).
SS7 is an entire architecture that performs out-of-band signaling (that is, signaling in which the conversation and the signaling take place over different paths) in support of the information-exchange functions that are necessary in the PSTN, such as call establishment, billing, and routing. Database access messages convey information between toll centers and centralized databases to permit real-time access to billing-related information and other services. The SS7 architecture defines the procedures for the setup, ongoing management, and clearing of a call, and it allows you to pass along customer-related information (for example, the identity of the caller, the primary carrier chosen) that helps in routing calls. The efficiency of the network also results in faster call setup times and provides for more efficient use of the circuits when carrying the voice or data traffic. In addition, SS7 supports services that require signaling during a call as it is occurring—not in the same band as the conversation.
SS7 permits the telephone company to offer one database to several switches, thereby freeing up switch capacity for other functions, and this is what makes SS7 the foundation for INs and advanced intelligent networks (AINs). It is also the foundation for network interconnection and enhanced services. Without SS7, we would not be able to enjoy the level of interoperability we have today. SS7 is also a key to the development of new generations of services on the Internet, particularly those that support traditional telephony services. To be able to accommodate features such as call forwarding, call waiting, and conference calling, you must be able to tap into the service logic that delivers those features. Until quite recently, the Internet has not been able to do this, but the year 2000 saw the introduction of SS7 gateways, which allow an interface between circuit-switched networks (with their powerful SS7 infrastructure) and the emerging packet-switched networks that need to be capable of handling the more traditional type of voice communications on a more cost-effective basis.
As Figure 4 shows, there are the three prerequisite components in the SS7 network: service switching points (SSPs), service control points (SCPs), and signal transfer points (STPs).

Figure 4. An SS7 network

SSPs
SSPs are the switches that originate and terminate calls. They receive signals from the CPE and perform call processing on behalf of a user. The user, by dialing particular digits, triggers the network to request certain services. For instance, if you preface a number with a toll-free prefix, that toll-free arrangement triggers the local exchange, or SSP, to initiate a database lookup to determine the physical address of that toll-free number (that is, where it resides in the network). The SSP reaches into the network to find the database that can translate the toll-free number into a physical address in order to then complete the toll-free call. The SSP does this by interacting with a device called the SCP, which is discussed shortly.
SSPs are typically implemented at local exchanges, access tandem offices, or toll centers that contain the network-signaling protocols. The SSP serves as the source and destination point for the SS7 messages.

SCPs
The second key component of SS7 is SCP. This is the network element that interfaces with the SSP as well as the STP. Most importantly, the SCP is the network element that contains the network configuration and call-completion database; in other words, it contains the service logic to act on the types of calls and features the users are requesting. SCPs are centralized nodes that contain service logic—basically software and databases—for the management of the call. They provide functions such as digit translation, call routing, and verification of credit cards. The SCPs receive traffic from the SSP via the STP and return responses, based on that query, via the STP.

STPs
The STP is responsible for translating the SS7 messages and then routing those messages between the appropriate network nodes and databases. Notice in Figure 4 that the SCPs and the STPs are both redundant, and that the links running between them are also redundant.

SS7 and the Internet
If a network loses its signaling system, it loses the capability to complete calls, as well as to do any form of billing or passing along of management information. This makes SS7 critical. The SS7 signaling data link, as mentioned earlier in the chapter, is a full-duplex digital transmission channel that operates at either 56Kbps or 64Kbps. A variety of other SS7 links are defined as well, and each has specific uses within the signaling network:
• A (access) links— An A link interconnects an STP with either an SSP or an SCP. The SSP and SCP, collectively, are referred to as the signaling endpoints. A message sent to and from the SSPs or SCPs first goes to its home STP, which, in turn, processes or routes the message.
• B (bridge) links, D (diagonal) links, and B/D links— A B link connects an STP to another STP. Typically, a quad of B links interconnect peer (or primary) STPs (for example, the STPs from one network to the STPs of another network). The distinction between a B link and a D link is rather arbitrary, and such links may be referred to as B/D links.
• C (cross) links— C links interconnect mated STPs.
• E (extended) links— E links provide enhanced reliability by providing a set of links from the SSP to a second STP pair.
• F (fully associated) links— F links are links that directly connect to signaling endpoints.

The PSTN Infrastructure

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The traditional PSTN infrastructure was specifically designed to support only voice communications. At the time this infrastructure was being designed, we had no notion of data communications. Initially the traffic type the PSTN was designed to support was continuous real-time voice.
Another variable that's important to the design of the PSTN has to do with the length of calls. Most voice calls are quite short, so the circuit switches in the PSTN are engineered for call durations of three minutes or less. The average Internet session, on the other hand, lasts around an hour. This means that increased Internet access through the PSTN has, in some locales, put a strain on the local exchanges. If a circuit switch is blocked because it is carrying a long Internet session, people may not be able to get a dial tone. There are several solutions to this problem. For example, we can apply intelligence in front of some exchanges so that calls destined for ISPs can be diverted over a packet-switched network to the ISP rather than being completed on a circuit-switched basis through the local exchange.

Yet another variable that's important to the design of the PSTN has to do with what it was designed to support. The capacities of the channels in the PSTN are of the narrowband generation—they are based on 64Kbps channels. The worldwide infrastructure to accommodate voice communications evolved to include a series of circuit switches. Different switches are used based on the locations to which they're connecting. The switches have a high degree of intelligence built into them, both for establishing the communications channels and for delivering the service logic to activate a growing array of features. In the traditional framework, the monolithic switches in the network had all the smarts. The switch manufacturer and the carrier worked together very closely, and the carrier was not able to introduce new features and services into a particular area until a software release was available for the switch platform through which the neighborhood was being serviced. Thus, carriers were often unable to roll out new services and features because they hadn't yet received the new software releases from the switch manufacturers. Over time, we have separated the functions of switching and connection establishment from the functions involved in the intelligence that enables various services and features to be activated.
The traditional PSTN is associated with highly developed, although not necessarily integrated, operational support systems (such as billing systems, provisioning systems, network management systems, customer contact systems, and security systems). These systems have very well-developed business processes and techniques for managing their environments. But the various systems' databases cannot yet all speak to one another to give one comprehensive view. (But at least those systems exist, unlike in the public Internet, where the operational support systems are only now beginning to emerge to help manage that environment.)
The backbone of the traditional PSTN was largely based on a generation that we call the Plesiochronous Digital Hierarchy (PDH), which includes the T-carrier, E-carrier, and J-carrier standards. The local loop of the PSTN was provisioned as a twisted-copper-pair analog subscriber line.
Service Providers
Many abbreviations and acronyms are used to define the various players and the parts of the network in which they play. Some telcos can and do fulfill more than one of these functions; the extent to which they can or do fulfill more than one of these functions partly depends on the policy, regulatory, and licensing conditions that prevail in different countries. The following terms are largely used in the United States, but they are important to the discussion in this chapter because they illustrate the functions service providers are addressing:
• PTO— PTO stands for public telecommunications operator, which is the name for an incumbent carrier in places other than the United States.
• VAN— VAN stands for value-added network provider. This term originated around 1970 and was applied to companies that were competing to provide telecommunications services, specifically with offerings focused on data communications and data networking. VANs provided more than a simple pipe from Point A to Point B. They provided some additional intelligence in the network, to, for example, perform error detection and correction, or to convert protocols or languages that different computers speak so that you could have interoperability across the network.
• LEC— In the local environment we use the acronym LEC for local exchange carrier. There was originally no competition among LECs, but as soon as competition in the local loop picked up, LECs were segmented into ILECs, CLECs, and DCLECs.
• ILEC— The ILEC is the incumbent local exchange carrier, the original common carrier that either once had, or in some countries still has, monopoly rights in the local loop. For most residents in the United States, this would be one of the four "baby Bells"—Qwest Communications International, SBC Communications, BellSouth Corporation, and Verizon Communications.
• CLEC— The CLEC is the competitive local exchange carrier. CLECs came about as a result of the Telecommunications Act of 1996, which opened up competition in the local loop. The CLEC is the competitor to the ILEC. Although the decline of the telecommunications economy in 2000 and 2001 forced several CLECs out of business, there are still some CLECs in the United States, and they currently focus on delivering dial tone to business customers.
• DCLEC (or DLEC)— DCLEC stands for data competitive local exchange carrier. The DCLEC is a company that is specifically focused on supporting data services (for example, providers that offer DSL services to end users).
• ELEC— ELEC stands for Ethernet local exchange carrier. The ELEC specializes in providing Ethernet solutions in the local loop and metro area.
• IXC— The interexchange carrier (IXC) is the carrier for long-distance and international communications. AT&T Corporation, WorldCom, Sprint, Qwest, and Verizon are the primary IXCs in the United States. Unless certain stringent requirements imposed by the Federal Communications Commission are met, an IXC cannot offer long-distance services in the areas where it is also the ILEC.
• SP— Because so many lines are being blurred today by bundled services and bundled territories of operation, the basic term service provider (SP) is commonly used to refer generically to providers of different types of services.

Network Access
Figure 1 is a simple diagram of network access. On the left-hand side is the customer environment, which includes residences (single-line instruments being served by an access line) and business premises (with onsite telephone systems such as private branch exchange [PBXs] or key telephone systems—smaller site systems for installations where there are 50 or fewer employees). Those in the customer environment are connected to the PSTN via access lines. The access network, or the local loop we so often talk about, includes whatever equipment resides at the customer premise (that is, the customer premises equipment [CPE]), the access line leading to the local exchange, the components at the local exchange on which those access lines terminate (that is, the distribution cross-connects), and the logic used to help control the flow of traffic over the access lines. In the United States, competition is allowed in the local loop, and a myriad of players are interested in owning the local loop (for example, Internet service providers [ISPs], wireless operators, cable TV companies, power utilities). However, worldwide, the incumbent local providers continue to dominate the local loop, and, as usual, politics and economics are principal factors in delaying the mass deployment of high-speed residential access.

Figure 1 Network access

The local exchange, in the center of Figure 1, is the backbone, or the core, of the network. From the local exchange, we can establish connections into the other providers, such as IXCs for long distance, international carriers for overseas calls, cellular providers, and ISPs.
The underlying network access facilities can be either analog or digital loops, and they connect the exchanges to the customer premises. At the customer premises there are the network interfaces, CPE, premises distribution systems where wiring is cross-connected, and network interfaces. The equipment for providing switch access services includes line-termination cards, carrier and multiplexer equipment, and local exchange switching capabilities that support addressing, supervisory alerting, call progress, and other signaling functions.

Access Services
The main categories of access services are trunks, business lines for key telephone systems, centrex service, leased lines, and residential subscriber lines.
Trunks are used to provide connections into the PBX environment. There are three subcategories of trunks:
• Two-way local exchange trunks— On these trunks, traffic flows in both the incoming and outgoing directions.
• DID trunks— Direct inward dialing (DID) trunks are designed for only incoming calls. A benefit of DID trunks is that they enable the dialed number to ring directly on a user's phone rather than having to go through a centralized attendant. If the population knows whom they want to call directly, and if you want to ease the process of connecting the call, this can be a very useful feature. Another benefit of DID trunks is that they make it seem like a private line goes directly to the user, but with DID you can support perhaps 100 different numbers with a group of only 25 to 35 trunks (traffic engineering is used to determine the proper number of trunks).
• DOD trunks— Direct outward dialing (DOD) trunks are used specifically for outgoing calls. DOD trunks are used when you dial an access code such as the number 9 or the number 8 to get an outside-line dial tone before you can dial the actual number that you want to reach.
To service the key telephone systems, business lines connect the network termination at the user to the local exchange. Users that want to use the local exchange as if it were their PBX rent centrex trunks on a monthly basis. Large companies often access the network via leased lines, which can be a very expensive solution, and home users access the network via residential subscriber lines.
Access lines can either be in analog facilities or they can be digital carrier services. Analog transmission is often called plain old telephone service (POTS for short). Three main types of digital services are offered by using twisted-pair cable. The first type of digital services involves T-1 access (at 1.5Mbps), E-1 access (at 2.048Mbps), and J-1 access (at 1.544Mbps). The second type of digital services is narrowband ISDN (N-ISDN) services, including Basic Rate Interface (BRI) for residences and small businesses and Primary Rate Interface (PRI) for larger businesses. The third type of digital services is the xDSL subscriber lines and high-speed digital subscriber lines that enable the all-important applications of Internet access and multimedia exploration.

Transport Services
Transport services are the network switching, transmission, and related services that support information transfer between the originating and terminating access facilities. The underlying facilities include local exchanges and tandem switches, toll and transit switches, international gateways, and interoffice transmission equipment. Transport services include switched services, nonswitched services, and virtual private networks (VPNs).

Switched Services
There are two main types of switched services: public and private.
Switched public services include local calling, long-distance calling, toll-free calling, international calling, directory assistance, operator assistance, and emergency services.
Switched private services can be switchable either because they are deployed within the CPE or because they are deployed on a carrier basis. With CPE-based services, you can add capabilities to the telephone systems onsite in the PBXs—a feature called electronic tandem networking. For example, you can use electronic tandem networking to gain some flexibility in routing around congestion points: If the preferred leased line from Switch A to Switch B is occupied or not available, the switch can decide how to reroute that traffic to still reach Switch B, but through a different series of leased lines. However, because leased lines (also referred to as tie trunks) are mileage sensitive and dedicated to individual customers, they are very expensive; thus, not much private voice networking is done over tie trunks because there are several more attractive solutions, such as VPNs, which are discussed shortly.
With carrier-based switched private services, a centrex customer could partition and implement extensions across multiple local exchanges and in this way be able to switch traffic between those locations.

Nonswitched Services
Nonswitched services include leased lines, foreign exchange (FX) lines, and off-premises exchanges (OPXs). With leased lines, two locations or two devices are always on, using the same transmission path.
FX lines allow you to make a toll call appear to be a local call. For example, you might have a dedicated leased line that runs from your customer premise to a local exchange in a distant area where you call large numbers of customers. When anyone behind your PBX dials a number associated with that foreign local exchange, the PBX automatically selects the FX line. The dial tone the caller receives is actually coming from the distant local exchange, and the call proceeds as if it were a local call. The tradeoff with FX lines is that although you are not charged per call for your long-distance calls to the specified exchange, you pay a flat monthly fee for the leased line and you have to apply some traffic engineering to ensure that you're not making people wait for the FX line to become available. So with FX lines, you need to find the right balance point between reducing costs and ensuring a high level of service.
OPXs are used in distributed environments, such as a city government. Say that the city government has public works stations, libraries, fire stations, and parks and recreation facilities that are too far from the PBX to be served by the normal cabling. The city uses an OPX setup: It leases a circuit from the PBX to the off-premise location and ties it in as if it were part of that PBX. City government employees can then call one another, using their normal extension plan, their call accounting information can be accumulated so that cost allocations can be performed, and the employees can have access to the full suite of features that a business PBX offers.

VPNs
Although you might think that VPNs are related to the Internet or to Internet Protocol (IP) and are a somewhat new development, they actually originated in the circuit-switched network environment, with AT&T's software-defined network (SDN) in the early 1980s. A VPN is a concept, not a technology platform or a set of networking techniques. A VPN defines a network in which customer traffic is isolated over shared-service provider facilities, so as more customers share the same facilities, their costs go down. The purpose of a VPN, then, is to reduce the high cost of leased lines, while still providing high quality of service and guaranteeing that private traffic has capacity between locations. Figure 2 shows an example of a VPN.

Figure 2. An example of a VPN

The underlying facilities of a VPN include the carrier public network, augmented by network control points and service management systems. Under computer control, the traffic is then routed through the public network in a manner that makes the VPN service seem like a facilities-based private network. Access to the VPN can occur via dedicated access, leased lines, or carrier-switched access, using either an analog or a digital carrier.
The network control point represents a centralized database that stores a subscriber's unique VPN information. The network control point screens every call and then applies call processing in accordance with the customer-defined requirements. A common-channel signaling network connects the various network elements so that they can exchange information with each other in real-time.
A service management system is used to build and maintain the VPN database. It allows customers to program specific functions to accommodate their particular business applications. It transmits information to the network control points, with important instructions on a customer-by-customer basis. Thus, VPNs introduce to the realm of the PSTN a lower-cost alternative to building a private voice network.

PSTN Architecture
The PSTN includes a number of transmission links and nodes. There are basically four types of nodes: CPE nodes, switching nodes, transmission nodes, and service nodes.

CPE Nodes
CPE nodes generally refer to the equipment that's located at the customer site. The main function of CPE nodes is to transmit and receive user information. The other key function is to exchange control information with the network. In the traditional realm, this equipment includes PBXs, key telephone systems, and single-line telephones.

Switching Nodes
Switching nodes interconnect transmission facilities at various locations and route traffic through a network. They set up the circuit connections for a signal path, based on the number dialed. To facilitate this type of switching, the ITU standardized a worldwide numbering plan (based on ITU E.164) that essentially acts as the routing instructions for how to complete a call through the PSTN. The switching nodes include the local exchanges, tandem exchanges (for routing calls between local exchanges within a city), toll offices (for routing calls to or from other cities), and international gateways (for routing calls to or from other countries). Primary network intelligence is contained in the Class 4 switches (that is, toll offices switches) and Class 5 switches (that is, local exchange switches). The Class 4 toll switches provide long-distance switching and network features, and the Class 5 switches provide the local switching and telephony features that subscribers subscribe to. Figure 3 shows where the types of telephone exchanges are located.

Figure 3. Types of telephone exchanges

The Local Exchange
The local exchange (also called the Class 5 office or central office) is where communications common carriers terminate customer lines and locate the switching equipment that interconnects those lines. This class office represents the local network. Every subscriber line location in a local exchange is assigned a number, generally seven or eight digits. The first three (or four) digits represent the exchange and identify the local exchange switch that serves a particular telephone. The last four digits identify the individual line number, which is a circuit that is physically connected from the local exchange to the subscriber. The traditional local exchange switch can handle one or more exchanges, with each exchange capable of handling up to 10,000 subscriber lines, numbered 0000 to 9999. In large metropolitan areas, it is common to find one local exchange building housing more than one local exchange switch and for each switch to handle five or more exchanges. These offices are sometimes referred to as multi-entity buildings.

The Tandem Office
The tandem office, or junction network, is an exchange that is used primarily as a switching point for traffic between local exchanges in a metropolitan area. It is an office that is used to interconnect the local end offices over tandem trunks in a densely settled exchange area where it is not economical for a telephone company to provide direct interconnection between all end offices. The tandem office completes all calls between the end offices but is not directly connected to subscribers.

The Toll Office
The toll office (also called the trunk exchange or transit switch) is a telephone company switching center where channels and toll message circuits terminate—in other words, where national long-distance connections are made. This is usually one particular exchange in a city, but larger cities may have several exchanges where toll message circuits terminate.

The International Gateway
An international gateway is the point to and from which international services are available in each country. Protocol conversion may take place in the gateway; in ITU terminology, this is called a centre de transit (CT). C1 and C2 international exchanges connect only international circuits. CT2 exchanges switch traffic between regional groups of countries, and CT1 exchanges switch traffic between continents. CT3 exchanges connect switch traffic between the national PSTN and the international gateway.

Transmission Nodes
Transmission nodes are part of the transport infrastructure, and they provide communication paths that carry user traffic and network control information between the nodes in a network. The transmission nodes include the transmission media discussed in Chapter 3, as well as transport equipment, including amplifiers and/or repeaters, multiplexers, digital cross-connects, and digital loop carriers.

Service Nodes
Service nodes handle signaling, which is the transmission of information to control the setup, holding, charging, and releasing of connections, as well as the transmission of information to control network operations and billing. A very important area related to service nodes is the ITU standard specification Signaling System 7 (SS7), which is covered later in this chapter.

Wireless Transmission

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Our age has given rise to information junkies: people who need to be on-line all the time. For these mobile users, twisted pair, coax, and fiber optics are of no use. They need to get their hits of data for their laptop, notebook, shirt pocket, palmtop, or wristwatch computers without being tethered to the terrestrial communication infrastructure. For these users, wireless communication is the answer. In the following sections, we will look at wireless communication in general, as it has many other important applications besides providing connectivity to users who want to surf the Web from the beach.
Some people believe that the future holds only two kinds of communication: fiber and wireless. All fixed (i.e., nonmobile) computers, telephones, faxes, and so on will use fiber, and all mobile ones will use wireless.
Wireless has advantages for even fixed devices in some circumstances. For example, if running a fiber to a building is difficult due to the terrain (mountains, jungles, swamps, etc.), wireless may be better. It is noteworthy that modern wireless digital communication began in the Hawaiian Islands, where large chunks of Pacific Ocean separated the users and the telephone system was inadequate.

The Electromagnetic Spectrum
When electrons move, they create electromagnetic waves that can propagate through space (even in a vacuum). These waves were predicted by the British physicist James Clerk Maxwell in 1865 and first observed by the German physicist Heinrich Hertz in 1887. The number of oscillations per second of a wave is called its frequency, f, and is measured in Hz (in honor of Heinrich Hertz). The distance between two consecutive maxima (or minima) is called the wavelength, which is universally designated by the Greek letter l (lambda).
When an antenna of the appropriate size is attached to an electrical circuit, the electromagnetic waves can be broadcast efficiently and received by a receiver some distance away. All wireless communication is based on this principle.
In vacuum, all electromagnetic waves travel at the same speed, no matter what their frequency. This speed, usually called the speed of light, c, is approximately 3 x 108 m/sec, or about  foot (30 cm) per nanosecond. (A case could be made for redefining the foot as the distance light travels in a vacuum in 1 nsec rather than basing it on the shoe size of some long-dead king.) In copper or fiber the speed slows to about 2/3 of this value and becomes slightly frequency dependent. The speed of light is the ultimate speed limit. No object or signal can ever move faster than it.
The fundamental relation between f, , and c (in vacuum) is

Equation 2

Since c is a constant, if we know f, we can find , and vice versa. As a rule of thumb, when  is in meters and f is in MHz, f  300. For example, 100-MHz waves are about 3 meters long, 1000-MHz waves are 0.3-meters long, and 0.1-meter waves have a frequency of 3000 MHz.
The electromagnetic spectrum is shown in Fig. 2-11. The radio, microwave, infrared, and visible light portions of the spectrum can all be used for transmitting information by modulating the amplitude, frequency, or phase of the waves. Ultraviolet light, X-rays, and gamma rays would be even better, due to their higher frequencies, but they are hard to produce and modulate, do not propagate well through buildings, and are dangerous to living things. The bands listed at the bottom of Fig. 2-11 are the official ITU names and are based on the wavelengths, so the LF band goes from 1 km to 10 km (approximately 30 kHz to 300 kHz). The terms LF, MF, and HF refer to low, medium, and high frequency, respectively. Clearly, when the names were assigned, nobody expected to go above 10 MHz, so the higher bands were later named the Very, Ultra, Super, Extremely, and Tremendously High Frequency bands. Beyond that there are no names, but Incredibly, Astonishingly, and Prodigiously high frequency (IHF, AHF, and PHF) would sound nice.

Figure 2-11. The electromagnetic spectrum and its uses for communication.

The amount of information that an electromagnetic wave can carry is related to its bandwidth. With current technology, it is possible to encode a few bits per Hertz at low frequencies, but often as many as 8 at high frequencies, so a coaxial cable with a 750 MHz bandwidth can carry several gigabits/sec. From Fig. 2-11 it should now be obvious why networking people like fiber optics so much.
If we solve Eq. (2-2) for f and differentiate with respect to , we get

If we now go to finite differences instead of differentials and only look at absolute values, we get

Equation 2

Thus, given the width of a wavelength band, , we can compute the corresponding frequency band, f, and from that the data rate the band can produce. The wider the band, the higher the data rate. As an example, consider the 1.30-micron band of Fig. 2-6. Here we have =1.3 x 10-6 and  = 0.17 x 10-6,sof is about 30 THz. At, say, 8 bits/Hz, we get 240 Tbps.
Most transmissions use a narrow frequency band (i.e., f/f 1) to get the best reception (many watts/Hz). However, in some cases, a wide band is used, with two variations. In frequency hopping spread spectrum, the transmitter hops from frequency to frequency hundreds of times per second. It is popular for military communication because it makes transmissions hard to detect and next to impossible to jam. It also offers good resistance to multipath fading because the direct signal always arrives at the receiver first. Reflected signals follow a longer path and arrive later. By then the receiver may have changed frequency and no longer accepts signals on the previous frequency, thus eliminating interference between the direct and reflected signals. In recent years, this technique has also been applied commercially—both 802.11 and Bluetooth use it, for example.
As a curious footnote, the technique was co-invented by the Austrian-born sex goddess Hedy Lamarr, the first woman to appear nude in a motion picture (the 1933 Czech film Extase). Her first husband was an armaments manufacturer who told her how easy it was to block the radio signals then used to control torpedos. When she discovered that he was selling weapons to Hitler, she was horrified, disguised herself as a maid to escape him, and fled to Hollywood to continue her career as a movie actress. In her spare time, she invented frequency hopping to help the Allied war effort. Her scheme used 88 frequencies, the number of keys (and frequencies) on the piano. For their invention, she and her friend, the musical composer George Antheil, received U.S. patent 2,292,387. However, they were unable to convince the U.S. Navy that their invention had any practical use and never received any royalties. Only years after the patent expired did it become popular.
The other form of spread spectrum, direct sequence spread spectrum, which spreads the signal over a wide frequency band, is also gaining popularity in the commercial world. In particular, some second-generation mobile phones use it, and it will become dominant with the third generation, thanks to its good spectral efficiency, noise immunity, and other properties. Some wireless LANs also use it. We will come back to spread spectrum later in this chapter. For a fascinating and detailed history of spread spectrum communication, see (Scholtz, 1982).
For the moment, we will assume that all transmissions use a narrow frequency band. We will now discuss how the various parts of the electromagnetic spectrum of Fig. 2-11 are used, starting with radio.

Radio Transmission
Radio waves are easy to generate, can travel long distances, and can penetrate buildings easily, so they are widely used for communication, both indoors and outdoors. Radio waves also are omnidirectional, meaning that they travel in all directions from the source, so the transmitter and receiver do not have to be carefully aligned physically.
Sometimes omnidirectional radio is good, but sometimes it is bad. In the 1970s, General Motors decided to equip all its new Cadillacs with computer-controlled antilock brakes. When the driver stepped on the brake pedal, the computer pulsed the brakes on and off instead of locking them on hard. One fine day an Ohio Highway Patrolman began using his new mobile radio to call headquarters, and suddenly the Cadillac next to him began behaving like a bucking bronco. When the officer pulled the car over, the driver claimed that he had done nothing and that the car had gone crazy.
Eventually, a pattern began to emerge: Cadillacs would sometimes go berserk, but only on major highways in Ohio and then only when the Highway Patrol was watching. For a long, long time General Motors could not understand why Cadillacs worked fine in all the other states and also on minor roads in Ohio. Only after much searching did they discover that the Cadillac's wiring made a fine antenna for the frequency used by the Ohio Highway Patrol's new radio system.
The properties of radio waves are frequency dependent. At low frequencies, radio waves pass through obstacles well, but the power falls off sharply with distance from the source, roughly as 1/r2 in air. At high frequencies, radio waves tend to travel in straight lines and bounce off obstacles. They are also absorbed by rain. At all frequencies, radio waves are subject to interference from motors and other electrical equipment.
Due to radio's ability to travel long distances, interference between users is a problem. For this reason, all governments tightly license the use of radio transmitters, with one exception, discussed below.
In the VLF, LF, and MF bands, radio waves follow the ground, as illustrated in Fig. 2-12(a). These waves can be detected for perhaps 1000 km at the lower frequencies, less at the higher ones. AM radio broadcasting uses the MF band, which is why the ground waves from Boston AM radio stations cannot be heard easily in New York. Radio waves in these bands pass through buildings easily, which is why portable radios work indoors. The main problem with using these bands for data communication is their low bandwidth.

Figure 2-12. (a) In the VLF, LF, and MF bands, radio waves follow the curvature of the earth. (b) In the HF band, they bounce off the ionosphere.

In the HF and VHF bands, the ground waves tend to be absorbed by the earth. However, the waves that reach the ionosphere, a layer of charged particles circling the earth at a height of 100 to 500 km, are refracted by it and sent back to earth, as shown in Fig. 2-12(b). Under certain atmospheric conditions, the signals can bounce several times. Amateur radio operators (hams) use these bands to talk long distance. The military also communicate in the HF and VHF bands.

Microwave Transmission
Above 100 MHz, the waves travel in nearly straight lines and can therefore be narrowly focused. Concentrating all the energy into a small beam by means of a parabolic antenna (like the familiar satellite TV dish) gives a much higher signal-to-noise ratio, but the transmitting and receiving antennas must be accurately aligned with each other. In addition, this directionality allows multiple transmitters lined up in a row to communicate with multiple receivers in a row without interference, provided some minimum spacing rules are observed. Before fiber optics, for decades these microwaves formed the heart of the long-distance telephone transmission system. In fact, MCI, one of AT&T's first competitors after it was deregulated, built its entire system with microwave communications going from tower to tower tens of kilometers apart. Even the company's name reflected this (MCI stood for Microwave Communications, Inc.). MCI has since gone over to fiber and merged with WorldCom.
Since the microwaves travel in a straight line, if the towers are too far apart, the earth will get in the way (think about a San Francisco to Amsterdam link). Consequently, repeaters are needed periodically. The higher the towers are, the farther apart they can be. The distance between repeaters goes up very roughly with the square root of the tower height. For 100-meter-high towers, repeaters can be spaced 80 km apart.
Unlike radio waves at lower frequencies, microwaves do not pass through buildings well. In addition, even though the beam may be well focused at the transmitter, there is still some divergence in space. Some waves may be refracted off low-lying atmospheric layers and may take slightly longer to arrive than the direct waves. The delayed waves may arrive out of phase with the direct wave and thus cancel the signal. This effect is called multipath fading and is often a serious problem. It is weather and frequency dependent. Some operators keep 10 percent of their channels idle as spares to switch on when multipath fading wipes out some frequency band temporarily.
The demand for more and more spectrum drives operators to yet higher frequencies. Bands up to 10 GHz are now in routine use, but at about 4 GHz a new problem sets in: absorption by water. These waves are only a few centimeters long and are absorbed by rain. This effect would be fine if one were planning to build a huge outdoor microwave oven for roasting passing birds, but for communication, it is a severe problem. As with multipath fading, the only solution is to shut off links that are being rained on and route around them.
In summary, microwave communication is so widely used for long-distance telephone communication, mobile phones, television distribution, and other uses that a severe shortage of spectrum has developed. It has several significant advantages over fiber. The main one is that no right of way is needed, and by buying a small plot of ground every 50 km and putting a microwave tower on it, one can bypass the telephone system and communicate directly. This is how MCI managed to get started as a new long-distance telephone company so quickly. (Sprint went a completely different route: it was formed by the Southern Pacific Railroad, which already owned a large amount of right of way and just buried fiber next to the tracks.)
Microwave is also relatively inexpensive. Putting up two simple towers (may be just big poles with four guy wires) and putting antennas on each one may be cheaper than burying 50 km of fiber through a congested urban area or up over a mountain, and it may also be cheaper than leasing the telephone company's fiber, especially if the telephone company has not yet even fully paid for the copper it ripped out when it put in the fiber.

The Politics of the Electromagnetic Spectrum
To prevent total chaos, there are national and international agreements about who gets to use which frequencies. Since everyone wants a higher data rate, everyone wants more spectrum. National governments allocate spectrum for AM and FM radio, television, and mobile phones, as well as for telephone companies, police, maritime, navigation, military, government, and many other competing users. Worldwide, an agency of ITU-R (WARC) tries to coordinate this allocation so devices that work in multiple countries can be manufactured. However, countries are not bound by ITU-R's recommendations, and the FCC (Federal Communication Commission), which does the allocation for the United States, has occasionally rejected ITU-R's recommendations (usually because they required some politically-powerful group giving up some piece of the spectrum).
Even when a piece of spectrum has been allocated to some use, such as mobile phones, there is the additional issue of which carrier is allowed to use which frequencies. Three algorithms were widely used in the past. The oldest algorithm, often called the beauty contest, requires each carrier to explain why its proposal serves the public interest best. Government officials then decide which of the nice stories they enjoy most. Having some government official award property worth billions of dollars to his favorite company often leads to bribery, corruption, nepotism, and worse. Furthermore, even a scrupulously honest government official who thought that a foreign company could do a better job than any of the national companies would have a lot of explaining to do.
This observation led to algorithm 2, holding a lottery among the interested companies. The problem with that idea is that companies with no interest in using the spectrum can enter the lottery. If, say, a fast food restaurant or shoe store chain wins, it can resell the spectrum to a carrier at a huge profit and with no risk.
Bestowing huge windfalls on alert, but otherwise random, companies has been severely criticized by many, which led to algorithm 3: auctioning off the bandwidth to the highest bidder. When England auctioned off the frequencies needed for third-generation mobile systems in 2000, they expected to get about $4 billion. They actually received about $40 billion because the carriers got into a feeding frenzy, scared to death of missing the mobile boat. This event switched on nearby governments' greedy bits and inspired them to hold their own auctions. It worked, but it also left some of the carriers with so much debt that they are close to bankruptcy. Even in the best cases, it will take many years to recoup the licensing fee.
A completely different approach to allocating frequencies is to not allocate them at all. Just let everyone transmit at will but regulate the power used so that stations have such a short range they do not interfere with each other. Accordingly, most governments have set aside some frequency bands, called the ISM (Industrial, Scientific, Medical) bands for unlicensed usage. Garage door openers, cordless phones, radio-controlled toys, wireless mice, and numerous other wireless household devices use the ISM bands. To minimize interference between these uncoordinated devices, the FCC mandates that all devices in the ISM bands use spread spectrum techniques. Similar rules apply in other countries
The location of the ISM bands varies somewhat from country to country. In the United States, for example, devices whose power is under 1 watt can use the bands shown in Fig. 2-13 without requiring a FCC license. The 900-MHz band works best, but it is crowded and not available worldwide. The 2.4-GHz band is available in most countries, but it is subject to interference from microwave ovens and radar installations. Bluetooth and some of the 802.11 wireless LANs operate in this band. The 5.7-GHz band is new and relatively undeveloped, so equipment for it is expensive, but since 802.11a uses it, it will quickly become more popular.

Figure 2-13. The ISM bands in the United States.

Infrared and Millimeter Waves
Unguided infrared and millimeter waves are widely used for short-range communication. The remote controls used on televisions, VCRs, and stereos all use infrared communication. They are relatively directional, cheap, and easy to build but have a major drawback: they do not pass through solid objects (try standing between your remote control and your television and see if it still works). In general, as we go from long-wave radio toward visible light, the waves behave more and more like light and less and less like radio.
On the other hand, the fact that infrared waves do not pass through solid walls well is also a plus. It means that an infrared system in one room of a building will not interfere with a similar system in adjacent rooms or buildings: you cannot control your neighbor's television with your remote control. Furthermore, security of infrared systems against eavesdropping is better than that of radio systems precisely for this reason. Therefore, no government license is needed to operate an infrared system, in contrast to radio systems, which must be licensed outside the ISM bands. Infrared communication has a limited use on the desktop, for example, connecting notebook computers and printers, but it is not a major player in the communication game.

Lightwave Transmission
Unguided optical signaling has been in use for centuries. Paul Revere used binary optical signaling from the Old North Church just prior to his famous ride. A more modern application is to connect the LANs in two buildings via lasers mounted on their rooftops. Coherent optical signaling using lasers is inherently unidirectional, so each building needs its own laser and its own photodetector. This scheme offers very high bandwidth and very low cost. It is also relatively easy to install and, unlike microwave, does not require an FCC license.
The laser's strength, a very narrow beam, is also its weakness here. Aiming a laser beam 1-mm wide at a target the size of a pin head 500 meters away requires the marksmanship of a latter-day Annie Oakley. Usually, lenses are put into the system to defocus the beam slightly.
A disadvantage is that laser beams cannot penetrate rain or thick fog, but they normally work well on sunny days. However, the author once attended a conference at a modern hotel in Europe at which the conference organizers thoughtfully provided a room full of terminals for the attendees to read their e-mail during boring presentations. Since the local PTT was unwilling to install a large number of telephone lines for just 3 days, the organizers put a laser on the roof and aimed it at their university's computer science building a few kilometers away. They tested it the night before the conference and it worked perfectly. At 9 a.m. the next morning, on a bright sunny day, the link failed completely and stayed down all day. That evening, the organizers tested it again very carefully, and once again it worked absolutely perfectly. The pattern repeated itself for two more days consistently.
After the conference, the organizers discovered the problem. Heat from the sun during the daytime caused convection currents to rise up from the roof of the building, as shown in Fig. 2-14. This turbulent air diverted the beam and made it dance around the detector. Atmospheric ''seeing'' like this makes the stars twinkle (which is why astronomers put their telescopes on the tops of mountains—to get above as much of the atmosphere as possible). It is also responsible for shimmering roads on a hot day and the wavy images seen when one looks out above a hot radiator.

Figure 2-14. Convection currents can interfere with laser communication systems. A bidirectional system with two lasers is pictured here.

Data Link Layer

2009-08-30T11:03:00.009+07:00

The data link layer has a number of specific functions it can carry out. These functions include
1.Providing a well-defined service interface to the network layer.
2.Dealing with transmission errors.
3.Regulating the flow of data so that slow receivers are not swamped by fast senders.
To accomplish these goals, the data link layer takes the packets it gets from the network layer and encapsulates them into frames for transmission. Each frame contains a frame header, a payload field for holding the packet, and a frame trailer, as illustrated in Fig. 3-1. Frame management forms the heart of what the data link layer does. In the following sections we will examine all the above-mentioned issues in detail.

Figure 3-1. Relationship between packets and frames.

Although this chapter is explicitly about the data link layer and the data link protocols, many of the principles we will study here, such as error control and flow control, are found in transport and other protocols as well. In fact, in many networks, these functions are found only in the upper layers and not in the data link layer. However, no matter where they are found, the principles are pretty much the same, so it does not really matter where we study them. In the data link layer they often show up in their simplest and purest forms, making this a good place to examine them in detail.

Services Provided to the Network Layer
The function of the data link layer is to provide services to the network layer. The principal service is transferring data from the network layer on the source machine to the network layer on the destination machine. On the source machine is an entity, call it a process, in the network layer that hands some bits to the data link layer for transmission to the destination. The job of the data link layer is to transmit the bits to the destination machine so they can be handed over to the network layer there, as shown in Fig. 3-2(a). The actual transmission follows the path of Fig. 3-2(b), but it is easier to think in terms of two data link layer processes communicating using a data link protocol. For this reason, we will implicitly use the model of Fig. 3-2(a) throughout this chapter.

Figure 3-2. (a) Virtual communication. (b) Actual communication.

The data link layer can be designed to offer various services. The actual services offered can vary from system to system. Three reasonable possibilities that are commonly provided are
1.Unacknowledged connectionless service.
2.Acknowledged connectionless service.
3.Acknowledged connection-oriented service.
Let us consider each of these in turn.
Unacknowledged connectionless service consists of having the source machine send independent frames to the destination machine without having the destination machine acknowledge them. No logical connection is established beforehand or released afterward. If a frame is lost due to noise on the line, no attempt is made to detect the loss or recover from it in the data link layer. This class of service is appropriate when the error rate is very low so that recovery is left to higher layers. It is also appropriate for real-time traffic, such as voice, in which late data are worse than bad data. Most LANs use unacknowledged connectionless service in the data link layer.
The next step up in terms of reliability is acknowledged connectionless service. When this service is offered, there are still no logical connections used, but each frame sent is individually acknowledged. In this way, the sender knows whether a frame has arrived correctly. If it has not arrived within a specified time interval, it can be sent again. This service is useful over unreliable channels, such as wireless systems.
It is perhaps worth emphasizing that providing acknowledgements in the data link layer is just an optimization, never a requirement. The network layer can always send a packet and wait for it to be acknowledged. If the acknowledgement is not forthcoming before the timer expires, the sender can just send the entire message again. The trouble with this strategy is that frames usually have a strict maximum length imposed by the hardware and network layer packets do not. If the average packet is broken up into, say, 10 frames, and 20 percent of all frames are lost, it may take a very long time for the packet to get through. If individual frames are acknowledged and retransmitted, entire packets get through much faster. On reliable channels, such as fiber, the overhead of a heavyweight data link protocol may be unnecessary, but on wireless channels, with their inherent unreliability, it is well worth the cost.
Getting back to our services, the most sophisticated service the data link layer can provide to the network layer is connection-oriented service. With this service, the source and destination machines establish a connection before any data are transferred. Each frame sent over the connection is numbered, and the data link layer guarantees that each frame sent is indeed received. Furthermore, it guarantees that each frame is received exactly once and that all frames are received in the right order. With connectionless service, in contrast, it is conceivable that a lost acknowledgement causes a packet to be sent several times and thus received several times. Connection-oriented service, in contrast, provides the network layer processes with the equivalent of a reliable bit stream.
When connection-oriented service is used, transfers go through three distinct phases. In the first phase, the connection is established by having both sides initialize variables and counters needed to keep track of which frames have been received and which ones have not. In the second phase, one or more frames are actually transmitted. In the third and final phase, the connection is released, freeing up the variables, buffers, and other resources used to maintain the connection.
Consider a typical example: a WAN subnet consisting of routers connected by point-to-point leased telephone lines. When a frame arrives at a router, the hardware checks it for errors (using techniques we will study late in this chapter), then passes the frame to the data link layer software (which might be embedded in a chip on the network interface board). The data link layer software checks to see if this is the frame expected, and if so, gives the packet contained in the payload field to the routing software. The routing software then chooses the appropriate outgoing line and passes the packet back down to the data link layer software, which then transmits it. The flow over two routers is shown in Fig. 3-3.

Figure 3-3. Placement of the data link protocol.

The routing code frequently wants the job done right, that is, with reliable, sequenced connections on each of the point-to-point lines. It does not want to be bothered too often with packets that got lost on the way. It is up to the data link protocol, shown in the dotted rectangle, to make unreliable communication lines look perfect or, at least, fairly good. As an aside, although we have shown multiple copies of the data link layer software in each router, in fact, one copy handles all the lines, with different tables and data structures for each one.

Framing
To provide service to the network layer, the data link layer must use the service provided to it by the physical layer. What the physical layer does is accept a raw bit stream and attempt to deliver it to the destination. This bit stream is not guaranteed to be error free. The number of bits received may be less than, equal to, or more than the number of bits transmitted, and they may have different values. It is up to the data link layer to detect and, if necessary, correct errors.
The usual approach is for the data link layer to break the bit stream up into discrete frames and compute the checksum for each frame. (Checksum algorithms will be discussed later in this chapter.) When a frame arrives at the destination, the checksum is recomputed. If the newly-computed checksum is different from the one contained in the frame, the data link layer knows that an error has occurred and takes steps to deal with it (e.g., discarding the bad frame and possibly also sending back an error report).
Breaking the bit stream up into frames is more difficult than it at first appears. One way to achieve this framing is to insert time gaps between frames, much like the spaces between words in ordinary text. However, networks rarely make any guarantees about timing, so it is possible these gaps might be squeezed out or other gaps might be inserted during transmission.
Since it is too risky to count on timing to mark the start and end of each frame, other methods have been devised. In this section we will look at four methods:
1.Character count.
2.Flag bytes with byte stuffing.
3.Starting and ending flags, with bit stuffing.
4.Physical layer coding violations.
The first framing method uses a field in the header to specify the number of characters in the frame. When the data link layer at the destination sees the character count, it knows how many characters follow and hence where the end of the frame is. This technique is shown in Fig. 3-4(a) for four frames of sizes 5, 5, 8, and 8 characters, respectively.

Figure 3-4. A character stream. (a) Without errors. (b) With one error.

The trouble with this algorithm is that the count can be garbled by a transmission error. For example, if the character count of 5 in the second frame of Fig. 3-4(b) becomes a 7, the destination will get out of synchronization and will be unable to locate the start of the next frame. Even if the checksum is incorrect so the destination knows that the frame is bad, it still has no way of telling where the next frame starts. Sending a frame back to the source asking for a retransmission does not help either, since the destination does not know how many characters to skip over to get to the start of the retransmission. For this reason, the character count method is rarely used anymore.
The second framing method gets around the problem of resynchronization after an error by having each frame start and end with special bytes. In the past, the starting and ending bytes were different, but in recent years most protocols have used the same byte, called a flag byte, as both the starting and ending delimiter, as shown in Fig. 3-5(a) as FLAG. In this way, if the receiver ever loses synchronization, it can just search for the flag byte to find the end of the current frame. Two consecutive flag bytes indicate the end of one frame and start of the next one.

Figure 3-5. (a) A frame delimited by flag bytes. (b) Four examples of byte sequences before and after byte stuffing.

A serious problem occurs with this method when binary data, such as object programs or floating-point numbers, are being transmitted. It may easily happen that the flag byte's bit pattern occurs in the data. This situation will usually interfere with the framing. One way to solve this problem is to have the sender's data link layer insert a special escape byte (ESC) just before each ''accidental'' flag byte in the data. The data link layer on the receiving end removes the escape byte before the data are given to the network layer. This technique is called byte stuffing or character stuffing. Thus, a framing flag byte can be distinguished from one in the data by the absence or presence of an escape byte before it.
Of course, the next question is: What happens if an escape byte occurs in the middle of the data? The answer is that it, too, is stuffed with an escape byte. Thus, any single escape byte is part of an escape sequence, whereas a doubled one indicates that a single escape occurred naturally in the data. Some examples are shown in Fig. 3-5(b). In all cases, the byte sequence delivered after destuffing is exactly the same as the original byte sequence.
The byte-stuffing scheme depicted in Fig. 3-5 is a slight simplification of the one used in the PPP protocol that most home computers use to communicate with their Internet service provider. We will discuss PPP later in this chapter.
A major disadvantage of using this framing method is that it is closely tied to the use of 8-bit characters. Not all character codes use 8-bit characters. For example. UNICODE uses 16-bit characters, As networks developed, the disadvantages of embedding the character code length in the framing mechanism became more and more obvious, so a new technique had to be developed to allow arbitrary sized characters.
The new technique allows data frames to contain an arbitrary number of bits and allows character codes with an arbitrary number of bits per character. It works like this. Each frame begins and ends with a special bit pattern, 01111110 (in fact, a flag byte). Whenever the sender's data link layer encounters five consecutive 1s in the data, it automatically stuffs a 0 bit into the outgoing bit stream. This bit stuffing is analogous to byte stuffing, in which an escape byte is stuffed into the outgoing character stream before a flag byte in the data.
When the receiver sees five consecutive incoming 1 bits, followed by a 0 bit, it automatically destuffs (i.e., deletes) the 0 bit. Just as byte stuffing is completely transparent to the network layer in both computers, so is bit stuffing. If the user data contain the flag pattern, 01111110, this flag is transmitted as 011111010 but stored in the receiver's memory as 01111110. Figure 3-6 gives an example of bit stuffing.

Figure 3-6. Bit stuffing. (a) The original data. (b) The data as they appear on the line. (c) The data as they are stored in the receiver's memory after destuffing.

With bit stuffing, the boundary between two frames can be unambiguously recognized by the flag pattern. Thus, if the receiver loses track of where it is, all it has to do is scan the input for flag sequences, since they can only occur at frame boundaries and never within the data.
The last method of framing is only applicable to networks in which the encoding on the physical medium contains some redundancy. For example, some LANs encode 1 bit of data by using 2 physical bits. Normally, a 1 bit is a high-low pair and a 0 bit is a low-high pair. The scheme means that every data bit has a transition in the middle, making it easy for the receiver to locate the bit boundaries. The combinations high-high and low-low are not used for data but are used for delimiting frames in some protocols.
As a final note on framing, many data link protocols use a combination of a character count with one of the other methods for extra safety. When a frame arrives, the count field is used to locate the end of the frame. Only if the appropriate delimiter is present at that position and the checksum is correct is the frame accepted as valid. Otherwise, the input stream is scanned for the next delimiter.

Error Control
Having solved the problem of marking the start and end of each frame, we come to the next problem: how to make sure all frames are eventually delivered to the network layer at the destination and in the proper order. Suppose that the sender just kept outputting frames without regard to whether they were arriving properly. This might be fine for unacknowledged connectionless service, but would most certainly not be fine for reliable, connection-oriented service.
The usual way to ensure reliable delivery is to provide the sender with some feedback about what is happening at the other end of the line. Typically, the protocol calls for the receiver to send back special control frames bearing positive or negative acknowledgements about the incoming frames. If the sender receives a positive acknowledgement about a frame, it knows the frame has arrived safely. On the other hand, a negative acknowledgement means that something has gone wrong, and the frame must be transmitted again.
An additional complication comes from the possibility that hardware troubles may cause a frame to vanish completely (e.g., in a noise burst). In this case, the receiver will not react at all, since it has no reason to react. It should be clear that a protocol in which the sender transmits a frame and then waits for an acknowledgement, positive or negative, will hang forever if a frame is ever lost due to, for example, malfunctioning hardware.
This possibility is dealt with by introducing timers into the data link layer. When the sender transmits a frame, it generally also starts a timer. The timer is set to expire after an interval long enough for the frame to reach the destination, be processed there, and have the acknowledgement propagate back to the sender. Normally, the frame will be correctly received and the acknowledgement will get back before the timer runs out, in which case the timer will be canceled.
However, if either the frame or the acknowledgement is lost, the timer will go off, alerting the sender to a potential problem. The obvious solution is to just transmit the frame again. However, when frames may be transmitted multiple times there is a danger that the receiver will accept the same frame two or more times and pass it to the network layer more than once. To prevent this from happening, it is generally necessary to assign sequence numbers to outgoing frames, so that the receiver can distinguish retransmissions from originals.
The whole issue of managing the timers and sequence numbers so as to ensure that each frame is ultimately passed to the network layer at the destination exactly once, no more and no less, is an important part of the data link layer's duties. Later in this chapter, we will look at a series of increasingly sophisticated examples to see how this management is done.

Flow Control
Another important design issue that occurs in the data link layer (and higher layers as well) is what to do with a sender that systematically wants to transmit frames faster than the receiver can accept them. This situation can easily occur when the sender is running on a fast (or lightly loaded) computer and the receiver is running on a slow (or heavily loaded) machine. The sender keeps pumping the frames out at a high rate until the receiver is completely swamped. Even if the transmission is error free, at a certain point the receiver will simply be unable to handle the frames as they arrive and will start to lose some. Clearly, something has to be done to prevent this situation.
Two approaches are commonly used. In the first one, feedback-based flow control, the receiver sends back information to the sender giving it permission to send more data or at least telling the sender how the receiver is doing. In the second one, rate-based flow control, the protocol has a built-in mechanism that limits the rate at which senders may transmit data, without using feedback from the receiver. In this chapter we will study feedback-based flow control schemes because rate-based schemes are never used in the data link layer.
Various feedback-based flow control schemes are known, but most of them use the same basic principle. The protocol contains well-defined rules about when a sender may transmit the next frame. These rules often prohibit frames from being sent until the receiver has granted permission, either implicitly or explicitly. For example, when a connection is set up, the receiver might say: ''You may send me n frames now, but after they have been sent, do not send any more until I have told you to continue.'' We will examine the details shortly.

Ethernet

2009-08-29T14:12:00.019+07:00

The IEEE has standardized a number of local area networks and metropolitan area networks under the name of IEEE 802. Some people who believe in reincarnation think that Charles Darwin came back as a member of the IEEE Standards Association to weed out the unfit. The most important of the survivors are 802.3 (Ethernet) and 802.11 (wireless LAN). With 802.15 (Bluetooth) and 802.16 (wireless MAN), it is too early to tell. Please consult the 5th edition of this book to find out. Both 802.3 and 802.11 have different physical layers and different MAC sublayers but converge on the same logical link control sublayer (defined in 802.2), so they have the same interface to the network layer.

Ethernet Cabling
Since the name ''Ethernet'' refers to the cable (the ether), let us start our discussion there. Four types of cabling are commonly used, as shown in Fig. 4-13.
Figure 4-13. The most common kinds of Ethernet cabling.

Historically, 10Base5 cabling, popularly called thick Ethernet, came first. It resembles a yellow garden hose, with markings every 2.5 meters to show where the taps go. (The 802.3 standard does not actually require the cable to be yellow, but it does suggest it.) Connections to it are generally made using vampire taps, in which a pin is very carefully forced halfway into the coaxial cable's core. The notation 10Base5 means that it operates at 10 Mbps, uses baseband signaling, and can support segments of up to 500 meters. The first number is the speed in Mbps. Then comes the word ''Base'' (or sometimes ''BASE'') to indicate baseband transmission. There used to be a broadband variant, 10Broad36, but it never caught on in the marketplace and has since vanished. Finally, if the medium is coax, its length is given rounded to units of 100 m after ''Base.''
Historically, the second cable type was 10Base2, or thin Ethernet, which, in contrast to the garden-hose-like thick Ethernet, bends easily. Connections to it are made using industry-standard BNC connectors to form T junctions, rather than using vampire taps. BNC connectors are easier to use and more reliable. Thin Ethernet is much cheaper and easier to install, but it can run for only 185 meters per segment, each of which can handle only 30 machines.
Detecting cable breaks, excessive length, bad taps, or loose connectors can be a major problem with both media. For this reason, techniques have been developed to track them down. Basically, a pulse of known shape is injected into the cable. If the pulse hits an obstacle or the end of the cable, an echo will be generated and sent back. By carefully timing the interval between sending the pulse and receiving the echo, it is possible to localize the origin of the echo. This technique is called time domain reflectometry.
The problems associated with finding cable breaks drove systems toward a different kind of wiring pattern, in which all stations have a cable running to a central hub in which they are all connected electrically (as if they were soldered together). Usually, these wires are telephone company twisted pairs, since most office buildings are already wired this way, and normally plenty of spare pairs are available. This scheme is called 10Base-T. Hubs do not buffer incoming traffic. We will discuss an improved version of this idea (switches), which do buffer incoming traffic later in this chapter.
These three wiring schemes are illustrated in Fig. 4-14. For 10Base5, a transceiver is clamped securely around the cable so that its tap makes contact with the inner core. The transceiver contains the electronics that handle carrier detection and collision detection. When a collision is detected, the transceiver also puts a special invalid signal on the cable to ensure that all other transceivers also realize that a collision has occurred.
Figure 4-14. Three kinds of Ethernet cabling. (a) 10Base5. (b) 10Base2. (c) 10Base-T.

With 10Base5, a transceiver cable or drop cable connects the transceiver to an interface board in the computer. The transceiver cable may be up to 50 meters long and contains five individually shielded twisted pairs. Two of the pairs are for data in and data out, respectively. Two more are for control signals in and out. The fifth pair, which is not always used, allows the computer to power the transceiver electronics. Some transceivers allow up to eight nearby computers to be attached to them, to reduce the number of transceivers needed.
The transceiver cable terminates on an interface board inside the computer. The interface board contains a controller chip that transmits frames to, and receives frames from, the transceiver. The controller is responsible for assembling the data into the proper frame format, as well as computing checksums on outgoing frames and verifying them on incoming frames. Some controller chips also manage a pool of buffers for incoming frames, a queue of buffers to be transmitted, direct memory transfers with the host computers, and other aspects of network management.
With 10Base2, the connection to the cable is just a passive BNC T-junction connector. The transceiver electronics are on the controller board, and each station always has its own transceiver.
With 10Base-T, there is no shared cable at all, just the hub (a box full of electronics) to which each station is connected by a dedicated (i.e., not shared) cable. Adding or removing a station is simpler in this configuration, and cable breaks can be detected easily. The disadvantage of 10Base-T is that the maximum cable run from the hub is only 100 meters, maybe 200 meters if very high quality category 5 twisted pairs are used. Nevertheless, 10Base-T quickly became dominant due to its use of existing wiring and the ease of maintenance that it offers. A faster version of 10Base-T (100Base-T) will be discussed later in this chapter.
A fourth cabling option for Ethernet is 10Base-F, which uses fiber optics. This alternative is expensive due to the cost of the connectors and terminators, but it has excellent noise immunity and is the method of choice when running between buildings or widely-separated hubs. Runs of up to km are allowed. It also offers good security since wiretapping fiber is much more difficult than wiretapping copper wire.
Figure 4-15 shows different ways of wiring a building. In Fig. 4-15(a), a single cable is snaked from room to room, with each station tapping into it at the nearest point. In Fig. 4-15(b), a vertical spine runs from the basement to the roof, with horizontal cables on each floor connected to the spine by special amplifiers (repeaters). In some buildings, the horizontal cables are thin and the backbone is thick. The most general topology is the tree, as in Fig. 4-15(c), because a network with two paths between some pairs of stations would suffer from interference between the two signals.
Figure 4-15. Cable topologies. (a) Linear. (b) Spine. (c) Tree. (d) Segmented.

Each version of Ethernet has a maximum cable length per segment. To allow larger networks, multiple cables can be connected by repeaters, as shown in Fig. 4-15(d). A repeater is a physical layer device. It receives, amplifies (regenerates), and retransmits signals in both directions. As far as the software is concerned, a series of cable segments connected by repeaters is no different from a single cable (except for some delay introduced by the repeaters). A system may contain multiple cable segments and multiple repeaters, but no two transceivers may be more than 2.5 km apart and no path between any two transceivers may traverse more than four repeaters.
Manchester Encoding
None of the versions of Ethernet uses straight binary encoding with 0 volts for a 0 bit and 5 volts for a 1 bit because it leads to ambiguities. If one station sends the bit string 0001000, others might falsely interpret it as 10000000 or 01000000 because they cannot tell the difference between an idle sender (0 volts) and a 0 bit (0 volts). This problem can be solved by using +1 volts for a 1 and -1 volts for a 0, but there is still the problem of a receiver sampling the signal at a slightly different frequency than the sender used to generate it. Different clock speeds can cause the receiver and sender to get out of synchronization about where the bit boundaries are, especially after a long run of consecutive 0s or a long run of consecutive 1s.
What is needed is a way for receivers to unambiguously determine the start, end, or middle of each bit without reference to an external clock. Two such approaches are called Manchester encoding and differential Manchester encoding. With Manchester encoding, each bit period is divided into two equal intervals. A binary 1 bit is sent by having the voltage set high during the first interval and low in the second one. A binary 0 is just the reverse: first low and then high. This scheme ensures that every bit period has a transition in the middle, making it easy for the receiver to synchronize with the sender. A disadvantage of Manchester encoding is that it requires twice as much bandwidth as straight binary encoding because the pulses are half the width. For example, to send data at 10 Mbps, the signal has to change 20 million times/sec. Manchester encoding is shown in Fig. 4-16(b).
Figure 4-16. (a) Binary encoding. (b) Manchester encoding. (c) Differential Manchester encoding.

Differential Manchester encoding, shown in Fig. 4-16(c), is a variation of basic Manchester encoding. In it, a 1 bit is indicated by the absence of a transition at the start of the interval. A 0 bit is indicated by the presence of a transition at the start of the interval. In both cases, there is a transition in the middle as well. The differential scheme requires more complex equipment but offers better noise immunity. All Ethernet systems use Manchester encoding due to its simplicity. The high signal is + 0.85 volts and the low signal is - 0.85 volts, giving a DC value of 0 volts. Ethernet does not use differential Manchester encoding, but other LANs (e.g., the 802.5 token ring) do use it.
The Ethernet MAC Sublayer Protocol
The original DIX (DEC, Intel, Xerox) frame structure is shown in Fig. 4-17(a). Each frame starts with a Preamble of 8 bytes, each containing the bit pattern 10101010. The Manchester encoding of this pattern produces a 10-MHz square wave for 6.4 µsec to allow the receiver's clock to synchronize with the sender's. They are required to stay synchronized for the rest of the frame, using the Manchester encoding to keep track of the bit boundaries.
Figure 4-17. Frame formats. (a) DIX Ethernet. (b) IEEE 802.3.

The frame contains two addresses, one for the destination and one for the source. The standard allows 2-byte and 6-byte addresses, but the parameters defined for the 10-Mbps baseband standard use only the 6-byte addresses. The high-order bit of the destination address is a 0 for ordinary addresses and 1 for group addresses. Group addresses allow multiple stations to listen to a single address. When a frame is sent to a group address, all the stations in the group receive it. Sending to a group of stations is called multicast. The address consisting of all 1 bits is reserved for broadcast. A frame containing all 1s in the destination field is accepted by all stations on the network. The difference between multicast and broadcast is important enough to warrant repeating. A multicast frame is sent to a selected group of stations on the Ethernet; a broadcast frame is sent to all stations on the Ethernet. Multicast is more selective, but involves group management. Broadcasting is coarser but does not require any group management.
Another interesting feature of the addressing is the use of bit 46 (adjacent to the high-order bit) to distinguish local from global addresses. Local addresses are assigned by each network administrator and have no significance outside the local network. Global addresses, in contrast, are assigned centrally by IEEE to ensure that no two stations anywhere in the world have the same global address. With 48 - 2 = 46 bits available, there are about 7 x 1013 global addresses. The idea is that any station can uniquely address any other station by just giving the right 48-bit number. It is up to the network layer to figure out how to locate the destination.
Next comes the Type field, which tells the receiver what to do with the frame. Multiple network-layer protocols may be in use at the same time on the same machine, so when an Ethernet frame arrives, the kernel has to know which one to hand the frame to. The Type field specifies which process to give the frame to.
Next come the data, up to 1500 bytes. This limit was chosen somewhat arbitrarily at the time the DIX standard was cast in stone, mostly based on the fact that a transceiver needs enough RAM to hold an entire frame and RAM was expensive in 1978. A larger upper limit would have meant more RAM, hence a more expensive transceiver.
In addition to there being a maximum frame length, there is also a minimum frame length. While a data field of 0 bytes is sometimes useful, it causes a problem. When a transceiver detects a collision, it truncates the current frame, which means that stray bits and pieces of frames appear on the cable all the time. To make it easier to distinguish valid frames from garbage, Ethernet requires that valid frames must be at least 64 bytes long, from destination address to checksum, including both. If the data portion of a frame is less than 46 bytes, the Pad field is used to fill out the frame to the minimum size.
Another (and more important) reason for having a minimum length frame is to prevent a station from completing the transmission of a short frame before the first bit has even reached the far end of the cable, where it may collide with another frame. This problem is illustrated in Fig. 4-18. At time 0, station A, at one end of the network, sends off a frame. Let us call the propagation time for this frame to reach the other end . Just before the frame gets to the other end (i.e., at time -), the most distant station, B, starts transmitting. When B detects that it is receiving more power than it is putting out, it knows that a collision has occurred, so it aborts its transmission and generates a 48-bit noise burst to warn all other stations. In other words, it jams the ether to make sure the sender does not miss the collision. At about time 2, the sender sees the noise burst and aborts its transmission, too. It then waits a random time before trying again.
Figure 4-18. Collision detection can take as long as 2.

If a station tries to transmit a very short frame, it is conceivable that a collision occurs, but the transmission completes before the noise burst gets back at 2. The sender will then incorrectly conclude that the frame was successfully sent. To prevent this situation from occurring, all frames must take more than 2 to send so that the transmission is still taking place when the noise burst gets back to the sender. For a 10-Mbps LAN with a maximum length of 2500 meters and four repeaters (from the 802.3 specification), the round-trip time (including time to propagate through the four repeaters) has been determined to be nearly 50 µsec in the worst case, including the time to pass through the repeaters, which is most certainly not zero. Therefore, the minimum frame must take at least this long to transmit. At 10 Mbps, a bit takes 100 nsec, so 500 bits is the smallest frame that is guaranteed to work. To add some margin of safety, this number was rounded up to 512 bits or 64 bytes. Frames with fewer than 64 bytes are padded out to 64 bytes with the Pad field.
As the network speed goes up, the minimum frame length must go up or the maximum cable length must come down, proportionally. For a 2500-meter LAN operating at 1 Gbps, the minimum frame size would have to be 6400 bytes. Alternatively, the minimum frame size could be 640 bytes and the maximum distance between any two stations 250 meters. These restrictions are becoming increasingly painful as we move toward multigigabit networks.
The final Ethernet field is the Checksum. It is effectively a 32-bit hash code of the data. If some data bits are erroneously received (due to noise on the cable), the checksum will almost certainly be wrong and the error will be detected. The checksum algorithm is a cyclic redundancy check (CRC). It just does error detection, not forward error correction.
When IEEE standardized Ethernet, the committee made two changes to the DIX format. The first one was to reduce the preamble to 7 bytes and use the last byte for a Start of Frame delimiter, for compatibility with 802.4 and 802.5. The second one was to change the Type field into a Length field. Of course, now there was no way for the receiver to figure out what to do with an incoming frame, but that problem was handled by the addition of a small header to the data portion itself to provide this information. We will discuss the format of the data portion when we come to logical link control later in this chapter.
Unfortunately, by the time 802.3 was published, so much hardware and software for DIX Ethernet was already in use that few manufacturers and users were enthusiastic about converting the Type field into a Length field. In 1997 IEEE threw in the towel and said that both ways were fine with it. Fortunately, all the Type fields in use before 1997 were greater than 1500. Consequently, any number there less than or equal to 1500 can be interpreted as Length, and any number greater than 1500 can be interpreted as Type. Now IEEE can maintain that everyone is using its standard and everybody else can keep on doing what they were already doing without feeling guilty about it.

The Binary Exponential Backoff Algorithm
Let us now see how randomization is done when a collision occurs. After a collision, time is divided into discrete slots whose length is equal to the worst-case round-trip propagation time on the ether (2). To accommodate the longest path allowed by Ethernet, the slot time has been set to 512 bit times, or 51.2 µsec as mentioned above.
After the first collision, each station waits either 0 or 1 slot times before trying again. If two stations collide and each one picks the same random number, they will collide again. After the second collision, each one picks either 0, 1, 2, or 3 at random and waits that number of slot times. If a third collision occurs (the probability of this happening is 0.25), then the next time the number of slots to wait is chosen at random from the interval 0 to 2^3 - 1.
In general, after i collisions, a random number between 0 and 2i - 1 is chosen, and that number of slots is skipped. However, after ten collisions have been reached, the randomization interval is frozen at a maximum of 1023 slots. After 16 collisions, the controller throws in the towel and reports failure back to the computer. Further recovery is up to higher layers.
This algorithm, called binary exponential backoff, was chosen to dynamically adapt to the number of stations trying to send. If the randomization interval for all collisions was 1023, the chance of two stations colliding for a second time would be negligible, but the average wait after a collision would be hundreds of slot times, introducing significant delay. On the other hand, if each station always delayed for either zero or one slots, then if 100 stations ever tried to send at once, they would collide over and over until 99 of them picked 1 and the remaining station picked 0. This might take years. By having the randomization interval grow exponentially as more and more consecutive collisions occur, the algorithm ensures a low delay when only a few stations collide but also ensures that the collision is resolved in a reasonable interval when many stations collide. Truncating the backoff at 1023 keeps the bound from growing too large.
As described so far, CSMA/CD provides no acknowledgements. Since the mere absence of collisions does not guarantee that bits were not garbled by noise spikes on the cable, for reliable communication the destination must verify the checksum, and if correct, send back an acknowledgement frame to the source. Normally, this acknowledgement would be just another frame as far as the protocol is concerned and would have to fight for channel time just like a data frame. However, a simple modification to the contention algorithm would allow speedy confirmation of frame receipt (Tokoro and Tamaru, 1977). All that would be needed is to reserve the first contention slot following successful transmission for the destination station. Unfortunately, the standard does not provide for this possibility.
Ethernet Performance
Now let us briefly examine the performance of Ethernet under conditions of heavy and constant load, that is, k stations always ready to transmit. A rigorous analysis of the binary exponential backoff algorithm is complicated. Instead, we will follow Metcalfe and Boggs (1976) and assume a constant retransmission probability in each slot. If each station transmits during a contention slot with probability p, the probability A that some station acquires the channel in that slot is
Equation 4

A is maximized when p = 1/k, with A 1/e as k ∞. The probability that the contention interval has exactly j slots in it is A(1 - A)j - 1, so the mean number of slots per contention is given by

Since each slot has a duration 2, the mean contention interval, w, is 2/A. Assuming optimal p, the mean number of contention slots is never more than e, so w is at most 2e  5.4.
If the mean frame takes P sec to transmit, when many stations have frames to send,
Equation 4

When the second term in the denominator is large, network efficiency will be low. More specifically, increasing network bandwidth or distance (the BL product) reduces efficiency for a given frame size. Unfortunately, much research on network hardware is aimed precisely at increasing this product. People want high bandwidth over long distances (fiber optic MANs, for example), which suggests that Ethernet implemented in this manner may not be the best system for these applications. We will see other ways of implementing Ethernet when we come to switched Ethernet later in this chapter.
In Fig. 4-19, the channel efficiency is plotted versus number of ready stations for 2=51.2 µsec and a data rate of 10 Mbps, using Eq. (4-7). With a 64-byte slot time, it is not surprising that 64-byte frames are not efficient. On the other hand, with 1024-byte frames and an asymptotic value of e 64-byte slots per contention interval, the contention period is 174 bytes long and the efficiency is 0.85.

Figure 4-19. Efficiency of Ethernet at 10 Mbps with 512-bit slot times.

To determine the mean number of stations ready to transmit under conditions of high load, we can use the following (crude) observation. Each frame ties up the channel for one contention period and one frame transmission time, for a total of P + w sec. The number of frames per second is therefore 1/(P + w). If each station generates frames at a mean rate of  frames/sec, then when the system is in state k, the total input rate of all unblocked stations combined is k frames/sec. Since in equilibrium the input and output rates must be identical, we can equate these two expressions and solve for k. (Notice that w is a function of k.) A more sophisticated analysis is given in (Bertsekas and Gallager, 1992).
It is probably worth mentioning that there has been a large amount of theoretical performance analysis of Ethernet (and other networks). Virtually all of this work has assumed that traffic is Poisson. As researchers have begun looking at real data, it now appears that network traffic is rarely Poisson, but self-similar (Paxson and Floyd, 1994; and Willinger et al., 1995). What this means is that averaging over long periods of time does not smooth out the traffic. The average number of frames in each minute of an hour has as much variance as the average number of frames in each second of a minute. The consequence of this discovery is that most models of network traffic do not apply to the real world and should be taken with a grain (or better yet, a metric ton) of salt.
Switched Ethernet
As more and more stations are added to an Ethernet, the traffic will go up. Eventually, the LAN will saturate. One way out is to go to a higher speed, say, from 10 Mbps to 100 Mbps. But with the growth of multimedia, even a 100-Mbps or 1-Gbps Ethernet can become saturated.
Fortunately, there is an additional way to deal with increased load: switched Ethernet, as shown in Fig. 4-20. The heart of this system is a switch containing a high-speed backplane and room for typically 4 to 32 plug-in line cards, each containing one to eight connectors. Most often, each connector has a 10Base-T twisted pair connection to a single host computer.
Figure 4-20. A simple example of switched Ethernet.

When a station wants to transmit an Ethernet frame, it outputs a standard frame to the switch. The plug-in card getting the frame may check to see if it is destined for one of the other stations connected to the same card. If so, the frame is copied there. If not, the frame is sent over the high-speed backplane to the destination station's card. The backplane typically runs at many Gbps, using a proprietary protocol.
What happens if two machines attached to the same plug-in card transmit frames at the same time? It depends on how the card has been constructed. One possibility is for all the ports on the card to be wired together to form a local on-card LAN. Collisions on this on-card LAN will be detected and handled the same as any other collisions on a CSMA/CD network—with retransmissions using the binary exponential backoff algorithm. With this kind of plug-in card, only one transmission per card is possible at any instant, but all the cards can be transmitting in parallel. With this design, each card forms its own collision domain, independent of the others. With only one station per collision domain, collisions are impossible and performance is improved.
With the other kind of plug-in card, each input port is buffered, so incoming frames are stored in the card's on-board RAM as they arrive. This design allows all input ports to receive (and transmit) frames at the same time, for parallel, full-duplex operation, something not possible with CSMA/CD on a single channel. Once a frame has been completely received, the card can then check to see if the frame is destined for another port on the same card or for a distant port. In the former case, it can be transmitted directly to the destination. In the latter case, it must be transmitted over the backplane to the proper card. With this design, each port is a separate collision domain, so collisions do not occur. The total system throughput can often be increased by an order of magnitude over 10Base5, which has a single collision domain for the entire system.
Since the switch just expects standard Ethernet frames on each input port, it is possible to use some of the ports as concentrators. In Fig. 4-20, the port in the upper-right corner is connected not to a single station, but to a 12-port hub. As frames arrive at the hub, they contend for the ether in the usual way, including collisions and binary backoff. Successful frames make it to the switch and are treated there like any other incoming frames: they are switched to the correct output line over the high-speed backplane. Hubs are cheaper than switches, but due to falling switch prices, they are rapidly becoming obsolete. Nevertheless, legacy hubs still exist.
Fast Ethernet
At first, 10 Mbps seemed like heaven, just as 1200-bps modems seemed like heaven to the early users of 300-bps acoustic modems. But the novelty wore off quickly. As a kind of corollary to Parkinson's Law (''Work expands to fill the time available for its completion''), it seemed that data expanded to fill the bandwidth available for their transmission. To pump up the speed, various industry groups proposed two new ring-based optical LANs. One was called FDDI (Fiber Distributed Data Interface) and the other was called Fibre Channel [ ]. To make a long story short, while both were used as backbone networks, neither one made the breakthrough to the desktop. In both cases, the station management was too complicated, which led to complex chips and high prices. The lesson that should have been learned here was KISS (Keep It Simple, Stupid).
In any event, the failure of the optical LANs to catch fire left a gap for garden-variety Ethernet at speeds above 10 Mbps. Many installations needed more bandwidth and thus had numerous 10-Mbps LANs connected by a maze of repeaters, bridges, routers, and gateways, although to the network managers it sometimes felt that they were being held together by bubble gum and chicken wire.
It was in this environment that IEEE reconvened the 802.3 committee in 1992 with instructions to come up with a faster LAN. One proposal was to keep 802.3 exactly as it was, but just make it go faster. Another proposal was to redo it totally to give it lots of new features, such as real-time traffic and digitized voice, but just keep the old name (for marketing reasons). After some wrangling, the committee decided to keep 802.3 the way it was, but just make it go faster. The people behind the losing proposal did what any computer-industry people would have done under these circumstances—they stomped off and formed their own committee and standardized their LAN anyway (eventually as 802.12). It flopped miserably.
The 802.3 committee decided to go with a souped-up Ethernet for three primary reasons:
1.The need to be backward compatible with existing Ethernet LANs.
2.The fear that a new protocol might have unforeseen problems.
3.The desire to get the job done before the technology changed.
The work was done quickly (by standards committees' norms), and the result, 802.3u, was officially approved by IEEE in June 1995. Technically, 802.3u is not a new standard, but an addendum to the existing 802.3 standard (to emphasize its backward compatibility). Since practically everyone calls it fast Ethernet, rather than 802.3u, we will do that, too.
The basic idea behind fast Ethernet was simple: keep all the old frame formats, interfaces, and procedural rules, but just reduce the bit time from 100 nsec to 10 nsec. Technically, it would have been possible to copy either 10Base-5 or 10Base-2 and still detect collisions on time by just reducing the maximum cable length by a factor of ten. However, the advantages of 10Base-T wiring were so overwhelming that fast Ethernet is based entirely on this design. Thus, all fast Ethernet systems use hubs and switches; multidrop cables with vampire taps or BNC connectors are not permitted.
Nevertheless, some choices still had to be made, the most important being which wire types to support. One contender was category 3 twisted pair. The argument for it was that practically every office in the Western world has at least four category 3 (or better) twisted pairs running from it to a telephone wiring closet within 100 meters. Sometimes two such cables exist. Thus, using category 3 twisted pair would make it possible to wire up desktop computers using fast Ethernet without having to rewire the building, an enormous advantage for many organizations.
The main disadvantage of category 3 twisted pair is its inability to carry 200 megabaud signals (100 Mbps with Manchester encoding) 100 meters, the maximum computer-to-hub distance specified for 10Base-T. In contrast, category 5 twisted pair wiring can handle 100 meters easily, and fiber can go much farther. The compromise chosen was to allow all three possibilities, as shown in Fig. 4-21, but to pep up the category 3 solution to give it the additional carrying capacity needed.
Figure 4-21. The original fast Ethernet cabling.

The category 3 UTP scheme, called 100Base-T4, uses a signaling speed of 25 MHz, only 25 percent faster than standard Ethernet's 20 MHz (remember that Manchester encoding, as shown in Fig. 4-16, requires two clock periods for each of the 10 million bits each second). However, to achieve the necessary bandwidth, 100Base-T4 requires four twisted pairs. Since standard telephone wiring for decades has had four twisted pairs per cable, most offices are able to handle this. Of course, it means giving up your office telephone, but that is surely a small price to pay for faster e-mail.
Of the four twisted pairs, one is always to the hub, one is always from the hub, and the other two are switchable to the current transmission direction. To get the necessary bandwidth, Manchester encoding is not used, but with modern clocks and such short distances, it is no longer needed. In addition, ternary signals are sent, so that during a single clock period the wire can contain a 0, a 1, or a 2. With three twisted pairs going in the forward direction and ternary signaling, any one of 27 possible symbols can be transmitted, making it possible to send 4 bits with some redundancy. Transmitting 4 bits in each of the 25 million clock cycles per second gives the necessary 100 Mbps. In addition, there is always a 33.3-Mbps reverse channel using the remaining twisted pair. This scheme, known as 8B/6T (8 bits map to 6 trits), is not likely to win any prizes for elegance, but it works with the existing wiring plant.
For category 5 wiring, the design, 100Base-TX, is simpler because the wires can handle clock rates of 125 MHz. Only two twisted pairs per station are used, one to the hub and one from it. Straight binary coding is not used; instead a scheme called used4B/5Bis It is taken from FDDI and compatible with it. Every group of five clock periods, each containing one of two signal values, yields 32 combinations. Sixteen of these combinations are used to transmit the four bit groups 0000, 0001, 0010, ..., 1111. Some of the remaining 16 are used for control purposes such as marking frames boundaries. The combinations used have been carefully chosen to provide enough transitions to maintain clock synchronization. The 100Base-TX system is full duplex; stations can transmit at 100 Mbps and receive at 100 Mbps at the same time. Often 100Base-TX and 100Base-T4 are collectively referred to as 100Base-T.
The last option, 100Base-FX, uses two strands of multimode fiber, one for each direction, so it, too, is full duplex with 100 Mbps in each direction. In addition, the distance between a station and the hub can be up to 2 km.
In response to popular demand, in 1997 the 802 committee added a new cabling type, 100Base-T2, allowing fast Ethernet to run over two pairs of existing category 3 wiring. However, a sophisticated digital signal processor is needed to handle the encoding scheme required, making this option fairly expensive. So far, it is rarely used due to its complexity, cost, and the fact that many office buildings have already been rewired with category 5 UTP.
Two kinds of interconnection devices are possible with 100Base-T: hubs and switches, as shown in Fig. 4-20. In a hub, all the incoming lines (or at least all the lines arriving at one plug-in card) are logically connected, forming a single collision domain. All the standard rules, including the binary exponential backoff algorithm, apply, so the system works just like old-fashioned Ethernet. In particular, only one station at a time can be transmitting. In other words, hubs require half-duplex communication.
In a switch, each incoming frame is buffered on a plug-in line card and passed over a high-speed backplane from the source card to the destination card if need be. The backplane has not been standardized, nor does it need to be, since it is entirely hidden deep inside the switch. If past experience is any guide, switch vendors will compete vigorously to produce ever faster backplanes in order to improve system throughput. Because 100Base-FX cables are too long for the normal Ethernet collision algorithm, they must be connected to switches, so each one is a collision domain unto itself. Hubs are not permitted with 100Base-FX.
As a final note, virtually all switches can handle a mix of 10-Mbps and 100-Mbps stations, to make upgrading easier. As a site acquires more and more 100-Mbps workstations, all it has to do is buy the necessary number of new line cards and insert them into the switch. In fact, the standard itself provides a way for two stations to automatically negotiate the optimum speed (10 or 100 Mbps) and duplexity (half or full). Most fast Ethernet products use this feature to autoconfigure themselves.
Gigabit Ethernet
The ink was barely dry on the fast Ethernet standard when the 802 committee began working on a yet faster Ethernet (1995). It was quickly dubbed gigabit Ethernet and was ratified by IEEE in 1998 under the name 802.3z. This identifier suggests that gigabit Ethernet is going to be the end of the line unless somebody quickly invents a new letter after z. Below we will discuss some of the key features of gigabit Ethernet. More information can be found in (Seifert, 1998).
The 802.3z committee's goals were essentially the same as the 802.3u committee's goals: make Ethernet go 10 times faster yet remain backward compatible with all existing Ethernet standards. In particular, gigabit Ethernet had to offer unacknowledged datagram service with both unicast and multicast, use the same 48-bit addressing scheme already in use, and maintain the same frame format, including the minimum and maximum frame sizes. The final standard met all these goals.
All configurations of gigabit Ethernet are point-to-point rather than multidrop as in the original 10 Mbps standard, now honored as classic Ethernet. In the simplest gigabit Ethernet configuration, illustrated in Fig. 4-22(a), two computers are directly connected to each other. The more common case, however, is having a switch or a hub connected to multiple computers and possibly additional switches or hubs, as shown in Fig. 4-22(b). In both configurations each individual Ethernet cable has exactly two devices on it, no more and no fewer.
Figure 4-22. (a) A two-station Ethernet. (b) A multistation Ethernet.

Gigabit Ethernet supports two different modes of operation: full-duplex mode and half-duplex mode. The ''normal'' mode is full-duplex mode, which allows traffic in both directions at the same time. This mode is used when there is a central switch connected to computers (or other switches) on the periphery. In this configuration, all lines are buffered so each computer and switch is free to send frames whenever it wants to. The sender does not have to sense the channel to see if anybody else is using it because contention is impossible. On the line between a computer and a switch, the computer is the only possible sender on that line to the switch and the transmission succeeds even if the switch is currently sending a frame to the computer (because the line is full duplex). Since no contention is possible, the CSMA/CD protocol is not used, so the maximum length of the cable is determined by signal strength issues rather than by how long it takes for a noise burst to propagate back to the sender in the worst case. Switches are free to mix and match speeds. Autoconfiguration is supported just as in fast Ethernet.
The other mode of operation, half-duplex, is used when the computers are connected to a hub rather than a switch. A hub does not buffer incoming frames. Instead, it electrically connects all the lines internally, simulating the multidrop cable used in classic Ethernet. In this mode, collisions are possible, so the standard CSMA/CD protocol is required. Because a minimum (i.e., 64-byte) frame can now be transmitted 100 times faster than in classic Ethernet, the maximum distance is 100 times less, or 25 meters, to maintain the essential property that the sender is still transmitting when the noise burst gets back to it, even in the worst case. With a 2500-meter-long cable, the sender of a 64-byte frame at 1 Gbps would be long done before the frame got even a tenth of the way to the other end, let alone to the end and back.
The 802.3z committee considered a radius of 25 meters to be unacceptable and added two features to the standard to increase the radius. The first feature, called carrier extension, essentially tells the hardware to add its own padding after the normal frame to extend the frame to 512 bytes. Since this padding is added by the sending hardware and removed by the receiving hardware, the software is unaware of it, meaning that no changes are needed to existing software. Of course, using 512 bytes worth of bandwidth to transmit 46 bytes of user data (the payload of a 64-byte frame) has a line efficiency of 9%.
The second feature, called frame bursting, allows a sender to transmit a concatenated sequence of multiple frames in a single transmission. If the total burst is less than 512 bytes, the hardware pads it again. If enough frames are waiting for transmission, this scheme is highly efficient and preferred over carrier extension. These new features extend the radius of the network to 200 meters, which is probably enough for most offices.
In all fairness, it is hard to imagine an organization going to the trouble of buying and installing gigabit Ethernet cards to get high performance and then connecting the computers with a hub to simulate classic Ethernet with all its collisions. While hubs are somewhat cheaper than switches, gigabit Ethernet interface cards are still relatively expensive. To then economize by buying a cheap hub and slash the performance of the new system is foolish. Still, backward compatibility is sacred in the computer industry, so the 802.3z committee was required to put it in.
Gigabit Ethernet supports both copper and fiber cabling, as listed in Fig. 4-23. Signaling at or near 1 Gbps over fiber means that the light source has to be turned on and off in under 1 nsec. LEDs simply cannot operate this fast, so lasers are required. Two wavelengths are permitted: 0.85 microns (Short) and 1.3 microns (Long). Lasers at 0.85 microns are cheaper but do not work on single-mode fiber.
Figure 4-23. Gigabit Ethernet cabling.

Three fiber diameters are permitted: 10, 50, and 62.5 microns. The first is for single mode and the last two are for multimode. Not all six combinations are allowed, however, and the maximum distance depends on the combination used. The numbers given in Fig. 4-23 are for the best case. In particular, 5000 meters is only achievable with 1.3 micron lasers operating over 10 micron fiber in single mode, but this is the best choice for campus backbones and is expected to be popular, despite its being the most expensive choice.
The 1000Base-CX option uses short shielded copper cables. Its problem is that it is competing with high-performance fiber from above and cheap UTP from below. It is unlikely to be used much, if at all.
The last option is bundles of four category 5 UTP wires working together. Because so much of this wiring is already installed, it is likely to be the poor man's gigabit Ethernet.
Gigabit Ethernet uses new encoding rules on the fibers. Manchester encoding at 1 Gbps would require a 2 Gbaud signal, which was considered too difficult and also too wasteful of bandwidth. Instead a new scheme, called 8B/10B, was chosen, based on fibre channel. Each 8-bit byte is encoded on the fiber as 10 bits, hence the name 8B/10B. Since there are 1024 possible output codewords for each input byte, some leeway was available in choosing which codewords to allow. The following two rules were used in making the choices:
1.No codeword may have more than four identical bits in a row.
2.No codeword may have more than six 0s or six 1s.
These choices were made to keep enough transitions in the stream to make sure the receiver stays in sync with the sender and also to keep the number of 0s and 1s on the fiber as close to equal as possible. In addition, many input bytes have two possible codewords assigned to them. When the encoder has a choice of codewords, it always chooses the codeword that moves in the direction of equalizing the number of 0s and 1s transmitted so far. This emphasis of balancing 0s and 1s is needed to keep the DC component of the signal as low as possible to allow it to pass through transformers unmodified. While computer scientists are not fond of having the properties of transformers dictate their coding schemes, life is like that sometimes.
Gigabit Ethernets using 1000Base-T use a different encoding scheme since clocking data onto copper wire in 1 nsec is too difficult. This solution uses four category 5 twisted pairs to allow four symbols to be transmitted in parallel. Each symbol is encoded using one of five voltage levels. This scheme allows a single symbol to encode 00, 01, 10, 11, or a special value for control purposes. Thus, there are 2 data bits per twisted pair or 8 data bits per clock cycle. The clock runs at 125 MHz, allowing 1-Gbps operation. The reason for allowing five voltage levels instead of four is to have combinations left over for framing and control purposes.
A speed of 1 Gbps is quite fast. For example, if a receiver is busy with some other task for even 1 msec and does not empty the input buffer on some line, up to 1953 frames may have accumulated there in that 1 ms gap. Also, when a computer on a gigabit Ethernet is shipping data down the line to a computer on a classic Ethernet, buffer overruns are very likely. As a consequence of these two observations, gigabit Ethernet supports flow control (as does fast Ethernet, although the two are different).
The flow control consists of one end sending a special control frame to the other end telling it to pause for some period of time. Control frames are normal Ethernet frames containing a type of 0x8808. The first two bytes of the data field give the command; succeeding bytes provide the parameters, if any. For flow control, PAUSE frames are used, with the parameter telling how long to pause, in units of the minimum frame time. For gigabit Ethernet, the time unit is 512 nsec, allowing for pauses as long as 33.6 msec.
As soon as gigabit Ethernet was standardized, the 802 committee got bored and wanted to get back to work. IEEE told them to start on 10-gigabit Ethernet. After searching hard for a letter to follow z, they abandoned that approach and went over to two-letter suffixes. They got to work and that standard was approved by IEEE in 2002 as 802.3ae. Can 100-gigabit Ethernet be far behind?

IEEE 802.2: Logical Link Control
In contrast, we have not said a word about reliable communication. All that Ethernet and the other 802 protocols offer is a best-efforts datagram service. Sometimes, this service is adequate. For example, for transporting IP packets, no guarantees are required or even expected. An IP packet can just be inserted into an 802 payload field and sent on its way. If it gets lost, so be it.
Nevertheless, there are also systems in which an error-controlled, flow-controlled data link protocol is desired. IEEE has defined one that can run on top of Ethernet and the other 802 protocols. In addition, this protocol, called LLC (Logical Link Control), hides the differences between the various kinds of 802 networks by providing a single format and interface to the network layer. This format, interface, and protocol are all closely based on the HDLC protocol. LLC forms the upper half of the data link layer, with the MAC sublayer below it, as shown in Fig. 4-24.
Figure 4-24. (a) Position of LLC. (b) Protocol formats.

Typical usage of LLC is as follows. The network layer on the sending machine passes a packet to LLC, using the LLC access primitives. The LLC sublayer then adds an LLC header, containing sequence and acknowledgement numbers. The resulting structure is then inserted into the payload field of an 802 frame and transmitted. At the receiver, the reverse process takes place.
LLC provides three service options: unreliable datagram service, acknowledged datagram service, and reliable connection-oriented service. The LLC header contains three fields: a destination access point, a source access point, and a control field. The access points tell which process the frame came from and where it is to be delivered, replacing the DIX Type field. The control field contains sequence and acknowledgement numbers, very much in the style of HDLC, but not identical to it. These fields are primarily used when a reliable connection is needed at the data link level. For the Internet, best-efforts attempts to deliver IP packets is sufficient, so no acknowledgements at the LLC level are required.

Retrospective on Ethernet
Ethernet has been around for over 20 years and has no serious competitors in sight, so it is likely to be around for many years to come. Few CPU architectures, operating systems, or programming languages have been king of the mountain for two decades going on three. Clearly, Ethernet did something right. What?
Probably the main reason for its longevity is that Ethernet is simple and flexible. In practice, simple translates into reliable, cheap, and easy to maintain. Once the vampire taps were replaced by BNC connectors, failures became extremely rare. People hesitate to replace something that works perfectly all the time, especially when they know that an awful lot of things in the computer industry work very poorly, so that many so-called ''upgrades'' are appreciably worse than what they replaced.
Simple also translates into cheap. Thin Ethernet and twisted pair wiring is relatively inexpensive. The interface cards are also low cost. Only when hubs and switches were introduced were substantial investments required, but by the time they were in the picture, Ethernet was already well established.
Ethernet is easy to maintain. There is no software to install (other than the drivers) and there are no configuration tables to manage (and get wrong). Also, adding new hosts is as simple as just plugging them in.
Another point is that Ethernet interworks easily with TCP/IP, which has become dominant. IP is a connectionless protocol, so it fits perfectly with Ethernet, which is also connectionless. IP fits much less well with ATM, which is connection oriented. This mismatch definitely hurt ATM's chances.
Lastly, Ethernet has been able to evolve in certain crucial ways. Speeds have gone up by several orders of magnitude and hubs and switches have been introduced, but these changes have not required changing the software. When a network salesman shows up at a large installation and says: ''I have this fantastic new network for you. All you have to do is throw out all your hardware and rewrite all your software,'' he has a problem. FDDI, Fibre Channel, and ATM were all faster than Ethernet when introduced, but they were incompatible with Ethernet, far more complex, and harder to manage. Eventually, Ethernet caught up with them in terms of speed, so they had no advantages left and quietly died off except for ATM's use deep within the core of the telephone system.

Timbunan Pahala di Ramadhan

2009-08-27T14:05:00.003+07:00

Imam Thabrani meriwayatkan suatu hadits dari sahabat Ubadah bin Shamit r.a. yang menyatakan bahwa suatu hari di bulan Ramadlan Rasulullah saw. bersabda:
“Telah datang bulan Ramadlan kepada kalian, bulan barakah yang di dalamnya Allah men- datangi kalian. Maka turunlah rahmat. Dan dihapuskanlah kesalahan-kesalahan. Dan di bulan itu Allah mengabulkan doa. Di bulan itu Allah melihat (memperhatikan) perlombaan di antara kalian. Dan Allah membanggakan kalian kepada para malaikatNya. Maka perlihatkanlah kepada Allah kebaikan kalian sebab orang yang celaka adalah yang tidak mendapatkan rahmat Allah di dalamnya “.

Hadits tersebut menjelaskan secara gamblang bahwa bulan Ramadlan adalah bulan barakah. Allah SWT memberkahi orang-orang mukmin. Makanan dan minuman mereka men- jadi berkah. Meskipun sedikit, tetapi menge- nyangkan dan menghilangkan dahaga. Harta dan shadaqah mereka pun berkah, yakni ditambah dan diperbanyak oleh Allah SWT. Pendekatan diri mereka kepadaNya pun diberkahi dan amal-amal shalih mereka pun dilipat gandakan hingga 700 kali lipat. Allah SWT memuliakan mereka pada bulan ini lebih dari bulan-bulan lain. Pada bulan ini para hamba Allah merasakan sentuhan ketuhanan dengan turunnya rahmat, yang hanya diperoleh oleh hamba-hambaNya yang berpuasa. Kesalahan-kesalahan mereka dihapus dan pada akhir Ramadlan dosa-dosa mereka diampuni. Jadi berkat rahmat Allah Yang Maha Pengam- pun, bulan Ramadlan menjadi bulan cuci dosa tahunan bagi muslim yang berpuasa. Tentu yang dimaksud adalah dosa-dosa kecil sebagai- mana sabda Rasulullah saw. “Ramadlan hingga Ramadlan menjadi penghapus dosa di antara keduanya jika dijauhi dosa-dosa besar”.
Pada bulan Ramadlan Allah SWT men- jawab doa orang-orang muslim yang berpuasa sejak terbit fajar hingga ia berbuka puasa. Rasulullah saw. bersabda:
“Tiga kategori orang yang doa mereka tidak ditolak (oleh Allah SWT), orang yang berpuasa tatkala ia berbuka, Imam yang adil, dan doa orang yang dizhalimi. Allah mengangkat doa itu di atas awan dan pintu-pintu langit dibuka untuknya. Dan Allah Rabbul alamin berfirman: ‘Demi Kemuliaan dan KeagunganKu, niscaya aku akan menolongmu meskipun nanti” (HR. Ahmad dan Tirmidzi).
Dalam hadits di atas Rasulullah menye- butkan bahwa Allah SWT melihat serta mem- perhatikan, dan tentunya memberikan penilaian perlombaan amal shalih yang kita lakukan: “yanzhurullah ila tanaafusikum fiih”. Dalam penggalan hadits ini Allah mendorong kaum muslimin agar berlomba-lomba dalam melak- sanakan berbagai ketaatan, seperti shalat-shalat sunnah, dzikir, doa, dan melaksanakan umrah. Juga berlomba melaksanakan aktivitas-aktivitas kebajikan seperti memberikan shadaqah untuk fakir miskin dan orang-orang yang membutuh- kan, melaksanakan amar ma’ruf nahi mungkar, serta menyebarkan Islam di tengah-tengah masyarakat dengan jalan mengajarkan dan mendakwahkannya. Perlombaan yang disaksi- kan Allah SWT tersebut berlangsung sepanjang bulan Ramadlan. Dan perlombaan amal shalih ini dibanggakan oleh Allah SWT kepada para malaikat muqarrabin, makhluk Allah penghuni alam yang tinggi yang ketaatannya paten itu (QS. At Tahrim 6). Setiap muslim yang men- dengar sabda Rasulullah saw. itu tentu tak mau ketinggalan dalam perlombaan amal shalih tersebut. Dengan itu Allah mengangkat derajat- nya di sisiNya, bahkan Allah SWT menyebut- nyebut namanya di kalangan masyarakat yang tinggi (almala’u a’la) dan membanggakannya di hadapan para malaikatNya.
Oleh karena itu, Rasulullah dalam hadits di atas mengatakan: “Perlihatkanlah kebaikan kalian kepada Allah”. Seakan-akan Rasulullah saw. bersabda kepada kaum muslimin: “Turunlah kalian ke arena perlomba- an super akbar ini dengan penampilan paling prima!”. Inilah kesempatan kita memecahkan rekor-rekor dalam cabang-cabang amal shalih, dalam bidang-bidang ibadah dan pendekatan diri kepada Allah SWT, baik yang bersifat ketaatan ritual maupun aktivitas kebajikan di masyarakat. Apalagi cabang yang paling ber- gengsi di sisi Allah, yakni aktivitas menggusur hukum kufur dan mengembalikan posisi hukum Al Quran dengan mengembalikan Khilafah Islamiyyah. Termasuk yang paling bergengsi pula adalah melaksanakan kewajiban jihad fi sabilillah memerangi kaum kafir di bulan Rama- dlan. Yang terakhir ini seperti yang dilakukan oleh Rasulullah saw. dan para sahabatnya yang mulia. Mereka memerangi kaum Kafir Quraisy pertama kali dalam medan perang Badar pada bulan Ramadlan tahun 2 H. Mereka membe- baskan kota Makkah dari kedaulatan Quraisy pada tahun 8 H. Mereka menjebol benteng pertahanan terakhir kaum Yahudi di Khaibar pada bulan Ramadlan 6 H. Demikian pula kaum muslimin berikutnya, tak ketinggalan berjihad memerangi kaum kafir meskipun mereka sedang berpuasa menahan lapar dan haus. Pembebasan Andalusia dan pengusiran pasukan Tartar dari negeri Syam merupakan sebagian bukti sejarah kemuliaan dan keperka- saan mereka serta keberkahan aktivitas mereka.
Lipat Ganda Pahala
Salah satu keberkahan bulan Ramadlan adalah dilipatgandakannya pahala amalan sha- lih seorang muslim. Dalam hadits yang diriwayatkan oleh Imam Ibnu Khuzaimah antara lain disebutkan bahwa Rasulullah saw. bersabda:
“Siapa saja yang mendekatkan diri kepada Allah dengan perbuatan baik (sunnah/mandub) pada bulan Ramadlan, (ia diganjar pahala) sama seperti menunaikan suatu kewajiban (fardlu) pada bulan yang lain. Siapa saja yang menu- naikan kewajiban (fardlu) di bulan Ramadlan , (ia diganjar pahala) sama dengan orang yang mengerjakannya 70 kali kewajiban tersebut di bulan yang lain”.
Hadits ini kita tampilkan untuk mem- buat suatu proyeksi amal shalih agar kita bisa lebih teliti dan efektif dalam beramal. Sebab, Allah SWT sendiri telah mengabarkan akan menyeleksi kita, siapa yang terbaik amalannya. Dia berfirman:
“Dialah yang telah menciptakan hidup dan mati agar dia menguji siapa di antara kalian yang terbaik amalannya” (QS. Al Mulk 2).
Dari informasi hadits di atas dapat kita duga secara relatif –dan Allah SWT yang Maha Mengetahui dan paling berhak memberikan penilaian dan pahala– nilai-nilai aktivitas kaum muslimin yang berpuasa di bulan Ramadlan. Puasa Ramadlan sendiri tak ada bandingannya, karena Allah SWT sendiri yang bakal meng- hitung balasannya, sebagaimana disebut dalam sabda Rasulullah saw:
“Segala amal kebajikan anak Adam dilipat- gandakan pahalanya dengan 10 hingga 700 kali lipat. Allah berfirman: ‘kecuali puasa, puasa itu untukKu dan Aku (sendiri) yang akan mem- berikan pahala kepadanya. Dia telah mening- galkan syahwat dan makan minum lantaran Aku’…” (HR. Muslim).
Namun aktivitas-aktivitas lain yang kita lakukan sambil berpuasa perlu kita ketahui dan dugaan relatif kita tentunya adalah untuk mem- perkuat motivasi kita dalam melakukan amal shalih.
Jika kita melaksanakan shalat fardlu –dan ini harus tentunya– lima waktu selama sebulan penuh, maka kita insyaallah dipahalai sama dengan melaksanakannya 70 bulan atau kurang lebih 5 tahun sepuluh bulan pada bulan yang lain. Jika kita laksanakan shalat sunnat rawatib (pengiring shalat lima waktu), maka insyaallah kita dihitung sama dengan melak- sanakan shalat fardlu tersebut pada bulan yang lain. Jika kita melaksanakan shalat tarawih se- bulan penuh, maka kita insyaallah dihitung sama dengan melaksanakan shalat fardlu pada bulan yang lain dengan jumlah rakaat sesuai bilangan rakaat tarawih kita. Shalat-shalat sun- nah lainnya pun alangkah baiknya kita kerjakan karena insyaallah status pahalanya sama dengan shalat-shalat fardlu pada bulan-bulan yang lain. Sedangkan sekali melaksanakan shalat fardlu di masjid secara berjamaah, insya- allah dinilai sama dengan 70 kali shalat fardlu sendirian, artinya sama dengan 4900 kali shalat fardlu sendirian di bulan yang lain. Adapun memberikan pesan-pesan taqwa pada mimbar Jumat, insyaallah dinilai sama 70 kali berkhut- bah Jumat pada bulan yang lain.
Melaksanakan umrah di bulan Rama- dlan, insyaallah mendapatkan pahala setara dengan pahala menunaikan ibadah haji atau menurut suatu hadits riwayat Imam Muslim disebut seutama dengan pergi haji bersama Rasulullah saw. Tentu saja tanpa menggugur- kan kewajiban haji yang bersangkutan.
Membayar shadaqah sunnah di bulan Ramadlan, insyaallah mendapatkan pahala membayar zakat di bulan yang lain. Sedangkan membayar zakat di bulan Ramadlan, insyaallah mendapatkan pahala membayar zakat selama 70 tahun. Memberikan pinjaman kepada sau- dara muslim yang membutuhkan, insyaallah mendapatkan pahala membayar kewajiban naf- kah di bulan yang lain. Sedangkan membayar utang di bulan Ramadlan kiranya sebagaimana membayar nafkah wajib, insyaallah akan men- dapatkan pahala sama dengan 70 kali mem- bayar kewajiban tersebut.
Mengambil keputusan hukum atas perkara-perkara dengan menggunakan hukum Allah SWT dan bersikap adil di bulan Rama- dlan, insyaallah mendapatkan pahala seperti 70 kali mengambil keputusan dengan cara tersebut di bulan lain. Berdakwah mengajak kaum muslimin meninggalkan hukum kufur dan me- ngembalikan hukum Al Quran di bulan Rama- dlan, insyaallah akan mendapatkan pahala setara dengan melakukan dakwah seperti itu sebanyak 70 kali di bulan yang lain. Sedangkan sikap ikhlas penuh kerendahan hati mengambil keputusan politik untuk kembali kepada hukum Allah di bulan Ramadlan, insyaallah akan men- dapatkan pahala sebanyak 70 kali pengambilan keputusan yang luhur itu.
Berjihad memerangi pasukan kafir di bulan Ramadlan, insyaallah akan mendapatkan pahala 70 kali puncak ibadah di dalam Islam itu jika dikerjakan pada bulan yang lain. Berpatroli menjaga keamanan pasukan kaum muslimin di bulan Ramadlan, insyaallah dinilai sama dengan 70 kali patroli yang sama di bulan lain. Dan menjaga perbatasan atau menjadi murabithuun (QS. Ali Imran 200) sebulan penuh di bulan Ramadlan, insyaallah mendapat pahala setara dengan menjaga perbatasan selama 70 bulan di bulan yang lain.
Menyuruh seorang muslimah berjilbab pada saat pergi ke luar rumah pada bulan Ramadlan insyaallah mendapatkan pahala seperti menyuruhnya 70 kali pada bulan yang lain. Melarang seorang pemuda minum khamr, minum ecstasy, dan mengisap ganja di bulan Ramadlan, insyaallah mendapatkan pahala melarangnya 70 kali di bulan yang lain. Sedangkan menutup pabrik-pabrik barang- barang terlarang tersebut serta menutup jalur- jalur peredarannya dengan tanda tangan penguasa, insyaallah mendapatkan pahala yang jauh lebih besar. Orang yang menasihati penguasa agar lurus dengan perintah Allah SWT, dan melarang mereka mengingkari kete- tapan Allah SWT di bulan Ramadlan, insyaallah akan mendapatkan pahala setara dengan kalau ia lakukan 70 kali di bulan yang lain. Adapun kalau ia dibunuh penguasa pada bulan Rama- dlan lantaran tindakan luhurnya itu, insyaallah bagaikan ia mati syahid 70 kali di bulan yang lain.
Bertafaquh fiddin menuntut ilmu-ilmu Islam selama seminggu di bulan Ramadlan, insyaallah mendapat pahala setara dengan me- nuntutnya selama 70 minggu di bulan yang lain. Mempelajari dan meneliti sains dan teknologi selama sebulan di bulan Ramadlan, insyaallah dipahalai seperti mempelajari dan menelitinya selama 70 bulan di bulan yang lain.
Pendek kata, pada bulan ini kaum mus- limin panen pahala secara besar-besaran. Lebih-lebih pada malam kemuliaan (lailatul qadar), yang nilainya melebihi seribu bulan.
Teliti Sebelum Beramal
Kalau dalam hadits di atas Rasulullah saw. menyuruh kaum muslimin agar menampil- kan kebaikan mereka kepada Allah SWT di bulan Ramadlan ini, arti sebaliknya (mafhum mukhalafah) adalah beliau saw. melarang kita menampilkan keburukan-keburukan kita di bulan Ramadlan. Jika dalam bulan Ramadlan Allah SWT melipatgandakan pahala perbuatan baik kita, maka dapat kita mengerti kalau Allah akan menghukum berat pula siapa saja yang melakukan kemaksiatan di bulan Ramadlan. Jika Rasulullah saw. menghukum seseorang yang bergaul dengan istrinya di siang hari bulan Ramadlan dengan alternatif-alternatif yang berat, yaitu: melaksanakan puasa berturut-turut 2 bulan, atau membebaskan budak, atau mem- berikan makan kepada 60 orang fakir miskin; bagaimana pula hukuman untuk yang berzina di siang seperti itu? Bahkan untuk orang yang sengaja tidak berpuasa di bulan Ramadlan, tanpa alasan untuk mendapatkan rukhshah, Rasulullah saw. dengan tegas mengancam yang bersangkutan dengan predikat kafir dan puasa yang ditinggalkannya itu tak pernah dapat dilunasinya!
Abu Ya’la meriwayatkan hadits dari Ibnu Abbas bahwa Rasulullah saw. bersabda:
“Sendi-sendi dan dasar-dasar Islam itu ada tiga, atasnyalah didirikan Islam. Siapa saja yang me- ninggalkan salah satu darinya, maka ia dinilai kafir dan halal darahnya. Ketiga sendi itu ada- lah: mengakui bahwasanya tiada Tuhan yang wajib disembah kecuali Allah, shalat fardlu (lima waktu), dan puasa Ramadlan” (lihat Prof. Dr. T.M. Hasbi As Shiddieqi. Pedoman Puasa. hal 363).
Imam Bukhari meriwayatkan hadits dari Abu Hurairah bahwasanya Rasulullah saw. bersabda:
“Siapa saja yang berbuka suatu hari di bulan Ramadlan tanpa udzur dan tidak sakit, niscaya puasa yang ditinggalkannya itu tak dapat dilunasinya (qadla) dengan puasa sepanjang masa, sekalipun ia (sanggup) melakukannya”.
Dengan demikian, sebagai hamba Allah SWT yang yakin bahwa kita ini milikNya dan akan kembali pulang kepadaNya, maka hen- daknya kita selalu meneliti perbuatan yang akan kita amalkan. Apakah itu bernilai positif, nol, atau negatif di sisi Allah SWT. Jika kita ketemu perbuatan yang sama-sama positif, mana yang lebih besar nilainya di sisi Allah SWT, itu yang kita prioritaskan. Dengan ketelitian itu kita akan dapat mengeruk keuntungan yang sebesar- besarnya berupa pahala sebanyak-banyaknya dan kemuliaan dari Allah yang setinggi- tingginya. Insyaallah!.

Diet Dalam 13 Hari

2009-08-05T09:42:00.001+07:00

MENU DIET 13 HARI
PANTANGAN : GARAM (ASIN)
SYARAT :
1. Satu hari minum 8 gelas besar air putih (jangan air es), kurang lebih 2 liter air putih
2. Daging bistik digoreng dalam Danish Butter/Minyak Jagung/Butter unsalted (tawar)
3. Sayuran biasanya 1 ikat bayam, jangan direbus terlalu lama (tanpa garam), dan selada dimakan mentah
4. Makan malam terakhir pk. 18:00
5. Air putih dapat diminum setiap jam
6. Sebaiknya tinggal di rumah karena kita buang air kecil bisa mencapai 8x sehari
7. Buang air besar jarang sekali karena hampir semua makanan diserap oleh tubuh
8. Diet ini hanya menghilangkan lemak, jangan takut ada efek sampingan

KEUNTUNGAN :
1. Diet ini jika diikuti betul-betul (tidak menyimpang sedikitpun), akan menurunkan berat badan anda 7-8 kg
2. Diet ini telah diselidiki bahwa keadaan kimia di dalam tubuh semakin baik, sehingga tidak ada kemungkinan untuk bisa menjadi gemuk lagi
3. Asalkan setelah hari ke-14, makannya secara normal kembali (tidak berlebih-lebihan)
4. Diet ini cukup dilakukan sekali dalam setahun

YANG PERLU DIPERHATIKAN :
Apabila lupa, tiba-tiba makan sesuatu yang menyimpang dari diet ini, misalnya hari ke-4 atau misalnya sudah hampir selesai (hari ke-12), diet ini harus diulang lagi dari hari pertama. Sebab nantinya walaupun berat badan kita turun, tetapi kemungkinan untuk bisa gemuk lagi akan terjadi dengan adanya penyimpangan tersebut.

MENU DIET :
Hari ke-1 dan ke-8 :
Pagi : 1 cangkir kopi/teh dengan 1 sendok gula equal
Siang : 2 butir telur rebus matang + 1 ikat bayam direbus sebentar + 1 buah
tomat segar
Malam : 100 gr daging bistik goreng + slada diberi jeruk citrun/jeruk nipis

Hari ke-2 dan ke-9 :
Pagi : 1 cangkir kopi/teh dengan 1 sendok gula equal
Siang : 60 gr bistik goreng + slada + 1 buah segar
(jeruk/semangka/ belimbing/ jambu raksasa)
Malam : 120 gr ayam kukus + 1 gelas susu nonfat tanpa gula

Hari ke-3 dan ke-10 :
Pagi : 1 cangkir kopi/teh + 1 sendok gula equal + 1 potong roti tawar bakar
Siang : 2 butir telur rebus + 1 buah tomat + 1 buah segar + 1 ikat
bayam/kangkung
Malam : 120 gr ayam kukus + slada yang ditaburi jeruk citrun/nipis

Hari ke-4 dan ke-11 :
Pagi : 1 cangkir kopi/teh + 1 sendok gula equal + 1 potong roti tawar bakar
Siang : 1 telur rebus + 1 wortel (rebus sebentar) + 1 potong keju non fat
Malam : 1 mangkok penuh pepaya ditaburi jeruk citrun/nipis + 1 gelas susu non
fat tanpa gula

Hari ke-5 dan ke-12 :
Pagi : 1 wortel besar mentah diparut kasar ditaburi jeruk nipis
Siang : 120 gr ayam kukus dengan sedikit saus mentega yang tidak asin
Malam : 60 gr bistik goreng (minyak jagung) + selada mentah + bayam direbus sebentar

Hari ke-6 dan ke-13
Pagi : 1 cangkir kopi/teh tanda gula + 1 potong roti tawar bakar
Siang : 120 gr ayam kukus + selada yang ditaburi jeruk nipis
Malam : 2 telur rebus + 1 wortel besar mentah parut ditaburi jeruk nipis

Hari ke-7
Pagi : 1 cangkir kopi/teh tanpa gula equal
Siang : 60 gr bistik goreng + 1 buah segar
Malam : Dilarang Makan

Menu Sehat

2009-08-05T09:38:00.001+07:00

Agar tubuh kita sehat, maka setiap hari kita harus memilih asupan makanan yang benar-benar dibutuhkan oleh tubuh kita. Makanan yang lengkap dan kecukupan gizi.

Berikut adalah tips memilih maupun menyajikan menu makanan yang harus diperhatikan, agar tubuh tetap sehat setiap hari :

* Saat tubuh dalam kondisi kurang fit, perbanyaklah makan tempe, tahu, serta kacang-kacangan yang banyak mengandung protein. Perbanyak juga mengkonsumsi makan yang mengandung vitamin C dan vitamin E yang berfungsi sebagai antioksidan dalam tubuh yang bersifat mencegah oksidasi serta memperbaiki sel tubuh.
* Usahakan mengkonsumsi makanan seimbang setiap hari. Mengkonsumsi makanan pokok, sayuran, dan buah-buahan.
* Konsumsi makanan yang berbeda setiap hari untuk memenuhi kebutuhan gizi, serta vitamin dan mineral yang diperlukan oleh tubuh. Misalnya jika hari ini Anda terlalu banyak konsumsi daging, usahakan besok banyak mengkonsumsi sayuran dan buah-buahan dengan mengurangi konsumsi daging.
* Hindari terlalu sering mengkonsumi makanan instant maupun junk food.
* Jika Anda terpaksa banyak konsumsi makanan junk food, imbangi dengan perbanyak sayur dan dan buah-buahan.
* Kebersihan dalam mengolah makanan yang kita konsumsi sangat berpengaruh terhadap kesehatan kita.

Semoga bermanfaat.