bab 5

Bab 5 Data Encoding Sistem Komunikasi Data 1

William StallingsKomunikasi Data dan Komputer

Bab 5Data Encoding

Diterjemahkan oleh Andi SusiloE-mail: [email protected] Teknik, Jurusan Teknik Elektro, Peminatan Teknik TelekomunikasiUniversitas Krisnadwipayana, Jakarta, November 2005


Teknik-teknik Encoding

Digital data, digital signalAnalog data, digital signalDigital data, analog signalAnalog data, analog signal


Data Digital, Sinyal Digital

Sinyal digitalDiskrit, pulsa-pulsa tegangan diskontinuSetiap pulsa adalah merupakan elemen sinyalData biner di-encoding menjadi elemen-elemensinyal

Istilah-istilah (1)Unipolar

Semua elemen sinyal memiliki tanda yang sama

PolarKondisi logika satu diwakili oleh tegangan positif, kondisi yang lain oleh tegangan negatif


Istilah-istilah (2)

Data rateTarif dari pentransmisian data dalam bits per second (bps)

Durasi atau panjang dari sebuah bitWaktu yang dibutuhkan bagi transmitter untukmemancarkan bit

Modulation rateNilai saat level sinyal berubahDiukur dalam satuan baud = jumlah elemen sinyal per detik

Mark and Space (Nilai dan ruang)Biner 1 dan Biner 0


Penafsiran Sinyal-sinyal

Hal yang perlu diketahuiPewaktuan bit-bit – saat dimulai dan berakhirLevel-level sinyal

Faktor-faktor yang mempengaruhi keberhasilan dalampenafsiran sinyal-sinyal

Rasio sinyal terhadap noiseData rateBandwidth


Perbandingan dari skema-skema Encoding (1)

Spektrum SinyalKelemahan dari frekuensi tinggi adalah mengurangi bandwidth yang dibutuhkanKelemahan dari komponen dc mengijinkan kopling ac melaluitransformer, penyediaan isolasiMengkonsentrasikan daya di tengah bandwidth

ClockingPensinkronisasian transmitter dan receiverClock EksternalMekanisme sinkronisasi berbasis sinyal


Perbandingan dari skema-skema Encoding (2)

Deteksi ErrorDapat dibangun kepada peng-encoding-an sinyal

Interferensi sinyal dan kekebalan terhadap noiseBeberapa kode lebih baik daripada yang lainnya

Biaya dan kompleksitasSemakin tinggi tarif sinyal (& maka data rate) mengarah kepadabiaya yang lebih tinggiBeberapa kode membutuhkan tarif sinyal yang lebih besardaripada tarif data (data rate)


Skema-skema Encoding

Nonreturn to Zero-Level (NRZ-L)Nonreturn to Zero Inverted (NRZI)Bipolar -AMIPseudoternaryManchesterDifferential ManchesterB8ZSHDB3


Nonreturn to Zero-Level (NRZ-L)

Dua tegangan yang berbeda untuk bit 0 dan 1 Tegangan konstan selama interval bit

Tidak ada transisi I.e. no return to zero voltage

e.g. Tidak ada tegangan bagi nol (0), teganganpositif konstan bagi satu (1)Lebih sering dipakai, tegangan negatif untuknilai satu dan positif untuk nilai nolIni adalah NRZ-L


Nonreturn to Zero Inverted

Nonreturn to zero inverted on onesPulsa tegangan konstan selama durasi bitData di-encode sebagaimana ada atau tidaknyatransisi sinyal di permulaan waktu bitTransisi (dari rendah ke tinggi atau tinggi kerendah) dicatat sebagai biner 1Tidak ada transisi dicatat sebagai biner 0Sebuah contoh dari differential encoding


NRZ


Differential Encoding

Data direpresentasikan oleh perubahan daripadalevel-levelnyaDeteksi transisi lebih dapat diandalkan daripadalevelDidalam layout-layout transmisi yang kompleksadalah mudah untuk kehilangan indera polaritas


NRZ pros dan cons

ProsMudah bagi engineerPenggunaan bandwidth yang baik

ConsKomponen dcLemahnya kemampuan sinkronisasi

Digunakan untuk perekaman magnetikTidak sering digunakan untuk transmisi sinyal


Multilevel Binary

Memakai lebih dari dua levelBipolar-AMI

Nol diwakili oleh tidak ada jalur sinyalSatu diwakili oleh pulsa positif atau negatifPulsa-pulsa satu bergantian didalam polaritasTidak kehilangan sinkronisasi jika terdapat pita yang panjang dari satu (nol-nol masih menjadi masalah)Tidak bersih dari komponen dcBandwidth lebih rendahMudah dalam mendeteksi kesalahan


Pseudoternary

Satu diwakili oleh ketiadaan jalur sinyalNol diwakili oleh pergantian positif dan negatifTidak ada keuntungan dan kerugian melewatibipolar-AMI


Trade Off for Multilevel Binary

Tidak seefisien NRZSetiap elemen sinyal hanya mewakili satu bitDidalam sistem level 3 bisa mewakili log23 = 1.58 bitsReceiver harus bisa membedakan diantara ketiga level (+A, -A, 0)Membutuhkan sekitar 3dB lebih besar dari daya sinyaluntuk probablilitas kesalahan bit yang sama


Biphase

ManchesterTransition in middle of each bit periodTransition serves as clock and dataLow to high represents oneHigh to low represents zeroUsed by IEEE 802.3

Differential ManchesterMidbit transition is clocking onlyTransition at start of a bit period represents zeroNo transition at start of a bit period represents oneNote: this is a differential encoding schemeUsed by IEEE 802.5


Biphase Pros dan Cons

ConAt least one transition per bit time and possibly twoMaximum modulation rate is twice NRZRequires more bandwidth

ProsSynchronization on mid bit transition (self clocking)No dc componentError detection⌧Absence of expected transition


Modulation Rate


Scrambling

Use scrambling to replace sequences that would produce constant voltageFilling sequence

Must produce enough transitions to syncMust be recognized by receiver and replace with originalSame length as original

No dc componentNo long sequences of zero level line signalNo reduction in data rateError detection capability


B8ZS

Bipolar With 8 Zeros SubstitutionBased on bipolar-AMIIf octet of all zeros and last voltage pulse preceding was positive encode as 000+-0-+If octet of all zeros and last voltage pulse preceding was negative encode as 000-+0+-Causes two violations of AMI codeUnlikely to occur as a result of noiseReceiver detects and interprets as octet of all zeros


HDB3

High Density Bipolar 3 ZerosBased on bipolar-AMIString of four zeros replaced with one or two pulses


B8ZS and HDB3


Digital Data, Analog Signal

Public telephone system300Hz to 3400HzUse modem (modulator-demodulator)

Amplitude shift keying (ASK)Frequency shift keying (FSK)Phase shift keying (PK)


Teknik-teknik Modulasi


Amplitude Shift Keying

Values represented by different amplitudes of carrierUsually, one amplitude is zero

i.e. presence and absence of carrier is used

Susceptible to sudden gain changesInefficientUp to 1200bps on voice grade linesUsed over optical fiber


Frequency Shift Keying

Values represented by different frequencies (near carrier)Less susceptible to error than ASKUp to 1200bps on voice grade linesHigh frequency radioEven higher frequency on LANs using co-ax


FSK on Voice Grade Line


Phase Shift Keying

Phase of carrier signal is shifted to represent dataDifferential PSK

Phase shifted relative to previous transmission rather than some reference signal


Quadrature PSK

More efficient use by each signal element representing more than one bit

e.g. shifts of π/2 (90o)Each element represents two bitsCan use 8 phase angles and have more than one amplitude9600bps modem use 12 angles , four of which have two amplitudes


Performance of Digital to Analog Modulation Schemes

BandwidthASK and PSK bandwidth directly related to bit rateFSK bandwidth related to data rate for lower frequencies, but to offset of modulated frequency from carrier at high frequencies(See Stallings for math)

In the presence of noise, bit error rate of PSK and QPSK are about 3dB superior to ASK and FSK


Analog Data, Digital Signal

DigitizationConversion of analog data into digital dataDigital data can then be transmitted using NRZ-LDigital data can then be transmitted using code other than NRZ-LDigital data can then be converted to analog signalAnalog to digital conversion done using a codecPulse code modulationDelta modulation


Pulse Code Modulation(PCM) (1)

If a signal is sampled at regular intervals at a rate higher than twice the highest signal frequency, the samples contain all the information of the original signal

(Proof - Stallings appendix 4A)

Voice data limited to below 4000HzRequire 8000 sample per secondAnalog samples (Pulse Amplitude Modulation, PAM)Each sample assigned digital value


Pulse Code Modulation(PCM) (2)

4 bit system gives 16 levelsQuantized

Quantizing error or noiseApproximations mean it is impossible to recover original exactly

8 bit sample gives 256 levelsQuality comparable with analog transmission8000 samples per second of 8 bits each gives 64kbps


Nonlinear Encoding

Quantization levels not evenly spacedReduces overall signal distortionCan also be done by companding


Delta Modulation

Analog input is approximated by a staircase functionMove up or down one level (δ) at each sample intervalBinary behavior

Function moves up or down at each sample interval


Contoh- Modulasi Delta


Delta Modulation - Operation


Delta Modulation - Performance

Good voice reproduction PCM - 128 levels (7 bit)Voice bandwidth 4khzShould be 8000 x 7 = 56kbps for PCM

Data compression can improve on thise.g. Interframe coding techniques for video


Analog Data, Analog Signals

Why modulate analog signals?Higher frequency can give more efficient transmissionPermits frequency division multiplexing (chapter 8)

Types of modulationAmplitudeFrequencyPhase


Modulasi Analog


Spread Spectrum

Analog or digital dataAnalog signalSpread data over wide bandwidthMakes jamming and interception harderFrequency hoping

Signal broadcast over seemingly random series of frequencies

Direct SequenceEach bit is represented by multiple bits in transmitted signalChipping code


Required Reading

Stallings chapter 5

bab 5

Documents

data digital

data low

digital signal digital

digital signal analog

analog signal analog

yang sama bab

jumlah elemen sinyal

nrz bab