DATA COMMUNICATION(ELA…)

SIGNAL ENCODING TECHNIQUES1

ENCODING TECHNIQUES Digital data

Analog data

2

• Digital Signal• Analog Signal• Digital Signal• Analog Signal

ENCODING ONTO A DIGITAL SIGNAL

3

MODULATION ONTO AN ANALOG SIGNAL

4

DATA ENCODING CRITERIA

An increase in DR increases BER An increase in SNR decreases BER An increase in BW allows an increase in

DR

5

DATA ENCODING CRITERIA (CONT.)

The other factor that improves performance is the encoding scheme The encoding scheme is simply the mapping from

data bits to signal elements

6

DIGITAL DATA DIGITAL SIGNAL

DIGITAL DATA DIGITAL SIGNAL Receiver needs to know

Timing of bitsSignal levels

Factors affecting successful interpretation of signals

8

ENCODING SCHEMES Non-return to Zero-Level (NRZ-L) Non-return to Zero Inverted (NRZI) Bipolar-Alternate Mark Inversion (AMI) Pseudoternary Manchester Differential Manchester Bipolar with 8-Zeros Substitution

(B8ZS) High-Density Bipolar 3-zeros (HDB3) 9

COMPARING ENCODING SCHEMESSignal spectrum

With lack of high-frequency components, less bandwidth required

With no DC component, AC coupling via transformer possible

Concentrate power in the middle of the bandwidth

ClockingEase of determining beginning and end of

each bit position10

COMPARING ENCODING SCHEMESError detection

Can be built into signal encodingSignal interference and noise

immunityPerformance in the presence of noise

Cost and complexityThe higher the signal rate to achieve a

given data rate, the greater the costSome codes require signal rate greater

than data rate11

NRZ-L Two different voltages for 0 and 1 bits Voltage constant during bit interval No transition (i.e., no return to zero

voltage) Options:

Absence of voltage for zero, constant positive voltage for one

More often, negative voltage for one value and positive for the other

12

NRZI Inverted on ones Constant voltage pulse for duration of

bit Data encoded as presence or absence

of signal transition at beginning of bit timeTransition (low-to-high or high-to-low)

denotes a binary 1No transition denotes binary 0

An example of differential encoding 13

DIFFERENTIAL ENCODING In complex transmission layouts, it is easy to

lose sense of polarity Therefore

Data represented by changes (i.e., transitions) rather than levels

More reliable detection of transition rather than level

14

NONRETURN TO ZERO (NRZ)

15

0 1 0 0 1 1 0 0 0 1 1

NRZ-L

NRZI

NRZ – PROS AND CONS Pros

Easy to engineer Make good use of bandwidth

Cons DC component Lack of synchronization capability

Used for magnetic recording Not often used for signal transmission

16

BIPOLAR-AMI

Uses more than two levels “0” represented by no line signal “1” represented by positive or negative pulse,

pulses alternate in polarity No loss of sync if a long string of 1s (0s still a

problem) No net DC component

Because the “1” signals alternate in voltage from + to -

Lower bandwidth Easy error detection

Because pulses alternate in polarity 17

PSEUDOTERNARY Uses more than two levels

“1” represented by absence of line signal“0” represented by alternating positive

and negative levels No advantage or disadvantage over

bipolar-AMI

18

TRADE OFF FOR MULTILEVEL BINARY Not as efficient as NRZ

Technically, a 3 signal level system Log2 3 = 1.58 bits

However, each signal element only represents one bit

Receiver must distinguish between three levels (+A, -A, 0)

Requires ≈ 3dB more signal power for same probability of bit error

19

BIPOLAR-AMI & PSEUDOTERNARY

20

0 1 0 0 1 1 0 0 0 1 1

B-AMI

PT

MANCHESTER Transition in middle of each bit period Transition serves as clock AND data

Low-to-high represents “1” High-to-low represents “0”

Used in IEEE 802.3 (Ethernet LAN)

21

DIFFERENTIAL MANCHESTER Mid-bit transition is clocking only

Transition at start of a bit period represents “0” No transition at start of a bit period represents

“1” This is a differential encoding scheme Used in IEEE 802.5 (Token Ring LAN)

22

BIPHASE (MANCHESTER AND D-MANCHESTER)

23

0 1 0 0 1 1 0 0 0 1 1

Man

D-Man

BIPHASE – PROS AND CONS

ProsSynchronization on mid bit transition (self

clocking)No DC componentError detection

Absence of expected transition Cons

At least one transition per bit time and possibly two

Maximum modulation rate is twice NRZRequires more bandwidth 24

TRANSMISSION RATES

25

TRANSMISSION RATES

26

MODULATION RATE

Mbps1T1

Rsec1Tb

b Mbps1T1

Rsec1Tb

b

27

SCRAMBLING

Use scrambling to replace sequences that would produce constant voltage

Filling sequence Must produce enough transitions to syncMust be recognized by receiver and replaced

with originalSame length as original

Design Goals No DC component No long sequences of zero level line signal No reduction in data rate Error detection capability 28

BIPOLAR WITH 8 ZEROS SUBSTITUTION (B8ZS) Based on bipolar-AMI

If 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 noise

Receiver detects and interprets as octet of all zeros

29

HIGH DENSITY BIPOLAR 3 ZEROS (HDB3) Based on bipolar-AMI String of four zeros replaced with one or two

pulses

30

B8ZS AND HDB3

31

DIGITAL DATA ANALOG SIGNAL

DIGITAL DATA ANALOG SIGNAL Public telephone system

300Hz to 3400HzUse modem (modulator-demodulator)

Basic Encoding TechniquesAmplitude-shift keying (ASK)

Amplitude difference of carrier frequencyFrequency-shift keying (FSK)

Frequency difference near carrier frequencyPhase-shift keying (PSK)

Phase of carrier signal shifted

33

AMPLITUDE-SHIFT KEYING (ASK) One binary digit represented by presence of

carrier, at constant amplitude Other binary digit represented by absence of

carrier

where the carrier signal is A cos(2πfct)

34

ts tfA c2cos

0

1binary 0binary

ASK CHARACTERISTICS Susceptible to sudden gain changes Inefficient modulation technique On voice-grade lines, used up to 1200 bps Used to transmit digital data over optical

fiber

35

ASK – PRINCIPLE OF OPERATION

36

ASK – PRINCIPLE OF OPERATION

37

ASK BANDWIDTH REQUIREMENTS

38

ASK – EXAMPLE

Assuming ASK modulation is to be used, estimate the BW required of a channel to transmit at the following bit rates: 300bps, 1200bps and 4800bps, assuming

a) the fo of the sequence 101010… is to be receivedb) the fo and 3fo are to the received

Comment on your results in relation to the PSTN

39

Source: Halsall, F., Data Communications, Computer Networks and Open Systems, (USA: Addison-Wiley, 1996), pg. 61

BINARY FREQUENCY-SHIFT KEYING (BFSK)

Two binary digits represented by two different frequencies near the carrier frequency

where f1 and f2 are offset from carrier frequency fc by equal but opposite amounts

40

ts tfA 12cos

tfA 22cos

1binary

0binary

BFSK CHARACTERISTICS Less susceptible to error than ASK On voice-grade lines, used up to 1200bps Used for high-frequency (3 to 30 MHz) radio

transmission Can be used at higher frequencies on LANs

that use coaxial cable

41

BFSK – PRINCIPLE OF OPERATION

42

BFSK – PRINCIPLE OF OPERATION

43

BFSK BANDWIDTH REQUIREMENTS

44

BFSK – EXAMPLE

45

PHASE-SHIFT KEYING (PSK) Two-level PSK (BPSK)

Uses two phases to represent binary digits

Differential PSK

46

ts tfA c2cos tfA c2cos 0binary

1binary

tfA c2cos tfA c2cos

QUADRATURE PSK More efficient use by each signal element

representing more than one bit Uses shifts separated by multiples of /2 (90o) Each element represents two bits Can use 8 phase angles and have more than one

amplitude 9600bps modems use 12 angles , four of which

have two amplitudes

47

QUADRATURE PSK Quadrature PSK (QPSK)

Each element represents more than one bit

48

ts

11

3cos 24cA f t

01

00

10

cos 24cA f t

3cos 24cA f t

cos 24cA f t

PHASE-SHIFT KEYING (PSK)

Multilevel PSKUsing multiple phase angles with each

angle having more than one amplitude, multiple signals elements can be achieved

D = modulation rate, baud R = data rate, bps M = number of different signal elements = 2L

L = number of bits per signal element49

MR

LRD

2log

PSK BANDWIDTH REQUIREMENTS

50

PERFORMANCE OF DIGITAL TO ANALOG MODULATION SCHEMES Bandwidth

ASK and PSK bandwidth directly related to bit rate

FSK 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, the bit error

rates of PSK and QPSK are about 3dB superior to ASK and FSK 51

ANALOG DATA DIGITAL SIGNAL

ANALOG DATA DIGITAL SIGNAL Conversion of analog data into digital data

Digitization Analog to digital conversion done using a

CODEC Basic encoding techniques

Pulse code modulation (PCM) Delta modulation (DM)

53

ANALOG DATA DIGITAL SIGNAL Once analog data have been converted to

digital data, the digital data: can be transmitted using NRZ-L can be encoded as a digital signal using a code

other than NRZ-L can be converted to an analog signal, using

previously discussed techniques

54

PULSE CODE MODULATION

Based on the sampling theorem 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

Each analog sample is assigned a binary code Analog samples are referred to as pulse amplitude

modulation (PAM) samples The digital signal consists of blocks of n bits,

where each n-bit number is the amplitude of a PCM pulse

Voice data limited to below 4000Hz Requires 8000 samples per second 55

PULSE CODE MODULATION 4 bit system gives 16 levels Quantized

Quantizing error or noiseApproximations mean it is impossible to

recover original signal exactly 8 bit sample gives 256 levels Quality comparable with analog

transmission 8000 samples per second of 8 bits

each gives 64kbps56

VOICE DIGITIZATION PROCESS

57

SamplingCircuit

Quantizerand

Compander

PAM Signal PCM SignalSampling

Clock

AnalogVoiceSignal

DigitalVoiceSignal

PAM Pulse Analog ModulationPCM Pulse Code Modulation

PULSE CODE MODULATION

58

EXAMPLES OF DIGITIZED SIGNALS

Digital VoicePCM 128 to 256 quantization levels (64

kbps)Adaptive Differential PCM (ADPCM) 32 kbps

Digital TV (5.5 MHz signal)11 x 106 samples/sec256 to 1024 quantization levelsData rates ≈ 100 Mbps

HDTV (35 MHz signal)70 x 106 samples/secData rates of up to hundreds of Mbps

59

BASIC ENCODING TECHNIQUES

Analog data to analog signal Amplitude modulation (AM) Angle modulation

Frequency modulation (FM) Phase modulation (PM)

Spread Spectrum Frequency Hopping Direct Sequence

60