data communication -SignalEncodingTechniques

8/3/2019 data communication -SignalEncodingTechniques

1/62

Dat a Com m unic a t ion

Chapt er 5

Signal Enc oding Tec hniques


2/62

Enc oding Tec hniques Digital data, digital signal

Analog data, digital signal

Digital data, analog signal

Analog data, analog signal


3/62

Signal Enc oding Tec hniques


4/62

Digi t a l Dat a, Dig i t a l Signa l Digital signal

Discrete, discontinuous voltage pulses

Each pulse is a signal element

Binary data encoded into signal elements


5/62

Term s (1) Unipolar

All signal elements have same sign

Polar

One logic state represented by positive voltage theother by negative voltage

Data rate

Rate of data transmission in bits per second

Duration or length of a bitTime taken for transmitter to emit the bit


6/62

Term s (2) Modulation rate

Rate at which the signal level changes

Measured in baud = signal elements per second

Mark and Space

Binary 1 and Binary 0 respectively


7/62

In t erpret ing Dig i t a l Signals Receiver Need to know

Timing of bits - when they start and end

Signal levels

Factors affecting successful interpreting of

signalsSignal to noise ratio (SNR).

Data rate

BandwidthEncoding scheme.


8/62

Com par ison of Enc oding

Sc hem es (1) Signal Spectrum

Lack of high frequencies reduces required bandwidth

Lack of dc component allows ac coupling viatransformer, providing isolation, reduce interference.

Concentrate transmitted power in the middle of the

channel bandwidth to reduce distortion.

Clocking

Synchronizing transmitter and receiverExternal clock

Sync mechanism based on transmitted signal.


9/62

Com par ison of Enc oding

Sc hem es (2) Error detection

Can be built into signal encoding scheme.

Signal interference and noise immunity

Some codes are better than others

Cost and complexityHigher signal rate (& thus data rate) lead to higher

costs

Some codes require signal rate greater than data rate


10/62

Enc oding Sc hem es Nonreturn to Zero-Level (NRZ-L)

Nonreturn to Zero Inverted (NRZI)

Bipolar AMI (Alternate mark inversion)

Pseudoternary

Manchester

Differential Manchester

B8ZS(bipolar with 8-zeros substitution) HDB3( high density bipolar-3 zeros)


11/62

Nonre t urn t o Zero -Level (NRZ-L) Two different voltages for 0 and 1 bits

Voltage constant during bit interval

no transition I.e. no return to zero voltage

e.g. Absence of voltage for zero, constant

positive voltage for one More often, negative voltage for one and

positive for zero.

This is NRZ-L


12/62

Nonret urn t o Zero Inver t ed (NRZI) Nonreturn to zero inverted on ones

Constant voltage pulse for duration of bit

Data encoded as presence or absence of signaltransition at beginning of bit time

Transition (low to high or high to low) denotes abinary 1

No transition denotes binary 0

An example of differential encoding


13/62

NRZ


14/62

Dif ferent ia l Enc oding Data represented by changes rather than levels

More reliable detection of transition rather thanlevel

In complex transmission layouts it is easy to

lose sense of polarity


15/62

NRZ pros and c ons Pros

Easy to engineer

Make good use of bandwidth (most of energyconcentrated in the middle of transmission BW).

Cons

dc component

Lack of synchronization capability

Used for magnetic recording (data storage). Not often used for signal transmission.


16/62

Mul t i leve l B inary Use more than two voltage levels

Bipolar-AMI

zero represented by no line signal

one represented by positive or negative pulse

one pulses alternate in polarityNo loss of sync if a long string of ones (zeros still a

problem)

No net dc componentLower bandwidth than NRZ.

Easy error detection


17/62

Pseudoternary

One represented by absence of line signal

Zero represented by alternating positive andnegative

No advantage or disadvantage over bipolar-AMI


18/62

Bipolar-AMI and Pseudot ernary

0 1 0 0 1 1 0 0 0 1 1


19/62

Trade Of f for Mul t i level B inary

Not as efficient as NRZ

Each signal element only represents one bit

In a 3 level system could represent log23 = 1.58 bits

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

Requires approx. 3dB more signal power for sameprobability of bit error


20/62

Biphase

Manchester

Transition in middle of each bit duration

Transition serves as clock and data Low to high represents one

High to low represents zero

Used by IEEE 802.3

Differential Manchester

Midbit transition is clocking only

Transition at start of a bit period represents zero

No transition at start of a bit period represents one

Note: this is a differential encoding scheme

Used by IEEE 802.5


21/62

Enc oding Sc hem es


22/62

Manc hest er Enc oding


23/62

Di f ferent ia l Manc hest er

Encoding


24/62

Biphase Cons and Pros

Cons

At least one transition per bit time and possibly two

Maximum modulation rate is twice data rate.

Requires more bandwidth

ProsSynchronization on mid bit transition (self clocking)

No dc component

Error detection Absence of expected transition


25/62

Modula t ion Rat e

2

R RD

L Log M = =


26/62

Scrambl ing

Use scrambling to replace sequences that wouldproduce constant voltage

Filling sequence Must produce enough transitions to sync

Must be recognized by receiver and replace with original

Same length as original No dc component

No long sequences of zero level line signal

No reduction in data rate Error detection capability


27/62

B8ZS

Bipolar With 8 Zeros Substitution

Based on bipolar-AMI

If octet of all zeros and last voltage pulse preceding was positiveencode as 000+-0-+

If octet of all zeros and last voltage pulse preceding was negativeencode as 000-+0+-

Causes two violations of AMI code Unlikely to occur as a result of noise

Receiver detects and interprets as octet of all zeros

Polarity of preceding

pulse

substitution

+ 000+-0-+

- 000-+0+-


28/62

HDB3

High Density Bipolar 3 Zeros

Based on bipolar-AMI

String of four zeros replaced with one or twopulses

Number of ones since last substitution

Polarity of preceding

pulse

odd even

- 000- +00+

+ 000+ -00-


29/62

B8ZS and HDB3


30/62

Digi t a l Dat a, Analog Signa l

Public telephone system

300Hz to 3400Hz

Use modem (modulator-demodulator)

Amplitude shift keying (ASK)

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


31/62

Modula t ion Tec hniques


32/62

Am pl i t ude Sh i f t K ey ing

encode 0/1 by different carrier amplitudesusually have one amplitude zero

susceptible to sudden gain changes inefficient

used forup to 1200bps on voice grade lines

very high speeds over optical fiber

{ cos(2 ):binary 10 :binary 0( ) c A f t s t =


33/62

Binary Frequenc y Shi f t K ey ing

most common is binary FSK (BFSK)

two binary values represented by two different

frequencies (near carrier) less susceptible to error than ASK

used for

up to 1200bps on voice grade lines high frequency radio

even higher frequency on LANs using co-ax

{ 12

cos(2 ):binary 1

cos(2 ):binary 0( )A f t

A f t s t

=


34/62

FSK on Voic e Grade L ine


35/62

Mult ip le FSK

More than two frequencies used

More bandwidth efficient than BFSK.

More susceptible to error Each signal element represents more than one bit.

2

( ) cos 2 , 1

(2 1 )

the difference frequancy

L=# of bits per signal element.

log

i i

i c d

d

s t A f t i M

f f i M f

f

M L

= = +

=

=


36/62

Phase Shi f t K eying

phase of carrier signal is shifted to represent data

binary PSK(BPSK)

two phases represent two binary digits

differential PSK(DPSK)

phase shifted relative to previous transmission ratherthan some reference signal

{ cos(2 ):binary 1cos(2 ):binary 0( ) c c A f t

A f t s t

=


37/62

Dif fe rent ia l PSK (DPSK )


38/62

Quadrat ure PSK (QPSK )

get more efficient use of BW if each signalelement represents more than one bit

eg. shifts of/2 (90o)each element represents two bits

split input data stream in two & modulate onto carrier

& phase shifted carrier can use 8 phase angles & more than one

amplitude

9600bps modem uses 12 angles, four of which havetwo amplitudes

Offset QPSK (orthogonal OQPSK)

Delay in Q stream


39/62

QPSK and OQPSK Modu la t ors

1 1( ) ( ) cos 2 ( )sin 22 2

c cs t I t f t Q t f t =

E l f QPSK d OQPSK


40/62

Ex am ples of QPSK and OQPSK

Waveforms

P f f Di i l A l


41/62

Per form anc e o f Dig i t a l t o Ana log

Modu lat ion Sc hem es

bandwidth

ASK/PSK bandwidth directly relates to bit rate

multilevel PSK gives significant improvements

in presence of noise:

bit error rate of PSK and QPSK are about 3dBsuperior to ASK and FSK

for MFSK & MPSK have tradeoff between bandwidthefficiency and error performance

Q adrat re Am pl i t de


42/62

Quadrat ure Am pl i t ude

Modula t ion

QAM used on asymmetric digital subscriber line(ADSL) and some wireless

Combination of ASK and PSK

Logical extension of QPSK

Send two different signals simultaneously onsame carrier frequency

Use two copies of carrier, one shifted 90

Each carrier is ASK modulated

Two independent signals over same medium

Demodulate and combine for original binary output


43/62

QAM Modulat or

1 2( ) ( ) cos 2 ( )sin 2c cs t d t f t d t f t = +


44/62

QAM Leve ls

Two level ASK

Each of two streams in one of two states

Four state system

Essentially QPSK

Four level ASK

Combined stream in one of 16 states

64 and 256 state systems have been

implemented Improved data rate for given bandwidth

Increased potential error rate


45/62

Analog Dat a, Dig i t a l Signa l

Digitization

Conversion of analog data into digital data

Digital data can then be transmitted using NRZ-L

Digital data can then be transmitted using code otherthan NRZ-L

Digital data can then be converted to analog signal

Analog to digital conversion done using a codec

Pulse code modulation

Delta modulation


46/62

Digi t izing Ana log Dat a


47/62

Pulse Code Modu la t ion(PCM) (1)

sampling theorem:If a signal is sampled at regular intervals at a rate

higher than twice the highest signal frequency, thesamples contain all information in original signal

Voice data limited to below 4000Hz

Require 8000 sample per second Analog samples (Pulse Amplitude Modulation,

PAM)

Each sample assigned digital value


48/62

Pulse Code Modu la t ion(PCM) (2)

4 bit system gives 16 levels

Quantized

Quantizing error or noise

Approximations mean it is impossible to recoveroriginal exactly

8 bit sample gives 256 levels

Quality comparable with analog transmission

8000 samples per second of 8 bits each gives64kbps


49/62

PCM Ex am ple


50/62

PCM Bloc k Diagram


51/62

Nonl inear Enc oding

Quantization levels not evenly spaced

Reduces overall signal distortion

Can also be done by companding


52/62

Effec t of Non-Linear Coding


53/62

Typic a l Com panding Func t ions


54/62

Del t a Modula t ion

Analog input is approximated by a staircasefunction

Move up or down one level () at each sampleinterval

Binary behavior

Function moves up or down at each sample interval

hence can encode each sample as single bit

1 for up or 0 for down


55/62

Del t a Modula t ion Ex am ple


56/62

Del t a Modulat ion Operat ion


57/62

PCM verses Del t a Modulat ion

DM has simplicity compared to PCM but has worse SNR

issue of bandwidth usedeg. for good voice reproduction with PCM

want 128 levels (7 bit) & voice bandwidth 4khz need 8000 x 7 = 56kbps

data compression can improve on this still growing demand for digital signals

use of repeaters, TDM, efficient switching

PCM preferred to DM for analog signals


58/62

Analog Dat a, Analog Signa ls

Why modulate analog signals?

Higher frequency can give more efficient transmission

Permits frequency division multiplexing (chapter 8)

Types of modulation

Amplitude

Frequency

Phase

Analog


59/62

Analog

Modula t ion


60/62

Am pl i t ude m odula t ion (AM) The AM signal is

The transmitted power

Transmission bandwidth

DSBSC

SSB

( ) [1 ( )]cos 2 , where ( ) ( )a c as t n x t f t m t n x t = + =

: modulation indexif 1, then ( ) can be recoverd

from the envolpe [1 ( )] of ( )

a

a

a

nn m t

n x t s t

+

2

12

at c

np p

= +

2 , where is bandwidth of ( )T B B B m t =


61/62


62/62

Angle m odulat ion(2)

Modulation index of FM and PM is

Carsons rule

f

B

=

2( 1) 2( )T B B f B= + = +

data communication -SignalEncodingTechniques

Documents

Transcript of data communication -SignalEncodingTechniques