data and computer communications chapter 5 – signal...

Data and Computer Communications

Chapter 5 – Signal Encoding Techniques

Computer EngineeringKyungpook National University

Terminology Encoding

Data → Signal Decoding

Signal →Data Data rate (R)

Rate, in bits per second (bps) that data are transmitted Duration or length of a bit

Time taken for transmitter to emit the bit Tb = 1/R (R = Transmission rate or data rate)

Modulation rate (signaling rate) Rate at which the signal level is changed; the rate is

expressed in baud, which means signal elements per second


Signal Encoding Techniques Digital data =>

Digital signal Analog data =>

Digital signal

Digital data => Analog signal

Analog data => Analog signal

sender receiver

link


Digital Data, Digital Signal

Digital signalDiscrete, discontinuous voltage pulsesEach pulse is a signal elementBinary data encoded into signal elements


Interpreting Digital Signals

Tasks involved in interpreting digital

signal at the receiver:

Timing of bits - when they start and end

(synchronization)

Signal levels or transitions

Factors affecting signal interpretation:

Signal to noise ratio

Data rate

Bandwidth


Synchronization The receiver must find beginning/end of each

signal To prevent timing drift between transmitter and

receiver, their clocks must somehow be synchronized

Out of sync

In sync

Sending time of each signal at sender

Picking-up time of each signal at receiver


Encoding Schemes


Biphase Encoding Manchester

Has transition in middle of each bit period Low to high represents “1” High to low represents “0” Transition serves as clock and data Used by IEEE 802 LAN standard

Differential Manchester Transition at start of bit period representing “0” No transition at start of bit period representing “1” Mid-bit transition is clocking only


Synchronization vs Transition Embed the clocking information in the data signal

So, transition provides clocking (timing) information Receiver adjusts its picking-up time by referring to transition

times More transitions facilitates synchronization

Actual transition

Receiver expects transitions here using its own clockTime drift

1st bit 2nd bit 3rd bit

DataPreamble Headersync Frame = Data plus header (control

information)


Comparison of Encoding Schemes• A good signal design should

concentrate the transmitted power in the middle of the transmission bandwidth

Signal spectrum

Signal spectrum

• Need to synchronize transmitter and receiver either with an external clock or sync mechanism

ClockingClocking

• Responsibility of a layer of logic above the signaling level that is known as data link control

Error detection

Error detection

• Certain codes perform better in the presence of noise

Signal interference

and noise immunity

Signal interference

and noise immunity

• The higher the signaling rate the greater the cost

Cost and complexityCost and

complexity

Less high frequency components, less distortion

Less DC (Direct Component), easier AC (Alternative Component) coupling

For better synchronization capability

More transitions, easier synchronization


Signal SpectrumNRZ = Nonreturn to zero levelNRZI = Nonreturn to zero invertedAMI = Alternate mark inversionB8ZS = Bipolar with 8 zeros substitutionHDB3 = High density bipolar - 3 zeros

f = frequency (Hz)R = data rate (bps)

Mea

n sq

uare

vol

tage

per

uni

t ba

ndw

idth

Normalized frequency ( f / R )

NRZ-L,NRZI

B8ZS, HDB3

AMI, Pseudoternary

Manchester,Differential Manchester


NRZ vs ManchesterNRZ Manchester

Spectrum

HighFrequency

Less (Make good use of givenbandwidth)

More (Requires more bandwidth)

DC Yes No

SynchronizationCapability No Yes (self clocking)

Pros Easy to engineer More reliable (easy error detection)

Cons Not often used for signal transmission

Maximum modulation rate is twice NRZ


Digital Data, Analog Signal

Main use is public telephone systemHas freq range of 300Hz to 3400HzUse modem (modulator-demodulator)

Encoding techniquesChange shape of carrier depending on data (cf) carrier = signal carrying dataAmplitude shift keying (ASK) Frequency shift keying (FSK)Phase shift keying (PSK)


ASK: Encode 0/1 by different amplitudes of carrier “0” = zero amplitude “1” = high amplitude

FSK: Two binary values represented by two different frequencies “0” = low frequency “1” = high frequency Less susceptible to error than

ASK

PSK: Phase of carrier signal is shifted to represent data “0” = No change of phase “1” = 180o change of phase

ASK, FSK, PSK


Quadrature PSK Each signal element represents two bits

00 - Shifts of 0o

01 - Shifts of 90o

10 - Shifts of 180o

11 - Shifts of 270o

Split input data stream in two & modulate onto carrier


Quadrature Amplitude Modulation Combination of ASK and PSK

Each carrier is ASK and PSK modulated simultaneously Receiver extracts two original bits from one signal by

demodulation

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

Phase shift

0o 90o 180o 270o

Amplitude Low 000 010 100 110

High 001 011 101 111


Example : QAM Signal Elements

45°

0°

90°

135°

180°

225°

270°

315°

1010

10000010

0111

0011

1111

1011

1100

01001110

0110

0001 1001

0101

0000

1101

Total 16 states Phase shift - 8 states

(0o,45o,90o,135o,180o,225o,270o,

315o) Amplitude - 2 states (high, low)

Sender modulates 4 bits into one signal

Receiver extracts four original bits from one signal by demodulation

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


Analog Data, Digital Signal

Digitization is conversion of analog data into digital data Sampling

How often the analog signal should be sampled?? (Sampling frequency)

QuantizationHow many bits are used to express each sample??

Samples3.0

1.4

6.2

1.32.8

5.94.1

31

6

13

64 Quantized samples

0 1 1 0 0 1 1 1 0 0 0 1 0 1 1 1 1 0 1 0 0 Digitized output

Analog data

Sampling times


Sampling Theorem Sampling frequency > 2* (the highest frequency

contained in the signal) “If a signal is sampled at a rate higher than twice the

highest signal frequency, the samples contain all information in original signal”


Example

For 4Khz voice data, how many samples per sec? Sampling frequency = 2*4000 = 8000 samples per sec A sample per 125us

Quantization 1 sample is expressed by 1 byte Values : 0 ~ 255

So, digital output = 8000*8bits/s = 64Kbps Digital recording for 1 hour

64K * 3600sec = 230.4Mbits = 28.8Mbytes


ExampleDigitizer

1010111…111

Manchester codeAnalog data Sampled data Digitized data


Analog Data, Analog Signals Modulate carrier with analog data

Carrier

Data

Amplitude Modulation

Frequency Modulation

Phase Modulation


Summary

Digital data, digital signalsBiphase (Manchester, Differential Manchester)Comparison

Analog data, digital signalsSamplingQuantization

Digital data, analog signalsASK, FSK, PSKQuadrature amplitude modulation

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