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CSCI 6431

C N kiComputer Networking:Physical Layer Signal Encoding Techniques

George Blankenship

Physical Layer Signal Encoding Techniques

George Blankenship 1

Lecture Outline• Digital data digital signal• Digital data, digital signal

– Digital signals– Self-clocked signals– Error detection

• Digital data, analog signalM d l ti– Modulation

• Analog data, digital signal– Digitization of analog dataDigitization of analog data– PCM

• Analog data, analog signal

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Digital Data, Digital Signal

• Digital signal– Discrete, discontinuous voltage pulsesDiscrete, discontinuous voltage pulses– Each pulse is a signal element– Binary data encoded into signal elementsBinary data encoded into signal elements

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Encoding Schemes

• Nonreturn to Zero– Nonreturn to Zero-Level (NRZ-L)– Nonreturn to Zero Inverted (NRZI)

• Multilevel– Bipolar -AMI– Pseudoternary– Manchester– Differential Manchester

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Nonreturn to Zero• Voltage constant during bit interval

– no transition (no return to zero voltage)

• NRZ-Level– Two different voltages for 0 and 1 bits– Unipolar

• Absence of voltage for zero• constant positive voltage for one

– Bipolarp• Negative voltage for one value• Positive for the other

• NRZI– Data encoded as presence or absence of signal transition at beginning of

bit time– Transition (low to high or high to low) denotes a binary 1– No transition denotes binary 0

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y

NRZ

• Pros– Easy to engineer– Make good use of bandwidth

• Cons– Lack of synchronization capability

• Used for magnetic recording• Not often used for signal transmission

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Not often used for signal transmission

Multilevel Binary• Use more than two levelsUse more than two levels

– No net dc component– Lower bandwidth– No loss of sync with a long string of ones/zerosNo loss of sync with a long string of ones/zeros – Zeros/ones still a problem– Easy error detection

• Bipolar AMI• Bipolar-AMI– Zero represented by no line signal– One represented by positive or negative pulse

P d t• Pseudoternary– One represented by absence of line signal– Zero represented by alternating positive and negative

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– One pulses alternate in polarity

Bipolar-AMI and Pseudoternary

0 1 0 0 1 1 0 0 0 1 1

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Trade Off for Multilevel Binary

• Not as efficient as NRZ– Each signal element only represents one bitEach 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

same probability of bit error

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Manchester Encoding

Manchester• Transition in middle of each bit period

Differential Manchester• Transition in middle of each bit period is clockp

• Transition serves as clock and data• Low to high represents one• High to low represents zero

p• Transition at start of a bit period is data• Transition represents zero• No transition represents one

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g p• Used by IEEE 802.3

p• Used by IEEE 802.5

Mulitlevel Pros and Cons

• Con– At least one transition per bit time and possibly two– Maximum modulation rate is twice NRZ– Requires more bandwidth

• Pros– Synchronization on mid bit transition (self clocking)

N d– No dc component– Error detection

• Absence of expected transition

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Absence of expected transition

Modulation Rate

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Digital Data, Analog Signal

• Public telephone system– 300Hz to 3400Hz300Hz to 3400Hz– Use modem (modulator-demodulator)

• Amplitude shift keying (ASK)• Amplitude shift keying (ASK)• Frequency shift keying (FSK)• Phase shift keying (PSK)

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Shift Keying• Amplitude Shift Keying (ASK)p y g ( )

– Values represented by different amplitudes of carrier– Usually, one amplitude is zero– Susceptible to sudden gain changes

Inefficient– Inefficient– Used over optical fiber

• Frequency Shift Keying (FSK)Binary FSK (BFSK)– Binary FSK (BFSK)

– Multiple FSK

• Phase Shift Keying (PSK)Phase of carrier signal is shifted to represent data– Phase of carrier signal is shifted to represent data

– Binary PSK– Differential PSK

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Modulation Techniques

• ASK– Amplitude Modulation

T lit d (0/1)– Two amplitudes (0/1)

• FSK– Frequency Modulationq y– Two frequencies (0/1)

• PSKPh l t bit– Phase selects bit

– Phase A (0)– Phase B (1)

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Differential PSK

• Phase shift is key• Phase shift indicates transitionPhase shift indicates transition• No transition – 0• Transition - 1

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Quadrature PSK

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

Wi h hif f /2 (90 ) h l– With shifts of /2 (90o) each element represents two bits

– Can use 8 phase angles and have more than one p gamplitude

– 9600 bps modem use 12 angles , four of which have two amplitudestwo amplitudes

• Offset QPSK (orthogonal QPSK)– Delay in Q stream

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Quadrature AmplitudeQuadrature Amplitude Modulation

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

bi i f d• Combination of ASK and PSK• Logical extension of QPSK

S d t diff t i l i lt l• Send two different signals simultaneously on same carrier frequency– Use two copies of carrier, one shifted 90°Use two copies of carrier, one shifted 90– Each carrier is ASK modulated– Two independent signals over same medium

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– Demodulate and combine for original binary output

Analog Data, Digital Signal

• 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

other than NRZ Lother than NRZ-L• Digital data can then be converted to analog signal• Analog to digital conversion done using a codec• Analog to digital conversion done using a codec

– Pulse code modulation– Delta modulation

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Digitizing Analog Data

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Pulse Code Modulation (PCM)

• 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 originalsamples contain all the information of the original signal

• Voice data limited to below 4000HzVoice data limited to below 4000Hz• Require 8000 sample per second• Analog samples (Pulse Amplitude Modulation, g p ( p ,

PAM)• Each sample assigned digital value

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PCM Encoding

• 4 bit system gives 16 levels• Quantized

– Quantizing error or noise– Approximations mean it is impossible to recover

original exactlyoriginal exactly• 8 bit sample gives 256 levels• Quality comparable with analog transmissionQ y p g• 8000 samples per second of 8 bits each gives

64kbps

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PCM Example

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PCM Block Diagram

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Delta Modulation

• Analog input is approximated by a staircase function

• Move up or down one level () at each sample intervalsample interval

• Binary behaviorF ti d t h l– Function moves up or down at each sample interval

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Delta Modulation - example

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Delta Modulation - Performance

• Good voice reproduction – PCM - 128 levels (7 bit)PCM 128 levels (7 bit)– Voice bandwidth 4khz– Should be 8000 x 7 = 56kbps for PCMShould be 8000 x 7 56kbps for PCM

• Data compression can improve on thisI t f di t h i f id– e.g. Interframe coding techniques for video

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Analog Data, Analog Signals

• Why modulate analog signals?– Higher frequency can give more efficient

transmission– Permits frequency division multiplexing

• Types of modulation– Amplitude– Frequency– Phase

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AnalogAnalog Modulation

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Suggested Reading

• Stallings chapter 5

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