Digital Transmission

Chaiporn Jaikaeo

Department of Computer EngineeringKasetsart University

01204325 Data Communications and Computer Networks

Based on lecture materials from Data Communications and Networking, 5th ed.,Behrouz A. Forouzan, McGraw Hill, 2012.

Revised 2021-05-07

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Outline• Line coding• Encoding considerations•DC components in signals

• Synchronization•Various line coding methods•Analog to digital conversion

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Line Coding•Process of converting binary data to digital signal

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Signal vs. Data Elements1 data element = 1 symbol

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Encoding Considerations• Signal spectrum

◦ Lack of DC components◦ Lack of high frequency components

•Clocking/synchronization• Error detection•Noise immunity•Cost and complexity

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DC Components•DC components in signals are not desirable

◦ Cannot pass thru certain devices◦ Leave extra (useless) energy on the line◦ Voltage built up due to stray capacitance in long cables

tSignal with DC component

v

tSignal without DC component

v

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t

0 1 0 0 0 1 1 0 1 1

Synchronization• To correctly decode a signal, receiver and sender must

agree on bit interval

t

0 1 0 0 1 1 0 1 Sender sends:01001101

Receiver sees:0100011011

v

v

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Providing Synchronization• Separate clock wire

• Self-synchronization

Sender Receiverdata

clock

t

0 1 0 0 1 1 0 1v

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Line Coding Methods•Unipolar

◦ Uses only one voltage level (one side of time axis)

•Polar◦ Uses two voltage levels (negative and positive)◦ E.g., NRZ, RZ, Manchester, Differential Manchester

•Bipolar◦ Uses three voltage levels (+, 0, and –) for data bits

•Multilevel

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Unipolar• Simplest form of line coding•Only one polarity of voltage is used• E.g., polarity assigned to 1 (TTL)

t

0 1 0 0 1 1 0 05V

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Polar Encoding• Two voltage levels (+,-) represent data bits•Most popular four

◦ Nonreturn-to-Zero (NRZ)◦ Return-to-Zero (RZ)◦ Manchester◦ Differential Manchester

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NRZ Encoding• Nonreturn to Zero

◦ NRZ-L (NRZ-Level): Signal level depends on bit value

◦ NRZ-I (NRZ-Invert): Signal is inverted if 1 is encountered

t

0 1 0 0 1 1 1 0

t

? 1 0 0 1 1 1 0

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RZ Encoding•Return to Zero

◦ Uses three voltage levels: +, - and 0, but only + and - represent data bits

◦ Half way thru each bit, signal returns to zero

t

0 1 0 0 1 1 0 0

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Manchester Encoding•Uses an inversion at the middle of each bit

◦ For bit representation◦ For synchronization

t

0 1 0 0 1 1 0 1 = 0

= 1

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Differential Manchester Encoding• The inversion on the middle of each bit is only for

synchronization• Transition at the beginning of each bit tells the value

t

0 1 0 0 1 1 0 1

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Bipolar Encoding•Bipolar encoding uses three voltage levels: +, - and 0• Each of all three levels represents a bit• E.g., Bipolar AMI (Alternate Mark Inversion)

◦ 0V always represents binary 0◦ Binary 1s are represented by alternating + and -

t

0 1 0 0 1 1 0 1

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BnZS Schemes• BnZS – Bipolar n-zero substitution

◦ Based on Bipolar AMI◦ n consecutive zeros are substituted with some +/- levels

◦ provides synchronization during long sequence of 0s◦ E.g., B8ZS

t

1 1 0 0 0 0 0 0 0 0 1 1 0 0 0 0 0 1 0

Bipolar AMI

B8ZS

V B 0 V000 B

V – Bipolar violationB – Valid bipolar signal

t

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mBnL Schemes•m data elements are substituted with n signal elements

• 2B1Q (two binary, 1 quaternary)

• 8B6T (eight binary, six ternary)

t

00 11 01 10 01 10 11 00

-3

-1

+1

+3

Bit sequence Voltage level

00 -3

01 -1

10 +3

11 +1

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Block Coding• Improves the performance of line coding•Provides

◦ Synchronization◦ Error detection

Division Substitution LineCoding

:001011010001

:

…01011010001… :101100101101010

:

t

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Data Code Data Code0000 11110 1000 100100001 01001 1001 100110010 10100 1010 101100011 10101 1011 101110100 01010 1100 110100101 01011 1101 110110110 01110 1110 111000111 01111 1111 11101

Data CodeQ (Quiet) 00000I (Idle) 11111H (Halt) 00100J (start delimiter) 11000K (start delimiter) 10001T (end delimiter) 01101S (Set) 11001R (Reset) 00111

4B/5B Encoding Table

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Analog to Digital Conversion•Pulse Amplitude Modulation (PAM)

◦ Converts an analog signal into a series of pulses by sampling

PAM

Analog signal PAM signal(Sampled analog data)

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Pulse Code Modulation (PCM)•Converts an analog signal into a digital signal

◦ PAM◦ Quantization◦ Binary encoding◦ Line coding

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PCM: Quantization

1 2 3 4 5 6 70Input

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4

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Output

•Converts continuous values of data to a finite number of discrete values

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PCM: Quantization

Quantization

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Quantization Error•Assume sine-wave input and uniform quantization

◦ nb is the number of bits per sample

•Known as the 6 dB/bit approximation

See also: http://en.wikipedia.org/wiki/Quantization_error#Quantization_noise_model

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SolutionWe can calculate the number of bits as

Telephone companies usually assign 7 or 8 bits per sample.

Example: Quantization Error•A telephone subscriber line must have an SNRdB above 40.

What is the minimum number of bits per sample?

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PCM: Binary Encoding•Maps discrete values to binary digits

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PCM: The Whole Process

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Minimum Sampling Rate

Sampling rate must be greater thantwice the highest frequency

•Nyquist Theorem:

t

sampling interval

Ex. Find the maximum samplinginterval for recording human voice(freq. range 300Hz – 3000Hz)

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Nyquist’s Sampling Theorem

See also: Wagon-wheel effect

Sampling demonstration

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Example: Sampling and Bit Rate•Calculate the minimum bit rate for recording human voice,

if each sample requires 60 levels of precision

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Summary• Line coding and block coding•Digital signal consideration

◦ Bit rate◦ Symbol rate◦ DC component◦ Synchronization

•Analog-to-digital conversion◦ Pulse-Code Modulation◦ Minimum sampling frequency