Digital transmission
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Digital Transmission. Outline. Line coding Encoding considerations DC components in signals Synchronization Various line coding methods. Line Coding. Process of converting binary data to digital signal. Signal Levels vs. Data Levels. Number of signal levels

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Digital transmission

Digital Transmission


Outline

Outline

  • Line coding

  • Encoding considerations

  • DC components in signals

  • Synchronization

  • Various line coding methods


Line coding

Line Coding

  • Process of converting binary data to digital signal


Signal levels vs data levels

Signal Levels vs. Data Levels

  • Number of signal levels

    • Number of different voltage levels allowed in a signal

  • Number of data levels

    • Number of voltage levels that actually represent data values


Signal vs data elements

Signal vs. Data Elements


Pulse rate vs bit rate

00

11

01

10

01

10

11

00

+3

+1

t

-1

-3

One pulse(one signal element)

Pulse Rate vs. Bit Rate

b – number of bits per pulse

L – number of different signal elements

BitRate = PulseRate× b = PulseRate × log2L

Bit rate  Bits per second

Pulse rate  Baud (pulses or signals per second)


Pulse rate vs bit rate1

Pulse Rate vs. Bit Rate

  • Example: In Manchester Encoding, if the bit rate is 10 Mbps, what is the pulse rate?

0

1

0

0

1

1

0

1

t

One pulse(one signal element)

One bit


Encoding considerations

Encoding Considerations

  • Signal spectrum

    • Lack of DC components

    • Lack of high frequency components

  • Clocking/synchronization

  • Error detection

  • Noise immunity

  • Cost and complexity


Dc components

0

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0

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1

0

1

t

0

1

0

0

1

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0

1

t

DC Components

  • DC components in signals are not desirable

    • Cannot pass thru certain devices

    • Leave extra (useless) energy on the line

Signal with DC component

Signal without DC component


Synchronization

0

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1

Sender sends:

01001101

t

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0

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1

t

Synchronization

  • To correctly decode a signal, receiver and sender must agree on bit interval

Receiver sees:

0100011011


Providing synchronization

data

Sender

Receiver

clock

0

1

0

0

1

1

0

1

t

Providing Synchronization

  • Separate clock wire

  • Self-synchronization


Line coding methods

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


Unipolar

0

1

0

0

1

1

0

0

t

Unipolar

  • Simplest form of digital encoding

    Rarely used

  • Only one polarity of voltage is used

  • E.g., polarity assigned to 1


Polar encoding

Polar Encoding

  • Two voltage levels (+,-) represent data bits

  • Most popular four

    • Nonreturn-to-Zero (NRZ)

    • Return-to-Zero (RZ)

    • Manchester

    • Differential Manchester


Nrz encoding

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0

t

0

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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

N = Bit rate

Save = Average signal rate


Rz encoding

0

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0

0

1

1

0

0

t

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

?


Manchester encoding

= 0

0

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0

1

t

= 1

Manchester Encoding

  • Uses an inversion at the middle of each bit

    • For bit representation

    • For synchronization


Differential manchester encoding

0

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0

1

t

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


Bipolar encoding

0

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1

1

0

1

t

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 -


B n zs schemes

1

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Bipolar AMI

t

0

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V

B

0

V

B

B8ZS

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

V – Bipolar violation

B – Valid bipolar signal


Other schemes

00

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01

10

01

10

11

00

+3

+1

t

-1

-3

Bit sequenceVoltage level

00-3

01-1

10+3

11+1

Other Schemes

  • mBnL

    • m data elements are substituted with n signal elements

    • E.g., 2B1Q (two binary, 1 quaternary)


Block coding

t

Block Coding

  • Improves the performance of line coding

  • Provides

    • Synchronization

    • Error detection

Division

Substitution

LineCoding

…01011010001…

:

001011010001

:

:

101100101101010

:


4b 5b encoding table

4B/5B Encoding Table


Analog to digital conversion

PAM

PAM signal(Sampled analog data)

Analog signal

Analog to Digital Conversion

  • Pulse Amplitude Modulation (PAM)

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


Pulse code modulation pcm

Pulse Code Modulation (PCM)

  • Converts an analog signal into a digital signal

    • PAM

    • Quantization

    • Binary encoding

    • Line coding


Pcm quantization

6

4

Output

2

0

1

2

3

4

5

6

7

Input

PCM: Quantization

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


Pcm quantization1

PCM: Quantization

Quantization


Quantization error

Quantization Error

  • Assume sine-wave input and uniform quantization

    • Known as the 6 dB/bit approximation

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


Example quantization error

Example: Quantization Error

  • A telephone subscriber line must have an SNRdB above 40. What is the minimum number of bits per sample?

Solution

We can calculate the number of bits as

Telephone companies usually assign 7 or 8 bits per sample.


Pcm binary encoding

PCM: Binary Encoding

  • Maps discrete values to binary digits


Pcm the whole process

PCM: The Whole Process


Minimum sampling rate

t

sampling interval

Minimum Sampling Rate

  • Nyquist Theorem:

Sampling rate must be greater than twice the highest frequency

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


Nyquist s sampling theorem

Nyquist’s Sampling Theorem

See also: Wagon-wheel effect

Demo: sampling.py


Sampling and bit rate

Sampling and Bit Rate

  • Ex.Calculate the minimum bit rate for recoding human voice, if each sample requires 60 levels of precision


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