dc (digital transmission)

8/6/2019 DC (Digital Transmission)

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Digital Transmission


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Digital Signals

Digital limited

number of defined

values

Use binary (0s and

1s) to encode

information

Less affected byinterference (noise);

fewer errors


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Line Coding

Process of converting binary data to a digital

signal


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Signal Level versus Data Level

Signal level number

of values allowed in a

particular signal

Data level number ofvalues used to

represent data Note: figure b should say three

signal levels, two data levelsNote: error in text

fig 4.2


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DC Components

Direct-current (dc)

components or zero

frequencies are

undesirable Some systems do not

allow passage of a dc

component; may

distort the signal andcreate output errors

DC component is extra

energy and is useless


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Self-Synchronization

The receivers bit

intervals must

correspond exactly to

senders bit intervals. A self synchronizing

signal includes timing

information (in the shape

of transitions in the

signal to notify thereceiver about the start,

middle or end of a pulse)

in the data being

transmitted to preventmisinterpretation.

Mismatch, shorter bit duration at receiver


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Unipolar

Simplest method; inexpensive

Uses only one voltage level (polarity)

Polarity is usually assigned to binary 1; a 0 is

represented by zero voltage


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Unipolar

Potential problems:

DC component (average amplitude of

encoded signal is non zero)

Lack of synchronization


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Polar

Uses two voltage levels, one positive and

one negative

Alleviates DC component Variations

Nonreturn to zero (NRZ)

Return to zero (RZ) Manchester

Differential Manchester


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Nonreturn to Zero (NRZ)

Value of signal is always positive ornegative

NRZ-L

Signal level depends on bit represented;positive usually means 0, negative usuallymeans 1

Problem: synchronization of long streams of

0s or 1s NRZ-I (NRZ-Invert)

Inversion of voltage represents a 1 bit

0 bit represented by no change

Allows for synchronization


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NRZ-L and NRZ-I Encoding


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Return to Zero (RZ)

In NRZ-I, long strings of 0s may still be a

problem

May include synchronization as part of thesignal for both 1s and 0s

How?

Must include a signal change during each bit

Uses three values: positive, negative, and

zero

1 bit represented by positive-to-zero

0 bit represented by negative-to-zero


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RZ Encoding


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RZ Encoding

Disadvantage

Requires two signal changes to encode each

bit; more bandwidth necessary


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Manchester

Uses an inversion at the middle of each bit intervalfor both synchronization and bit representation

Negative-to-positive represents binary 1

Positive-to-negative represents binary 0 Achieves same level of synchronization with only 2

levels of amplitude


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Differential Manchester

Inversion at middle of bit interval is used for

synchronization

Presence or absence of additional transition at

beginning of interval identifies the bit Transition means binary 0; no transition means 1

Requires two signal changes to represent binary 0;

only one to represent 1


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Bipolar Encoding

Uses three voltage levels: positive,

negative, and zero

Zero level represents binary 0; 1s arerepresented with alternating positive and

negative voltages, even when not

consecutive

This scheme is called Alternate mark

inversion (AMI).


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Bipolar AMI

Neutral, zero voltage represents binary 0

Binary 1s represented by alternating positive

and negative voltages


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Digitization of an Analog Signal


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Summary

Line coding process conversion of binary

data to a digital signal Different methods are used to provide

reliability, synchronization, and higher data

rates

Analog-to-digital conversion through

sampling, quantizing, and line coding

dc (digital transmission)

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