Chapter 2
The Physical Layer, Encoding Schemes:
-Physical Transmission of Bits-
What To Do Against Attenuation,
Distortion and Noise?
A media adapted to the distance/bit
rate (we see it earlier…)
Encoding schemes (we well see it now)
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Encoding Techniques
Encoding schemes deal with how to transport
bits over the physical media…
We must deal and manage many issues to well
represent and interpret the bits:




Timing of bits (start and end, duration, Signal
levels)
Clocking ( Synchronizing transmitter and receive,
External clock, Sync mechanism based on signal)
Error detection, Signal interference and Noise
immunity
Cost and complexity Chapter 2
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Encoding Techniques
Encoding techniques depend on the type of
data to transmit and the medium being used:

Digital data on digital signal (our focus in this brief
description...)

Analog data on digital signal

Digital data on analog signal

Analog data on analog signal
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Digital Data, Digital Signal
Digital signal:

Discrete, discontinuous voltage pulses

Each pulse is a signal element

Binary data encoded into signal elements
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A Simple Encoding Scheme
0 is Vo (some voltage)
1 is V1
Example : let us encode 10011101
Clock
Vo
V1
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Problem?
Let us now try to encode two bytes that
come one 2 seconds after the other:
10011101 and 00001101
What is the problem with this encoding
scheme?
What is the solution?
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Bipolar Encoding (RZ Signal)
+0.85V
HightV + NoV –» 1bit
1 is 0
0
LowV + NoV –» 0bit
0 is
-0.85V
Example : let us encode 01100010
Clock
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Problem?
The problem with this encoding scheme
is that we have to return to 0 each time
the transmission is done which
generates some complexity and cost in
implementing and managing this
technique
We should look for a better technique…
We have 2 alternatives:
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Alternative 1: Non-Return to Zero
Two different voltages for 0 and 1 bits
Voltage constant during bit interval

no transition i.e. no return to zero voltage
e.g. Absence of voltage for zero, constant
positive voltage for one
More often, negative voltage for one value and
positive for the other
This is NRZ also known as Manchester
Encoding...
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Alternative 1: NRZ-Manchester Encoding
1 is
0 is
+0.85V
0
HightV + LowV –» 1bit
-0.85V
LowV + HighV –» 0bit
0
Example : let us encode 01100010
Clock
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Alternative 2: Differential Encoding
Data represented by changes trends
rather than levels of voltage
More reliable detection of transition
rather than level
In complex transmission layouts it is
easy to lose sense of polarity
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Differential Manchester Encoding
0 is
1 is
0
(Presence of transition)
0
(Abscence of transition)
A transition in the middle of the bit is required
anyway… Example : let us encode 10011101
Clock
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