2.1 - encoding techniques (2)

7/27/2019 2.1 - Encoding Techniques (2)

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An encoding technique is a system thatallows alphanumeric characters and symbolsto be coded in terms of bits and transmittedover a LAN.

Encoding techniques to be discussed:

Binary encoding

Manchester encoding

Differential Manchester Encoding


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Digital networks and devices make use of

digital signals (not analogue signals).

The medium may therefore carry two distinctsignals representing binary 1 or binary 0.

binary 1 + 12 voltsbinary 0 - 12 volts


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Manchester encoding is a synchronous clockencoding technique.

It is used by the OSI physical layer (to be

discussed further on) to encode the clock anddata of a synchronous bit stream.

In this technique, the actual binary data to be

transmitted over the cable is not sent as asequence of logic 1's and 0's (knowntechnically as Non Return to Zero (NRZ)).


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Instead, the bits are translated into a slightly

different format that solves a problem whenusing straight binary encoding (i.e. NRZ).

The problem with straight binary encoding isthat long runs of 1s or 0s make it difficult

for the receiver to distinguish one 0 or one 1from another.


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Manchester encoding allows the receiver toextract the clock signal and correctly decodethe value and timing of each bit.

Manchester encoding follows the followingrules:

Logic 1 indicated by a 0 to 1 transition

Logic 0 indicated by a 1 to 0 transition


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The diagram shows a typical Manchesterencoded signal with the corresponding binaryrepresentation of the data (0,0,1,0,1,1) beingsent.

Bit Boundaries


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In the example in the previous slide, a logic 1is indicated by a 0 to 1 transition at thecentre of the bit and a logic 0 is indicated bya 1 to 0 transition at the centre of the bit.

Note that signal transitions do not alwaysoccur at the 'bit boundaries' (the division

between one bit and another), but that thereis always a transition at the centre of each bit.


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The problem with frequent transitions is that

the Manchester coded signal consumes morebandwidth than the original signal (in NRZ).

Manchester encoding is used as the physicallayer of an Ethernet LAN, where the additional

bandwidth is not a significant issue.


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Differential Manchester Encoding is a method

of encoding data in which data and clocksignals are combined to form a single datastream.

In differential Manchester encoding, if a "1" is

represented by one transition, a "0" isrepresented by two transitions, and viceversa.


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Therefore: A '1' bit is indicated by making the first half of the signal

equal to the last half of the previous bit's signal, i.e. notransition at the start of the bit-time.

A '0' bit is indicated by making the first half of the signalopposite to the last half of the previous bit's signal, i.e. azero bit is indicated by a transition at the beginning ofthe bit-time.

In the middle of the bit-time there is always a transition,

whether from high to low, or low to high. A reversed scheme is possible, and no advantage is

given by using either scheme.


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It differs from Manchester Encoding by detecting

the presence or absence of transitions to indicatethe logical value.

The advantage is that detecting transitions isoften less error-prone than comparing against athreshold in a noisy environment.

Differential Manchester is specified in the IEEE802.5 standard for token ring LANs, and is usedfor many other applications, including magneticand optical storage.

2.1 - encoding techniques (2)

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