Manchester code: Difference between revisions

Revision as of 08:34, 10 April 2022

Revision as of 23:47, 17 April 2022

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There are more complex codes, such as [[8B/10B encoding]], that use less [[bandwidth (signal processing)|bandwidth]] to achieve the same data rate but may be less tolerant of frequency errors and [[jitter]] in the transmitter and receiver reference clocks.{{citation needed|date=November 2015}}

==Encoding and decoding==

[[Image:Manchester encoding both conventions.svg|thumb|650px|An example of Manchester encoding showing both [[Manchester code#Conventions for representation of data|conventions for representation of data]]]]

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There are two opposing conventions for the representations of data.

The first of these was first published by G. E. Thomas in 1949 and is followed by numerous authors (e.g., [[Andrew S. Tanenbaum|Andy Tanenbaum]]).<ref name="tanenbaum">{{cite book |author-last=Tanenbaum |author-first=Andrew S. |author-link=Andrew S. Tanenbaum |title=Computer Networks |edition=4th |publisher=[[Prentice Hall]] |date=2002 |pages=[https://archive.org/details/computernetworks00tane_2/page/274 274–275] |isbn=0-13-066102-3 |url=https://archive.org/details/computernetworks00tane_2/page/274 }}</ref> It specifies that for a 0 bit the signal levels will be low-highlow–high (assuming an amplitude physical encoding of the data) -– with a low level in the first half of the bit period, and a high level in the second half. For a 1 bit the signal levels will be high-lowhigh–low. This is also known as Manchester II or Biphase-L code.

The second convention is also followed by numerous authors (e.g., [[William Stallings]])<ref name="stallings">{{cite book |author-last=Stallings |author-first=William |author-link=William Stallings |title=Data and Computer Communications |edition=7th |publisher=[[Prentice Hall]] |date=2004 |pages=[https://archive.org/details/datacomputercomm00stal_1/page/137 137–138] |isbn=0-13-100681-9 |url=https://archive.org/details/datacomputercomm00stal_1/page/137 }}</ref> as well as by [[IEEE 802.4]] (token bus) and lower speed versions of [[IEEE 802.3]] (Ethernet) standards. It states that a logic 0 is represented by a high-lowhigh–low signal sequence and a logic 1 is represented by a low-highlow–high signal sequence.

If a Manchester encoded signal is inverted in communication, it is transformed from one convention to the other. This ambiguity can be overcome by using [[differential Manchester encoding]].

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Encoding conventions are as follows:

* Each bit is transmitted in a fixed time (the "period").

* A <code>0</code> is expressed by a low-to-high transition, a <code>1</code> by high-to-low transition (according to G. E. Thomas's convention—inconvention – in the IEEE 802.3 convention, the reverse is true).<ref name="Manchesterencoding">{{Cite journal |author-last1=Forster |author-first1=R. |title=Manchester encoding: Opposing definitions resolved |doi=10.1049/esej:20000609 |journal=Engineering Science & Education Journal |volume=9 |issue=6 |pages=278–280 |date=2000}}</ref>

* The transitions which signify <code>0</code> or <code>1</code> occur at the midpoint of a period.

* Transitions at the start of a period are overhead and don't signify data.