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According to [[Cisco]], "Manchester encoding introduces some difficult frequency-related problems that make it unsuitable for use at higher data rates".<ref>{{citation |url=http://docwiki.cisco.com/wiki/Ethernet_Technologies |title=Ethernet Technologies |publisher=[[Cisco Systems]] |access-date=2017-09-12 |quote=Manchester encoding introduces some difficult frequency-related problems that make it unsuitable for use at higher data rates.}}</ref><!--Difficulties are in meeting [[Title 47 CFR Part 15]] and other RF emissions requirements.-->
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.{{cn|date=November 2015}}
== Encoding and decoding ==
[[Image:Manchester encoding both conventions.svg|thumb
Manchester code always has a transition at the middle of each bit period and may (depending on the information to be transmitted) have a transition at the start of the period also. The direction of the mid-bit transition indicates the data. Transitions at the period boundaries do not carry information. They exist only to place the signal in the correct state to allow the mid-bit transition.
=== Conventions for representation of data ===▼
[[Image:Manchester code.png|frame|right|256px|Encoding of <tt>11011000100</tt> in Manchester code (as per G. E. Thomas)]]▼
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=274–275 |isbn=0-13-066102-3}}</ref> It specifies that for a 0 bit the signal levels will be low-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-low.▼
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=137–138 |isbn=0-13-100681-9}}</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-low signal sequence and a logic 1 is represented by a low-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]].▼
===Decoding===
The existence of guaranteed transitions allows the signal to be self-clocking, and also allows the receiver to align correctly; the receiver can identify if it is misaligned by half a bit period, as there will no longer always be a transition during each bit period. The price of these benefits is a doubling of the bandwidth requirement compared to simpler [[non-return-to-zero|NRZ]] coding schemes (or see also [[NRZI]]).
===Encoding===
{| class="wikitable" style="text-align:center;"
|+Encoding data using [[exclusive or]] logic (802.3 convention)<ref>{{citation |url=https://www.maximintegrated.com/en/app-notes/index.mvp/id/3435 |title=Manchester Data Encoding for Radio Communications |access-date=2018-05-28}}</ref>
|-
! Original data
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|-
| rowspan=2 | 0
| rowspan=4 | =▼
| 0▼
| rowspan=4 | XOR <br/>⊕
▲| 0
▲| rowspan=4 | =
| 0
|-
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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' convention—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 |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.
▲Manchester code always has a transition at the middle of each bit period and may (depending on the information to be transmitted) have a transition at the start of the period also. The direction of the mid-bit transition indicates the data. Transitions at the period boundaries do not carry information. They exist only to place the signal in the correct state to allow the mid-bit transition. The existence of guaranteed transitions allows the signal to be self-clocking, and also allows the receiver to align correctly; the receiver can identify if it is misaligned by half a bit period, as there will no longer always be a transition during each bit period. The price of these benefits is a doubling of the bandwidth requirement compared to simpler [[non-return-to-zero|NRZ]] coding schemes (or see also [[NRZI]]).
▲=== Conventions for representation of data ===
▲[[Image:Manchester code.png|frame|right|256px|Encoding of <tt>11011000100</tt> in Manchester code (as per G. E. Thomas)]]
▲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=274–275 |isbn=0-13-066102-3}}</ref> It specifies that for a 0 bit the signal levels will be low-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-low.
▲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=137–138 |isbn=0-13-100681-9}}</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-low signal sequence and a logic 1 is represented by a low-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]].
== See also ==
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