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Encyclopedia > Manchester code

In telecommunication, Manchester code (also known as Phase Encoding, or PE) is a form of data communications line code in which each bit of data is signified by at least one voltage level transition. Manchester encoding is therefore considered to be self-clocking, which means that accurate synchronization of a data stream is possible. Each bit is transmitted over a predefined time period. Copy of the original phone of Graham Bell at the Musée des Arts et Métiers in Paris Telecommunication is the transmission of signals over a distance for the purpose of communication. ... In telecommunication, a line code is a code chosen for use within a communications system for transmission purposes. ... This article is about the unit of information. ... International safety symbol Caution, risk of electric shock (ISO 3864), colloquially known as high voltage symbol. ... In telecommunications and electronics, a self-clocking signal is one that can be decoded without need for a separate clock signal or other source of synchronization. ... Synchronization is a problem in timekeeping which requires the coordination of events to operate a system in unison. ...


However, there are today many more sophisticated codes (8B/10B encoding) which accomplish the same aims with less bandwidth overhead, and less synchronisation ambiguity in pathological cases. In telecommunications, 8B/10B is a line code that maps 8-bit symbols to 10-bit symbols to achieve DC-balance and bounded disparity, and yet provide enough state changes to allow reasonable clock recovery. ... This article or section does not cite its references or sources. ...

Contents

Use

Manchester coding has been adopted into many efficient and widely used telecommunications standards, such as Ethernet. Ethernet is a large and diverse family of frame-based computer networking technologies for local area networks (LANs). ...


Features

Manchester coding provides a simple way to encode arbitrary binary sequences without ever having long periods without level transitions, thus preventing the loss of clock synchronization, or bit errors from low-frequency drift on poorly-equalized analog links (see ones-density). For information about computer bandwidth management, see Equalization (computing). ... Wikipedia does not yet have an article with this exact name. ...


If transmitted as an AC signal it ensures that the DC component of the encoded signal is zero, again preventing baseline drift of the repeated signal, making it easy to regenerate and preventing waste of energy. City lights viewed in a motion blurred exposure. ... Direct current (DC or continuous current) is the continuous flow of electricity through a conductor such as a wire from high to low potential. ...


Description

An example of Manchester encoding showing both conventions
An example of Manchester encoding showing both conventions

Summary: Image File history File links Manchester_encoding_both_conventions. ... Image File history File links Manchester_encoding_both_conventions. ... In telecommunication, Manchester code (also known as Phase Encoding, or PE) is a form of data communications line code in which each bit of data is signified by at least one voltage level transition. ...

  • Time is divided into periods, and one bit is transmitted per period
  • A "0" is expressed by increasing current, a "1" by decreasing current (according to G.E. Thomas' convention--in the IEEE 802.3 convention, the reverse is true)
  • The transitions signifying 0 or 1 occur at the midpoint of a period
  • Transitions at the beginning of a period are overhead and don't signify data.

Manchester codes always have a transition at the middle of each bit period, and depending on the state of the signal, may have a transition at the beginning of the period as well. The direction of the mid-bit transition is what carries the data, with a low-to-high transition indicating one binary value, and a high-to-low transition indicating the other. Transitions that don't occur mid-bit don't carry useful information, and exist only to place the signal in a state where the necessary mid-bit transition can take place. Though this allows the signal to be self-clocking, it essentially doubles the bandwidth requirements compared to NRZ coding schemes (or see also NRZI). Contrast with: return-to-zero. ... Categories: Stub ...


The first half of a bit period is the true bit value and the second half is the complement of the true bit value. This is in contrast with non-return-to-zero coding schemes. Contrast with: return-to-zero. ...


Manchester Encoding as Binary Phase Shift Keying

Manchester encoding is just a special case of Binary Phase Shift Keying, where the data to be transmitted controls the phase of a square wave carrier that is at the data rate in frequency. Therefore, it is extremely easy to generate such a signal digitally. Binary Phase Shift Keying, or BiPhase Shift Keying, (BPSK) is a method of encoding and/or transmitting data on top of a carrier. ... To meet Wikipedias quality standards, this article or section may require cleanup. ... A carrier wave, or carrier is a waveform (usually sinusoidal) that is modulated (modified) to represent the information to be transmitted. ...


To control the consumed amount of bandwidth, a filter can be used to reduce the bandwidth to as low as 1Hz per bit/second, and still not lose any information during transmission. However, for practical reasons (and to further control bandwidth consumption, especially on crowded radio bands), most BPSK modulators choose a carrier frequency which is much higher than the data rate, thus resulting in tighter, more easily filtered bandwidths. The property of 1Hz/bit/second is preserved, however. Television signal splitter consisting of a hi-pass and a low-pass filter. ... Binary Phase Shift Keying, or BiPhase Shift Keying, (BPSK) is a method of encoding and/or transmitting data on top of a carrier. ...


Conventions for representation of data

Encoding of 11011000100 in Manchester code (as per G. E. Thomas)
Encoding of 11011000100 in Manchester code (as per G. E. Thomas)

There are two opposing conventions for the representations of data. Simple example of Manchester encoding, drawn by myself File links The following pages link to this file: Manchester code Categories: BSD images ...


The first of these was first published by G. E. Thomas in 1949 and is followed by numerous authors (e.g., Andrew S. Tanenbaum). 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. Dr. Andrew Stuart Andy Tanenbaum (born 1944) is a professor of Computer Science at Vrije Universiteit, Amsterdam in the Netherlands. ...


The second convention is also followed by numerous authors (e.g., Stallings) as well as by IEEE 802.4 (token bus) and old revisions of IEEE 802.3 (Ethernet - most modern deployments use PAM based medium) 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. IEEE 802. ... IEEE 802. ... Pulse-amplitude modulation, acronym PAM, is a form of signal modulation where the message information is encoded in the amplitude of a series of signal pulses. ...


If a Manchester encoded signal gets inverted somewhere along the communication path, it transforms from one variant to another. However, such cases can be easily overcome by using differential Manchester encoding. Differential Manchester encoding (also known as CDP; Conditional DePhase encoding) is a method of encoding data in which data and clock signals are combined to form a single self-synchronizing data stream. ...


Shortcomings

A consequence of the transitions for each bit is that the bandwidth requirement for Manchester encoded signals is doubled compared with asynchronous communications, and the signal spectrum is considerably wider. Although Manchester encoding is a highly reliable form of communication, the bandwidth requirements are seen as a disadvantage, and most modern communication takes place with protocols with more modern line codes which achieve the same goals with better coding performance and smaller bandwidth requirements. This article or section does not cite its references or sources. ... Asynchronous communication is sending data without synchronization to an external clock. ... In telecommunication, signalling (or signaling) has the following meanings: The use of signals for controlling communications. ... Legend: γ = Gamma rays HX = Hard X-rays SX = Soft X-Rays EUV = Extreme ultraviolet NUV = Near ultraviolet Visible light NIR = Near infrared MIR = Moderate infrared FIR = Far infrared Radio waves: EHF = Extremely high frequency (Microwaves) SHF = Super high frequency (Microwaves) UHF = Ultra high frequency VHF = Very high frequency HF = High...


One consideration with Manchester encoding is synchronising the receiver with the transmitter. At first sight it might seem that a half bit period error would lead to an inverted output at the receiver end, but further consideration reveals that on typical data this will lead to code violations. The hardware used can detect these code violations, and use this information to synchronise accurately on the correct interpretation of the data.


References

William Stallings is an American computer scientist, best known for his textbooks on computer science topics such as operating systems, computer networks, computer organization, and cryptography. ... Pearson can mean Pearson PLC the media conglomerate. ... Federal Standard 1037C entitled Telecommunications: Glossary of Telecommunication Terms is a U.S. Federal Standard, issued by the General Services Administration pursuant to the Federal Property and Administrative Services Act of 1949, as amended. ... MIL-STD-188 is a series of U.S. military standards relating to telecommunications. ...

Further reading

http://www.erg.abdn.ac.uk/users/gorry/course/phy-pages/man.htmlCoded Mark Inversion CMI line coding In telecommunication Coded Mark Inversion (CMI) is a Return-to-zero (RZ) line code, in which zero bit is encoded as zero-to-one transition at the pulses center and one is encoded as constant level for entire pulse time. ...


  Results from FactBites:
 
Manchester: Definition and Much More from Answers.com (5148 words)
Manchester was the center of the Manchester school of economics and the Anti-Corn-Law League, led by Cobden and John Bright.
Manchester is situated within a bowl-shaped land area, bordered to the north and east by the Pennine moors and to the south by the Cheshire Plain.
The main motorways serving Manchester are the M56 (to the airport, Chester and the M6 southbound to Birmingham), the M61 (to Bolton, Preston and the M6 northbound to Lancaster), the M62 (to Liverpool and Leeds) and the M66 (to Bury).
Manchester code: Information from Answers.com (1011 words)
In telecommunication, Manchester code (also known as Phase Encoding, or PE) is a form of data communications line code in which each bit of data is signified by at least one voltage level transition.
Manchester encoding is therefore considered to be self-clocking, which means that accurate synchronisation of a data stream is possible.
Manchester encoding is just a special case of Binary Phase Shift Keying, where the data to be transmitted controls the phase of a square wave carrier that is at the data rate in frequency.
  More results at FactBites »

 
 

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