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8b/10b encoding
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 means that the difference between the counts of ones and zeros in a string of ''at least'' 20 bits is no more than two, and that there are not more than five ones or zeros in a row. This helps to reduce the demand for the lower bandwidth limit of the channel necessary to transfer the signal.
An 8b/10b code can be implemented in various ways, where the design may focus on specific parameters such as hardware requirements, DC-balance, etc. One implementation was designed by K. Odaka for the DAT digital audio recorder.〔Method and apparatus for encoding binary data, October 1984.〕 Kees Schouhamer Immink designed an 8b/10b code for the DCC audio recorder.〔Method of transmitting information, encoding device for use in the method, and decoding device for use in the method, June 1986.〕 The IBM implementation was described in 1983 by Al Widmer and Peter Franaszek.〔Byte oriented DC balanced (0,4) 8B/10B partitioned block transmission code, December 1984.〕
== How it works for the IBM code==
As the scheme name suggests, eight bits of data are transmitted as a 10-bit entity called a ''symbol'', or ''character''. The low five bits of data are encoded into a 6-bit group (the 5b/6b portion) and the top three bits are encoded into a 4-bit group (the 3b/4b portion). These code groups are concatenated together to form the 10-bit symbol that is transmitted on the wire. The ''data symbols'' are often referred to as D.x.y where x ranges over 0–31 and y over 0–7. Standards using the 8b/10b encoding also define up to 12 ''special symbols'' (or ''control characters'') that can be sent in place of a ''data symbol''. They are often used to indicate start-of-frame, end-of-frame, link idle, skip and similar link-level conditions. At least one of them (i.e. a "comma" symbol) needs to be used to define the alignment of the 10 bit symbols. They are referred to as K.x.y and have different encodings from any of the D.x.y symbols.
Because 8b/10b encoding uses 10-bit symbols to encode 8-bit words, some of the possible 1024 (10 bit, 210) codes can be excluded to grant a run-length limit of 5 consecutive equal bits and grant that the difference of the count of zeros and ones is no more than two. Some of the 256 possible 8-bit words can be encoded in two different ways. Using these alternative encodings, the scheme is able to achieve long-term DC-balance in the serial data stream. This permits the data stream to be transmitted through a channel with a high-pass characteristic, for example Ethernet's transformer-coupled unshielded twisted pair or optical receivers using automatic gain control.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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