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In telecommunications, 4B5B is a form of data communications line code. 4B5B maps groups of 4 bits of data onto groups of 5 bits for transmission. These 5-bit words are predetermined in a dictionary and they are chosen to ensure that there will be sufficient transitions in the line state to produce a self-clocking signal.
Prior to 6250 bpi tapes, 1600 bpi tapes satisfied these constraints using a technique called phase encoding (PE), which was only 50% efficient. For 6250 bpi GCR tapes, a (0, 2) RLL code is used, or more specifically a 4 / 5 (0, 2) block code [3] sometimes also referred to as GCR (4B-5B) encoding. [6]
The 5b/6b code is a paired disparity code, and so is the 3b/4b code. Each 6- or 4-bit code word has either equal numbers of zeros and ones (a disparity of zero), or comes in a pair of forms, one with two more zeros than ones (four zeros and two ones, or three zeros and one one, respectively) and one with two less.
In the table below, the column "ISO 8859-1" shows how the file signature appears when interpreted as text in the common ISO 8859-1 encoding, with unprintable characters represented as the control code abbreviation or symbol, or codepage 1252 character where available, or a box otherwise. In some cases the space character is shown as ␠.
This mapping from 4 bits to three ternary states is given in a table known as Modified Monitoring State 43 (MMS43). A competing encoding technique, used for the ISDN basic rate interface where 4B3T is not used, is 2B1Q. The sync sequence used is the 11-symbol Barker code, +++−−−+−−+− or its reverse, −+−−+−−−+++.
Consult the following table to see if you’ll need additional forms: ... Line 9: Add lines 1, 2b, 3b, 4b, 5b, 6b, 7 and 8, and then enter the amount on this line. Line 10a/10b/10c: ...
It refers to "comma codes" in sections "Encoding tables" and "3b/4b code (fghj)". It also refers to "comma sequences" in section "3b/4b code (fghj)". In section "Control symbols" is refers to them as "comma symbols" but also has one reference to "comma sequences" although this is for part of a symbol.
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