4B5B

From Wikipedia, the free encyclopedia
Jump to: navigation, search

In telecommunication, 4B5B is a form of data communications Block Coding. 4B5B maps groups of four bits onto groups of 5 bits, with a minimum density of 1 bits in the output. When NRZI-encoded, the 1 bits provide necessary clock transitions for the receiver. For example, a run of 4 bits such as 0000 contains no transitions and that causes clocking problems for the receiver. 4B/5B solves this problem by assigning each block of 4 consecutive bits an equivalent word of 5 bits. These 5 bit words are pre-determined in a dictionary and they are chosen to ensure that there will be at least two transitions per block of bits.

A collateral effect of the code is that more bits are needed to send the same information than with 4 bits. An alternate to using 4B5B coding is to use a scrambler. Depending on the standard or specification of interest, there may be several 4b5b characters left unused. The presence of any of the "unused" characters in the data stream can be used as an indication that there is a fault somewhere in the link. Therefore, the unused characters can actually be used to detect errors in the data stream.

4B5B was popularized by Fiber distributed data interface (FDDI) in the mid-1980s, and was later adopted by

  • 100BASE-TX standard defined by IEEE 802.3u in 1995 and
  • AES10-2003 MADI [1] (Multichannel Audio Digital Interface).

The name "4B5B" is generally taken to mean the FDDI version. Other 4-to-5-bit codes have been used for magnetic recording and are known as group code recording (GCR).

On optical fiber, the 4B5B output is NRZI-encoded. FDDI over copper (CDDI) uses MLT-3 encoding instead, as does 100BASE-TX.

Command Characters[edit]

The following character sets are sometimes referred to as command characters.

Control Character 5b symbols Purpose
JK 11000 10001 Sync, Start delimiter
II 11111 11111 Not Used
TT 01101 01101 FDDI end delimiter
TS 01101 11001 Not Used
IH 11111 00100 SAL
TR 01101 00111 100BASE-TX end delimiter
SR 11001 00111 Not Used
SS 11001 11001 Not Used
HH 00100 00100 HDLC0
HI 00100 11111 HDLC1
HQ 00100 00000 HDLC2
RR 00111 00111 HDLC3
RS 00111 11001 HDLC4
QH 00000 00100 HDLC5
QI 00000 11111 HDLC6
QQ 00000 00000 HDLC7

(HDLC = High-Level Data Link Control)

References[edit]


External links[edit]