4B5B

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 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.

Encoding table

Name	4b	5b	Description
0	0000	11110	hex data 0
1	0001	01001	hex data 1
2	0010	10100	hex data 2
3	0011	10101	hex data 3
4	0100	01010	hex data 4
5	0101	01011	hex data 5
6	0110	01110	hex data 6
7	0111	01111	hex data 7
8	1000	10010	hex data 8
9	1001	10011	hex data 9
A	1010	10110	hex data A
B	1011	10111	hex data B
C	1100	11010	hex data C
D	1101	11011	hex data D
E	1110	11100	hex data E
F	1111	11101	hex data F
Q	-NONE-	00000	Quiet (signal lost)
I	-NONE-	11111	Idle
J	-NONE-	11000	Start #1
K	-NONE-	10001	Start #2
T	-NONE-	01101	End
R	-NONE-	00111	Reset
S	-NONE-	11001	Set
H	-NONE-	00100	Halt

Note that normal data symbols begin with at most one 0 bit and end with at most two, so there can be at most three 0 bits in a row. Control symbols are used in combinations that also preserve this rule. Thus, 4B5B encoding is a (0,3) RLL code.

FDDI and 100BASE-TX begin frames with a JK pair. FDDI ends frames with a TT pair, while 100BASE-TX uses a TR pair.

Command Characters

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)

External links

• Simulator - 4B/5B Encoder line written in Matlab