4B5B

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. A collateral effect of the code is that 25% more bits are needed to send the same information.

An alternative to using 4B5B coding is to use a scrambler. Some systems use scramblers in conjunction with 4B5B coding to assure DC balance and improve electromagnetic compatibility.

Depending on the standard or specification of interest, there may be several 5-bit output codes left unused. The presence of any of the unused codes in the data stream can be used as an indication that there is a fault somewhere in the link. Therefore, the unused codes can be used to detect errors in the data stream.

Applications

4B5B was popularized by Fiber Distributed Data Interface (FDDI) in the mid-1980s. It was adopted for digital audio transmission by MADI in 1989.^[1] and by Fast Ethernet in 1995.

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 coded recording (GCR), but those are (0,2) run-length limited codes, with at most two consecutive zeros. 4B5B allows up to three consecutive zeros (a (0,3) RLL code), providing a greater variety of control codes.

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

The 4B5B encoding is also used for USB Power Delivery (USB-PD) communication,^[2] where it is sent over the USB-C CC pin (further encoded using biphase mark code) or the USB-A/B power lines (further encoded using frequency-shift keying).

Clocking

4B5B codes are designed to produce at least two transitions per 5 bits of output code regardless of input data. The transitions provide necessary transitions for the receiver to perform clock recovery. For example, a run of 4 bits such as 0000₂ using NRZI encoding contains no transitions and that may cause clocking problems for the receiver. 4B5B solves this problem by assigning the 4-bit block a 5-bit code, in this case, 11110₂.

There are eight 5-bit codes that have 3 consecutive 0s: 00000, 00001, 00010, 01000, 10000, 00011, 10001, 11000. This leaves 24 codes available.

Encoding table

Data 4B5B code (Hex) (Binary) 0 0000 11110 1 0001 01001 2 0010 10100 3 0011 10101 4 0100 01010 5 0101 01011 6 0110 01110 7 0111 01111

Data 4B5B code (Hex) (Binary) 8 1000 10010 9 1001 10011 A 1010 10110 B 1011 10111 C 1100 11010 D 1101 11011 E 1110 11100 F 1111 11101

Symbol 4B5B code Description FDDI Fast Ethernet USB-PD H 00100 Halt Yes Yes No I 11111 Idle Yes Yes No J 11000 Start #1 Yes Yes Yes K 10001 Start #2 Yes Yes Yes L 00110 Start #3 No No Yes Q 00000 Quiet (loss of signal) Yes Yes No R 00111 Reset Yes Yes Yes S 11001 Set Yes No Yes T 01101 End (terminate) Yes Yes Yes

Three consecutive zero bits only appear in normal data when a code ending with two 0 bits (2, E) is followed by a code beginning with a 0 bit (1, 4, 5, 6, 7), so will always appear separated by multiples of the 5-bit encoded symbol length (and never separated by a single symbol). Violations of this property are used for special synchronization codes.

Command characters

The following codes are sometimes referred to as command characters. They are commonly used in pairs, although USB-PD uses 4-symbol sequences to begin its packets.

Control character	5b symbols	Purpose
JK	11000 10001	Sync, Start delimiter
I	11111	100BASE-X idle marker
T	01101	USB-PD end delimiter
TT	01101 01101	FDDI end delimiter
TS	01101 11001	Not used
IH	11111 00100	SAL
TR	01101 00111	100BASE-X end delimiter
SR	11001 00111	Not used
SS	11001 11001	Not used
H	00100	100BASE-X transmit error
JJJK	11000 11000 11000 10001	USB-PD Start Of Packet (SOP)
JJLL	11000 11000 00110 00110	USB-PD SOP′
JLJL	11000 00110 11000 00110	USB-PD SOP″
JSSL	11000 11001 11001 00110	USB-PD SOP′_Debug
JSLK	11000 11001 00110 10001	USB-PD SOP″_Debug
RRRS	00111 00111 00111 11001	USB-PD Hard Reset
RJRL	00111 11000 00111 00110	USB-PD Cable Reset

See also

• GCR 4B-5B encoding