4B5B

In telecommunication, 4B5B is a form of data communications Block Coding. 4B5B maps groups of 4 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 0000₂ contains no transitions and that causes clocking problems for the receiver. 4B5B 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 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 Multichannel Audio Digital Interface (MADI) .

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.

The 4B5B encoding is also used for USB Power Delivery communication on CC pin, over BMC protocol.

4B5B encoding table

Command characters

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

(HDLC = High-Level Data Link Control)

Line coding (digital baseband transmission)
Main articles	Unipolar encoding Bipolar encoding On-off keying
Basic line codes	Return to zero (RZ) Non-return-to-zero, level (NRZ/NRZ-L) Non-return-to-zero, inverted (NRZ-I) Non-return-to-zero, space (NRZ-S) Manchester Differential Manchester/biphase (Bi-φ)
Extended line codes	Conditioned Diphase 4B3T 4B5B 2B1Q Alternate mark inversion Modified AMI code Coded mark inversion MLT-3 encoding Hybrid ternary code 6b/8b encoding 8b/10b encoding 64b/66b encoding Eight-to-fourteen modulation Delay/Miller encoding TC-PAM
Optical line codes	Carrier-suppressed return-to-zero Alternate-phase return-to-zero
See also: Baseband Baud Bit rate Digital signal Digital transmission Ethernet physical layer Pulse modulation methods Pulse-amplitude modulation (PAM) Pulse code modulation (PCM) Serial communication Category:Line codes

External links

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