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Intelecommunications,4B5B is a form of data communicationsline code. 4B5B maps groups of 4bits 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 aself-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 ascrambler. Some systems use scramblers in conjunction with 4B5B coding to assureDC balance and improveelectromagnetic 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.

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

The name4B5B is generally taken to mean the FDDI version. Other 4-to-5-bit codes have been used for magnetic recording and are known asgroup 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 isNRZI-encoded. FDDI over copper (CDDI) usesMLT-3 encoding instead, as does 100BASE-TX Fast Ethernet.

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

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 performclock recovery. For example, a run of 4 bits such as 0000₂ usingNRZI 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.

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.

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

• GCR 4B-5B encoding

• Simulator - 4B/5B Encoder line written in Matlab
• CodSim 2.0: Open source simulator for Digital Data Communications Model at the University of Malaga written in HTML

• Telecommunications standards
• Line codes

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