4b-5b Definition


A line coding technique used in 100Base-TX, 100Base-FX, and Fiber Distributed Data Interface (FDDI) LANs. 4B/5B refers to the fact that every nibble of 4 Bits of data is encoded into 5 Bits of signal. Specified by the American National Standards Institute (ANSI) X3T9.5 committee recommendation for FDDI, 4B/5B is sometimes referred to as block coding, as a block of data bits is mapped into a block of signaling bits. This approach increases the number of bit patterns from 16 (2 4 = 16) to 32 (2 5 = 32), which means that each five-bit signal block includes enough clocking pulses and signal transitions to synchronize the network. 4B5B also provides some level of error detection. So, the signaling rate must be 125 MHz to support a unipolar code (such as that used with classic 10 Mbps Ethernet) with a signaling rate of 125 Mbps, which, in turn, supports a data rate of 100 Mbps.To address this issue, 100Base-TX uses an intermediate step known as multi-level transition (MLT) that reduces the carrier frequency to only 31.25 MHz. See also 100Base-TX, 100Base-FX, 8B/10B, ANSI, block coding, carrier, FDDI, LAN, MLT, quadbit, synchronize, and unipolar.

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