6b/8b encoding

In telecommunications, 6b/8b is a line code that expands 6-bit codes to 8-bit symbols for the purposes of maintaining DC-balance in a communications system.

Each 8-bit output symbol contains 4 zero bits and 4 one bits, so the code can, like a parity bit, detect all single-bit errors.

The number of binomial coefficient 8-bit patterns with 4 bits set is = 70. Further excluding the patterns 11110000 and 00001111, this allows 68 coded patterns: 64 data codes, plus 4 additional control codes.

Coding rules

The 64 possible 6-bit input codes can be classified according to their disparity, the number of 1 bits minus the number of 0 bits:

The 6-bit input codes are mapped to 8-bit output symbols as follows:

• The 20 6-bit codes with disparity 0 are prefixed with 10
  Example: 000111 → 10000111
  Example: 101010 → 10101010
• The 14 6-bit codes with disparity +2, other than 001111, are prefixed with 00
  Example: 010111 → 00010111
• The 14 6-bit codes with disparity −2, other than 110000, are prefixed with 11
  Example: 101000 → 11101000
• The remaining 20 codes: 12 with disparity ±4, 2 with disparity ±6, 001111, 110000, and the 4 control codes, are assigned to codes beginning with 01 as follows:

Note that no data symbol contains more than four consecutive matching bits, or begins or ends with more than three identical. Thus, the longest run of identical bits that will be produced is 6. (I.e. this is a (0,5) RLL code.)

Any occurrence of 6 consecutive identical bits constitutes a comma sequence or sync mark; it identifies the symbol boundaries because it must straddle the inter-symbol boundary.

See also

• 8b/10b encoding, another fixed-table system with a higher code rate but less error detection.
• 64b/66b encoding, linear feedback register.

External links

• Freepatents online, United States patent 6,876,315: DC-balanced 6B/8B transmission code with local parity.

v · d · eLine 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

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