In telecommunication, a longitudinal redundancy check (LRC) or horizontal redundancy check is a form of redundancy check that is applied independently to each of a parallel group of bit streams. The data must be divided into transmission blocks, to which the additional check data is added.

The term usually applies to a single parity bit per bit stream, although it could also be used to refer to a larger Hamming code. While simple longitudinal parity can only detect errors, it can be combined with additional error control coding, such as a transverse redundancy check, to correct errors.

A longitudinal redundancy check for a sequence of characters may be computed in software by the following algorithm:

       Set LRC = 0
       For each character c in the string
       do
           Set LRC = LRC XOR c
       end do

An 8-bit LRC such as this is equivalent to a cyclic redundancy check using the polynomial x⁸+1, but the independence of the bit streams is less clear when looked at that way.

Many protocols use such an XOR-based longitudinal redundancy check character, including the IEC 62056-21 standard for electrical meter reading, and the ACCESS.bus protocol.

References

• Federal Standard 1037C
• MIL-STD-188