Two-Dimensional Parity

On a link that uses two-dimensional parity bits for frame checking, the sending and receiving hosts examine each frame in the total packet transmission and create a parity byte that is evaluated to detect transmission errors.

For example, a host can create the parity byte for the following frame sequence by summing up each column (each bit position in the frame) and keeping only the least-significant bit:

 
 Frame 1       0 1 0 1 0 0 1
 Frame 2       1 1 0 1 0 0 1
 Frame 3       1 0 1 1 1 1 0
 Frame 4       0 0 0 1 1 1 0
 Frame 5       0 1 1 0 1 0 0
 Frame 6       1 0 1 1 1 1 1

 Parity Byte   1 1 1 1 0 1 1

If the sum of the bit values in a bit position is even, the parity bit for the position is 0. If the sum is odd, the parity bit is 1. This method is called even parity. Matching parity bytes on the originating and receiving hosts indicate that the packet was received without error.

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