Understanding Frame Check Sequences

All packets or frames within a network can be damaged by crosstalk or interference in the network's physical wires. The frame check sequence (FCS) is an extra field in each transmitted frame that can be analyzed to determine if errors have occurred. The FCS uses cyclic redundancy checks (CRCs), checksums, and two-dimensional parity bits to detect errors in the transmitted frames.

This topic contains the following sections:

• Cyclic Redundancy Checks and Checksums
• Two-Dimensional Parity

Cyclic Redundancy Checks and Checksums

On a link that uses CRCs for frame checking, the data source uses a predefined polynomial algorithm to calculate a CRC number from the data it is transmitting. The result is included in the FCS field of the frame and transmitted with the data. On the receiving end, the destination host performs the same calculation on the data it receives.

If the result of the second calculation matches the contents of the FCS field, the packet was sent and received without bit errors. If the values do not match, an FCS error is generated, the frame is discarded and the originating host is notified of the error.

Checksums function similarly to CRCs, but use a different algorithm.

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.

Related Topics

• Junos OS Feature Support Reference for SRX Series and J Series Devices
• Understanding Interface Physical Properties