Detecting Frame Errors with Frame Check Sequence (FCS)

All frames on a network are susceptible to errors, these can be caused by crosstalk between the wires in a cable or by external interference such as Radio Frequency and Electromagnetic Interference.

Fortunately the Data Link layer uses a Frame Check Sequence (FCS) to determine if errors have occurred during transmission. Frames have a FCS field which is used to store a value which is calculated for each frame. This field value is checked at the destinations Data Link layer against the same calculation performed by the source Data Link layer before the frame was sent.

If the FCS number included in the frame doesn’t match the recalculated number then an error has occurred during transmission, the frame is discarded, and the destination host requests it to be resent.

The Frame Check Sequence can use a number of different methods, however these are the most popular:

  • CRC – Cyclic redundancy Check – Polynomial calculations are performed on the data
  • Two Dimensional Parity – Uses a parity bit to make sure the data has not been corrupted.
  • Checksum – Sums the data to arrive at a total.

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