The Data Link Layer

   Error Detection and Correction: FCS

All protocols, except the Physical Layer, add header information to the data they transmit. However, the Data Link Layer also adds a trailer. The trailer typically contains a data field, called the Frame Check Sequence (FCS), that is used by the receiving Data Link Layer process to determine whether each frame has been transmitted without errors.

The sending Data Link Layer process creates the FCS by running the frame's data through a special calculation. The result of this calculation is a binary number that becomes that frame's FCS. When the destination Data Link process receives the frame, it runs the same calculation against the data. If the calculation produces the same binary number, it is likely the frame was not damaged in transit.

The FCS can only detect the most basic types of errors, such as a bit that has changed from a 1 to a 0. Thus, the Data Link Layer does not have complete responsibility for error detection. Other types of error correction can occur in higher layers.