Error detection discovers whether bit errors occurred during the transmission of the frame. To do this, most data-link protocols include a frame check sequence (FCS) or cyclical redundancy check (CRC) field in the data-link trailer. This field contains a value that is the result of a mathematical formula applied to the data in the frame.
An error is detected when the receiver plugs the contents of the received frame into a mathematical formula. Both the sender and the receiver of the frame use the same calculation, with the sender putting the results of the formula in the FCS field before sending the frame. If the FCS sent by the sender matches what the receiver calculates, the frame did not have any errors during transmission.
Error detection does not imply recovery; most data links, including IEEE 802.5 Token Ring and 802.3 Ethernet, do not provide error recovery. The FCS allows the receiving device to notice that errors occurred and then discard the data frame. Error recovery, which includes the resending of the data, is the responsibility of another protocol. For instance, TCP performs error recovery, as described in Chapter 6, "Fundamentals of TCP and UDP."
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