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Error control protocols such as LAPM use frames of variable lengths, each beginning with a header and ending with a [[frame check sequence]] trailer (a [[cyclic redundancy check]]). Correctly received packets are acknowledged, and unacknowledged packets are automatically retransmitted ([[Automatic repeat-request|ARQ]]).
The larger the payload included in each frame, the smaller the relative cost of transmitting the header and trailer bits. Usually LAPM adds only about 5 percent framing overhead. This is a noticeable improvement over the fixed 25% overhead of start and stop bits required by direct [[asynchronous serial communication]], but obviously such large frames impose a cost in the form of [[Latency (engineering)|communication latency]].
LAPM has an optional Selective reject (SREJ) functionality which allows it to resend only the corrupted frames, providing faster recovery from an error.
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