Link Access Procedure for Modems: Difference between revisions

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{{Short description|Error correction protocol for modems}}

'''Link Access Procedure for Modems''' ('''LAPM''') is part of the [[ITU-T V.42|V.42]] [[error correction]] [[Communications protocol|protocol]] for [[modems]].<ref>ITU-T Recommendation V.42: [http://www.itu.int/rec/T-REC-V.42/en Error-correcting procedures for DCEs using asynchronous-to-synchronous conversion]</ref>

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The larger the [[Payload (computing)|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''selective reject'' (''SREJ'') functionality which allows it to resend only the corrupted frames, providing faster recovery from an error.

The ITU-T V.42 LAPM procedure is considered more robust than the [[Microcom Networking Protocol]]s (MNP) that preceded it, and has a more sophisticated and powerful data compression option in V.42bis, allowing much greater data throughput.