Link Access Procedure for Modems: Difference between revisions

Revision as of 00:01, 28 June 2008

Revision as of 20:03, 7 January 2009

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'''Link Access Procedure for Modems''' ('''LAPM''') is part of the [[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>

LAPM is an error control protocol defined in ITU-T recommendations V.42. Like many [[data link layer]] protocols, it is a variant of [[HDLC]]. Like the [[Microcom Networking Protocol]]s that preceded it, LAPM uses cyclic redundancy checking (CRC) and retransmission of corrupted data (ARQ) to ensure data reliability.

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[[V.42bis]] added a string compression algorithm called BTLZ ([[British Telecom]] [[Lempel Ziv]]), in which frequently occurring strings of characters are replaced by code words. The "dictionary" of strings is dynamically maintained during transmission and keeps track of changes in the data; new strings are added and old strings are deleted. As the data may not always be compressible (for example encrypted or pre-compressed data) the modem has the ability to switch between compressed and uncompressed modes of operation. Compression performance is continually monitored and, if no compression is obtained, the modem transmits the data in uncompressed form. Even while in uncompressed mode, the modem monitors the characteristics of the data and switches compression back on as soon as some benefit can be achieved.

==References==

{{refs}}

==See also==

* [[Point-to-Point Protocol]] (PPP)