LAPM

Link Access Procedure for Modems (LAPM) is part of the V.42 error correction protocol for modems.LAPM is an error control protocol defined in ITU-T recommendations V.42. Like the MNP protocols, LAPM uses cyclic redundancy checking (CRC) and retransmission of corrupted data (ARQ) to ensure data reliability.Error control protocols such as LAPm use frames of variable lengths. The bigger data content handled by each frame, the less the relative share of the overhead bits. Usually LAPm adds only about 5 percent framing overhead.LAPm has an optional Selective reject (SREJ) functionality which allows it to resend only the corrupted frames. This in turn provides much faster recovery from an error than in Async protocol.The modems using LAPm saves up to 20% of the bandwidth as compared to the Async protocol.The ITU-T V.42 LAPM procedure is considered as more robust than MNP(Microcom Network Protocol), and has a more sophisticated and powerful data compression option in V.42bis, allowing much greater data throughput.V.42bis uses a string encoding algorithm called BTLZ, 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.
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Link Access Protocol for Modems

LINK ACCESS PROCEDURE FOR MODEMS. LAPM