Link Access Procedure for Modems: Difference between revisions

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LAPM has an optional 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 ProtocolsProtocol]]s (MNP) that preceded it, and has a more sophisticated and powerful data compression option in V.42bis, allowing much greater data throughput.
 
[[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.