Link Access Procedure for Modems: Difference between revisions

Revision as of 08:09, 3 March 2007

Revision as of 09:43, 14 April 2007

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<blockquote>

: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.

cyclic redundancy checking (CRC) and retransmission of corrupted data (ARQ)

: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.

to ensure data reliability.<br />

Error control protocols such as LAPm use frames of variable lengths.The bigger

: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.

data content handled by each frame, the less the relative share of the overhead

bits. Usually LAPm adds only about 5 percent framing overhead.<br />

:The modems using LAPm saves up to 20% of the bandwidth as compared to the Async protocol.

LAPm has a optional Selective reject (SREJ) functionality which allows it to

resend only the corrupted frames. This in turn provides much faster recovery

: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.

from an error than in Async protocol.<br />

The modems using LAPm saves up to 20% of the bandwidth as compared to the Async

: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.

protocol.<br />

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.<br />

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. Compre-

ssion 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. <br />

</blockquote>

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[[categoryCategory:modems]]

[[Category:Link access protocols]]

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