1 Answers
Link Access Procedure for Modems 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 many data link layer protocols, it is a variant of HDLC. Like the Microcom Networking Protocols that preceded it, LAPM uses cyclic redundancy checking and retransmission of corrupted data to ensure data reliability.
Error control protocols such as LAPM use frames of variable lengths, each beginning with a header and ending with a frame check sequence trailer. Correctly received packets are acknowledged, and unacknowledged packets are automatically retransmitted.
The larger the 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 communication latency.