LAPM
Encyclopedia
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 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.
Error control protocols such as LAPM use frames of variable lengths, each beginning with a header and ending with a frame check sequence
trailer (a cyclic redundancy check
). Correctly received packets are acknowledged, and unacknowledged packets are automatically retransmitted (ARQ).
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
.
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 Protocol
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.
Communications protocol
A communications protocol is a system of digital message formats and rules for exchanging those messages in or between computing systems and in telecommunications...
for modems.
LAPM is an error control protocol defined in ITU-T recommendations V.42. Like many data link layer
Data link layer
The data link layer is layer 2 of the seven-layer OSI model of computer networking. It corresponds to, or is part of the link layer of the TCP/IP reference model....
protocols, it is a variant of HDLC. Like the Microcom Networking Protocol
Microcom Networking Protocol
The MNP family of error-correcting protocols were commonly used on early high-speed modems. Originally developed for use on Microcom's own family of modems, the protocol was later openly licensed and used by most of the modem industry, notably the "big three", Telebit, USRobotics and Hayes...
s that preceded it, 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, each beginning with a header and ending with a frame check sequence
Frame Check Sequence
A frame check sequence refers to the extra checksum characters added to a frame in a communication protocol for error detection and correction. Frames are used to send upper-layer data and ultimately the user application data from a source to a destination. The data package includes the message...
trailer (a cyclic redundancy check
Cyclic redundancy check
A cyclic redundancy check is an error-detecting code commonly used in digital networks and storage devices to detect accidental changes to raw data...
). Correctly received packets are acknowledged, and unacknowledged packets are automatically retransmitted (ARQ).
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
Latency (engineering)
Latency is a measure of time delay experienced in a system, the precise definition of which depends on the system and the time being measured. Latencies may have different meaning in different contexts.-Packet-switched networks:...
.
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 Protocol
Microcom Networking Protocol
The MNP family of error-correcting protocols were commonly used on early high-speed modems. Originally developed for use on Microcom's own family of modems, the protocol was later openly licensed and used by most of the modem industry, notably the "big three", Telebit, USRobotics and Hayes...
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.