Link Access Procedure (LAP) Protocols

Link Access Procedure (LAP) is basically considered as an ITU family of Data Link Layer (DLL) protocols that are subsets of High-Level Data Link Control (HDLC). LAP is particularly derived from IBM’s System Development Life Cycle (SDLC). There are several LAP protocols. Each of these protocols is usually a subset of HDLC tailored for a particular purpose.

Some of them are given below :

These are explained as following below.

  1. Multilink Procedure (MLP) –
    MLP basically allows for Multilink Operations. It is an upper sublayer of the DLL that usually operates among Packet Layer and a multiplicity of SLP’s (Single Data Link Protocol) functions. It is generally an extension of LAPB that simply gives permission for multiple physical links along with providing better throughput.




  2. Link Access Procedure for Modems (LAPM) –
    LAPM has basically used in V.32 error-correcting modems and V.42 modems. Transmission among LAPM is synchronous even though transmission among computer and modem is asynchronous.

    With the help of bit-oriented synchronous techniques, LAPM generally transmits data in frames or packets. LAPM usually contains SREJ (Selective Reject) as an optional function that allows it to again send all of corrupted frames along with providing faster recovery from an error. Modem transmits or transfers data as frames even though all the attached or connected computer transfers data to LAPM modem as standard asynchronous input.

    To simply ensure data reliability i.e., data are reasonably complete and accurate or not, LAPM makes use of CRC (Cycle Redundancy Checking) and also of ARQ (Automatic Repeat Request). It is also been developed and established to apply some HDLC features to modems. It is also designed and created to do asynchronous-synchronous conversion, error detection and correction, and retransmission.


  3. Link Access Procedure for Frame Relay (LAPF) –
    LAPF basically provides data link for frame relay networks and is designed for use along with frame relay. It is easiest and simplest version of ISDN’s LAPD. It is also required or carrying data only and even there is no signaling at DLL for performing flow and error control. It is also a part of network’s communications protocol. LAPF simply ensures that whether frames are error-free and executed in correct sequence or not. It does not make use of C/R (Command/Response) bit, even though other higher-layer protocols that need application-specific do use this bit.


  4. Link Access Procedure for Half-Duplex (LAPX) –
    LAPX is basically used for ship-to-shore transmission.


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