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The LAP protocols are part of a group of data link layer protocols for framing and transmitting data across point-to-point links. LAP originates from IBM SDLC (Synchronous Data Link Control), which IBM submitted to the ISO for standardization. The ISO developed HDLC (High-level Data Link Control) from the protocol. Later, the CCITT (now referred to as the ITU) modified HDLC for use in its X.25 packet-switching network standard. It called the protocol LAP (Link Access Procedure), but later updated it and called it LAPB (LAP Balanced).

This section discusses LAPB and several derivatives of LAPB. You can also refer to "SDLC (Synchronous Data Link Control)" and "HDLC (High-level Data Link Control)" for additional information.

LAPB transmissions typically take place over physical point-to-point links. It is a full-duplex protocol, meaning that each station can send and receive commands and responses over separate channels to improve throughput. The protocol is bit oriented, meaning that the data is monitored bit by bit. Bit-oriented information in the LAPB frame defines the structure for delivering data and command/response messages between communicating systems. The frame format for LAPB is similar to the frame type for HDLC. Refer to that topic for more details.

As mentioned, LAPB is the data link protocol for X.25. Related LAP protocols for other data communication technologies are outlined next.

MLP is an extension of LAPB that allows for multiple physical links, thus providing better throughput. As shown in Figure L-3a, a device that has multiple LAPB links will implement MLP as an upper-layer management protocol to allocate frames to the links. MLP sees the multiple LAPB links as a pool of links for transmitting information from higher-layer protocols as frames. Higher-level software does not need to be aware that multiple links exist. The MLP layer handles distributing frames among the links, and thus gives upper layers full access to the links.

This is the data link protocol used by V.32 error- correcting modems. When two LAPM modems establish a session, as pictured in Figure L-3b, they transmit data in frames using bit-oriented synchronous techniques. An attached computer still sends data to the LAPM modems as standard asynchronous input, but the modem transmits it as frames.

LAPD is the protocol used on ISDN's (Integrated Services Digital Network) D channel. Call setup and other signaling takes place on the D channel. Data transmissions take place on B channels. LAPD is the ITU Q.921 protocol.

LAPF is designed for use with frame relay. It is similar to LAPD in its frame format except that there is no Control field in the frame. Thus, LAPF is used for carrying data only and there is no signaling at the data link layer for performing flow control and error control. End systems perform these functions in higher-layer protocols.

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