link access procedure D (ISDN) (LAP D)

The LAP-D protocol(Link Access Procedure D) is a security protocol for the D channel. It was developed for ISDN and ensures a controlled communication application between the terminal equipment and the public network. LAP-D is based on High Level Data Link Control( HDLC) and has been enhanced with essential features as a control protocol.

The LAP-D protocol has an address field enlarged to 16 bits with the Service Access Point Identifier( SAPI) and Terminal Equipment Identifier( TEI), and in addition it provides unacknowledged message transmissioninto the UI frame. According to the International Telecommunication Union( ITU), there are two frame formats; where one frame format transmits various control functions of the linklayer and the second frame format is used for transmitting control information of the network layer and for information transmission from higher layers.

The LAP-D protocol performs a number of important functions. For example, addressingISDN terminals with SAPI and TEI, TEI assignment and reassignment, call setup and disconnection, establishing individual connections and sending broadcast messages, controlling information transmission, providing a transparent transmission frame, detecting transmission errors and automatic block retransmission, and flow control.

LAP-D differs from the LAP-B protocol (LAP balanced) in its framing sequence. The LAP-D function is equivalent to the function of HDLC. LAP D is specified by the International Telecommunication Union (ITU) in Recommendations Q.920 and Q.921. It will probably be used as the basis for LAP M, the proposed ITU standard for error correction in modems.