3.3 Layer 2 Protocol Overview - WAN Protocols
3.3.10 ISDN frame format
Layer 2 of the ISDN signaling protocol is Link Access Procedure on the D channel, also known as LAPD. LAPD is similar to High-Level Data Link Control (HDLC) and Link Access Procedure, Balanced (LAPB). As the expansion of the LAPD abbreviation indicates, it is used across the D channel to ensure that control and signaling information flows and is received properly. The LAPD frame format (see Figure 1) is very similar to that of HDLC and, like HDLC, LAPD uses supervisory, information, and unnumbered frames. The LAPD protocol is formally specified in ITU-T Q.920 and ITU-T Q.921.

The LAPD Flag and Control fields are identical to those of HDLC. The LAPD Address field can be either 1 or 2 bytes long. If the extended address bit of the first byte is set, the address is 1 byte; if it is not set, the address is 2 bytes. The first address field byte contains the service access point identifier (SAPI), which identifies the portal at which LAPD services are provided to Layer 3. The C/R bit indicates whether the frame contains a command or a response. The terminal endpoint identifier (TEI) field identifies either a single terminal or multiple terminals. A TEI of all ones indicates a broadcast.

Note: ISDN troubleshooting is covered in Chapter 9.