|  | 5.2 | How ISDN Relates to the OSI Reference Model
| 5.2.3 | The ISDN data link layer
Layer 2 of the ISDN signaling protocol is Link Access Procedure on the D channel (LAPD). LAPD is similar to High-Level Data Link Control (HDLC) and Link Access Procedure, Balanced (LAPB). LAPD is used across the D channel to ensure that control and signaling information flows and is received properly. As the expansion of the LAPD abbreviation-Link Access Procedure on the D channel-indicates, it is used across the D channel to ensure that control and signaling information flows and is received properly. 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 Command/ Response (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. All 1s in the TEI field indicates a broadcast.