ISDN maps to the three lower layers of the OSI model – Network, Data Link, and Physical. However, different protocols exist at the Data Link and Network layers for B and D channels, as illustrated in the figure below. The functions handled at each OSI layer are described below.
- Physical Layer. The ISDN Physical layer is concerned with the actual sending and receiving of bits over variety of interfaces. For example, the I.430 standard is responsible for providing communication over S/T reference points, while the ANSI T1.601 standard defines communication over U interfaces in North America.
- Data Link Layer. ISDN D channels use the Link Access Procedure for D Channels (LAPD – Q.921) to frame signaling and control data at Layer 2. On the B channel, data can be framed in a variety of ways, including via PPP and HDLC.
- Network Layer. ISDN D channels handle call setup, termination, and maintenance at the OSI Network layer using the Q.931 protocol, which implements common signaling standards. On B channels, ISDN uses common Network layer protocols like IP, IPX, AppleTalk, and so forth.