ISDN spans the bottom three layers of the OSI reference model. As mentioned, ISDN uses a multitude of protocols that fall under those layers and govern its operation.

To communicate from the local terminal equipment to the ISDN switch in the central office (CO), ISDN uses a unique collection of protocols. ITU organizes these protocols in the following manner:

• E. series Describes telephone network standards as they relate to ISDN.

• I. series Describes theory, terminology, interfaces, and common techniques.

• Q. series Describes switching and signaling. For instance, Q.921 deals with Link Access Procedure on the D channel (LAPD) processes at Layer 2 of the OSI model. Q.931 deals with Layer 3 of the OSI model. The D channel uses Q.931 signaling.

After the completion of call setup and connection establishment, the ISDN process is identical to conventional calls. ISDN protocols come into play again when the call is disconnected between the local switch and the terminal equipment. This process is fast and typically doesn't affect user data.

ISDN Layer 1

Layer 1 encompasses the physical connection between the ISDN circuit and the CPE. This layer is shared by the B and D channels alike.

Now we can come back to the protocols we touched on earlier. ISDN Layer 1 is governed by the following protocols:

• I.430 For BRI across the S/T interface.

• I.431 For PRI.

• ANSI T1.601 For the BRI U interface. (The U interface is not standardized by ITU-T.)

ISDN Layer 2

Layer 2 deals with the B and D channels separately, offering functions unique to each channel. It specifies LAPD as the framing protocol used for the D channel. On the other hand, the B channel uses High-Level Data Link Control (HDLC) or PPP encapsulation.

Protocol assignment for Layer 2 is as follows:

• Q.920 Specifies the ISDN functions.

• Q.921 Specifies signaling over the network.

As is the case with the conventional LAN setting, the ISDN network needs the hardware addressing to take place between all the linked devices. ISDN Layer 2 is responsible for such addressing. In addition, there is further discrimination within each device when it comes to different processes running in that device. Therefore, a terminal endpoint identifier (TEI), dynamically assigned to each router by the switch at bootup, is used in tandem with a service access point identifier (SAPI), which is a way to identify the types of messages sent across the network.

ISDN Layer 3

At Layer 3, the D channel is controlled by the Q.931 protocol. The Q.931 protocol is a part of the Digital Subscriber Signaling System 1 (DSS1) protocol suite, which deals with message exchange.

The B channel specifications include support for the network layer protocols, such as IP, IPX, and AppleTalk