5.2 ISDN Protocol Layers
5.2.2 ISDN layer 2

With ISDN, all hardware addressing occurs in Layer 2, just like in a traditional LAN environment. It is possible to have up to eight ISDN terminals on an S/T bus. TEs are, therefore, differentiated from each other through terminal endpoint identifiers (TEIs) and service access point identifiers (SAPIs).

The TEI is a dynamic assignment to that device. In the United States, when you boot up a router, you make some type of request to the switch for a TEI. The switch assigns you a TEI, and you will communicate over the switch using the signaling that uses a SAPI. This is the same concept as is used in the 802.2 frames, in which you need to differentiate between the different processes that are running. You need some special identifiers to provide this discrimination between frames. Some of the messages sent over the ISDN network are for call setup or call teardown, and others are data. The SAPI is a way of prioritizing the calls or giving access to the network first.

Q.920 is the functional specification for ISDN. The actual communication takes place over the network and is specified in Q.921.

Examples of SAPI values are 0 for call-control procedure and 63 for Layer 2 management function. TEI groups assignments are 0-63 for non-automatic TEI assignment; 64-126 for automatic TEI assignment; and 127 for group assignment, or broadcast.

It is important to note that the BRI network processor module on a Cisco 4000 series router must have a dedicated S-bus connection for each of its ISDN ports. The BRI module cannot share any of its S-bus connections with other terminal equipment; therefore, no other terminals should be connected to the same S-bus as a BRI module connection.