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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.
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