User`s guide

190 XSR User’s Guide
Understanding ISDN Chapter 9
Configuring Integrated Services Digital Network (ISDN)
The number of B-channels is limited by the size of the standard trunk line
used in the region; T1 in North America and Japan and E1 most everywhere
else. Unlike BRI, PRI does not support a bus configuration, and only one
device can be connected to a PRI line - point-to-point service.
A single PRI connection is usually much less expensive than obtaining the
equivalent number of B-channels through multiple BRI connections. BRI and
PRI are used for the same applications, only the number of channels differ.
PRI is configured on the XSR by
controller t1/e1 sub-commands.
B-Channels
The XSR’s B-channels are 56 or 64 Kbps “pipes” also known as DSOs. B-
channels typically form circuit-switched connections. Just like a telephone
connection, a B-channel connection is an end-to-end physical circuit that is
temporarily dedicated to transferring data between two devices. The circuit-
switched nature of B-channel connections, combined with their reliability and
relatively high bandwidth, makes ISDN suitable for a range of applications
including video, fax, and data. They can be used to transfer any Layer 2 or
higher protocols across a link. The XSR employs PPP or Multilink PPP over
the switched BRI or PRI connections. For more information, refer to the PPP
and MLPPP chapters in this manual.
The router’s B-channels can also be configured as permanent or nailed-up
connections which are always up, as a leased-line application similar to the
channelized T1/E1 application.
D-Channel
The XSR’s D-channel is used for signaling, such as instructing the ISDN
carrier to set up or tear down a call along a B-channel, to ensure that a B-
channel is available to receive an incoming call, or to provide the signaling
information that is required for such features as caller identification. The D-
channel uses packet-switched connections, which are best adapted to the
intermittent but latency-sensitive nature of signaling traffic, thus accounting
for the vastly reduced call setup time of 1 to 2 seconds on ISDN calls (vs. 10 to
40 seconds using an analog modem).