User`s guide
XSR User’s Guide 191
Chapter 9 Understanding ISDN
Configuring Integrated Services Digital Network (ISDN)
Unlike the B-channel, which functions as a simple pipe for user data, the D-
channel is associated with higher level protocols, Layer 2: Q.921 and 3: Q.931
of the OSI model.
Q.931 is the call-control protocol component of this definition, although
various carriers tend to use variants. This Layer 3 signaling protocol is
transferred on the D-channel using Link Access Procedure-D-channel
(LAPD): Q.921, a Layer 2 HDLC-like protocol.
D-Channel Standards
The XSR supports several D-channel standards, which are enabled with the
isdn switch-type
command. The accepted standards and some associated
switches are:
Europe/ International: basic-net3 for BRI and primary-net5 for PRI
Japan: basic-ntt for BRI and primary-ntt for PRI
North America: basic-ni1 and basic-dms100 switches for BRI and
primary-ni2, primary-5ess, and primary-dms100 for PRI
D-Channel Signaling and Carrier Networks
When the ISDN carrier receives a Q.931 instruction from a remote location,
for example, to set up a call, it triggers network switches to set up an end-to-
end 64 Kbps B-channel between the source and the destination directory
number signaled by Q.931. The carrier's network uses a different signaling
system though. Signaling between remote ISDN devices and the public voice
and data network switches occurs using D-channel protocols such as Q.931,
which in turn is converted into Signaling System No. 7 (SS7) signals within
the carrier's digital voice and data networks. With SS7, carriers are able to
maintain clear channel 64 Kbps connections by communicating signaling data
in a distinct channel. The switch at the destination side of the network then
communicates with the remote ISDN device using its D-channel protocol.
Unfortunately, SS7 is not always fully implemented, leading to occasional
limitations when ISDN links traverse multiple switches. For instance, if one
switch does not fully support SS7 ISDN features, call setup and signaling
messages must be sent in-band or through the same communications channel
as the bearer service. In other words, 8 Kbps of a 64 Kbps B-channel must be
reserved for signaling, thus reducing available bandwidth.