Enterasys Networks Security Router X-PeditionTM - Manual (Page 267)

Understanding ISDN
XSR User’s Guide 11-3
which provides access to 23 B-channels in North America and Japan and 30 B-channels in Europe
and most of Asia, and a 64 Kbps D-channel in both.
Basic Rate Interface
The XSR’s BRI NIM provides two BRI ports. Each port has two 64 Kbps B-channels and one 16
Kbps D-channel. BRI is configured on the XSR by
interface bri sub-commands.
Primary Rate Interface
ISDN PRI is provisioned over T1 service in North America and Japan and includes one 64 Kbps D-
channel and 23 B-channels, and over E1 service includes 30 B-channels in most other parts of the
world.
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).
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.