User`s guide

192 XSR User’s Guide
Understanding ISDN Chapter 9
Configuring Integrated Services Digital Network (ISDN)
This explains the 56 in switched-56 services, which also use 8 Kbps of a 64
Kbps channel for signaling. Any ISDN call that passes through at least one
network which lacks full SS7 signaling, must then limit its B-channel traffic to
56 Kbps. In such cases the ISDN equipment on both ends must be configured
to put only 56 Kbps of data onto their 64 Kbps link. As networks have
continued to modernize, the use of 56 Kbps connection has diminished.
The XSR automatically adapts to the speed of incoming calls, whether 56 or 64
Kbps. When dialing over ISDN in North America, users can set the call speed
by specifying 64 (default) or 56 Kbps. If the network can not connect at 64
Kbps, it will be rejected and the router will try to redial (if redial attempts are
set). If users wish to be sure that their calls will succeed, the XSR will request
all outgoing calls be set at 56 Kbps. Consult “Configuring Dialer Services” on
page 135 for more detailed information.
To support 56 Kbps, communications equipment at both ends must support a
rate adaptation scheme which pads bandwidth above 56 Kbps with blank data,
using such schemes as V.110 or V.120 rate adaptation. This feature is usually
required whenever an ISDN call originates in, is destined for, or passes
through the U.S., where 56 Kbps ISDN connections are not uncommon.
ISDN Equipment Configurations
In a BRI configuration, an ISDN adapter, also known as a Terminal Adapter
(TE), connects directly to NT1 network terminating equipment. This device is
provided by a service provider except in North America where users must
supply their own NT1 or order a BRI U-interface NIM with a built-in NT1.
The NT1 delimits between U and S/T reference points. The U reference point
represents the last section of the network that connects the Central Office with
a customer’s premises while the S/T reference point represents the customer
premises’ wiring. S/T is a point-to-multipoint wiring configuration, that is,
the NTI can be connected to as many as eight TEs that contend for the two B
channels. Most XSR applications are critical and require point-to-point
connections with the ISDN service to ensure that the B channels are available
in a timely fashion. International users are limited to ordering the S/T NIM as
it is the only approved device for connection to the network. North American
users can order U or S/T NIMs depending on wiring premises’ requirements.