D-Channel Standards; D-Channel Signaling And Carrier Networks; Isdn Equipment Configurations - Enterasys Security Router X-PeditionTM User Manual

Understanding ISDN

D-Channel Standards

The XSR supports several D-channel standards, which are enabled with the
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.
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
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
11-4 Configuring Integrated Services Digital Network
"Configuring Dialer Services"
isdn switch-type
on page 10-1 for more