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Chapter 8
Troubleshooting ISDN
Services are available to send data over the D channel. These services are often
referred to as
always on
, reflecting the non-demand nature of this channel.
Always on services are often used to replace X.25 point-of-sale circuits and are
provisioned to offer 9.6Kbit bandwidth to the application.
The Local Loop
Although the majority of administrators troubleshoot only the local side of the
ISDN circuit, there is a remote side that is critical to the successful operation
of ISDN.
The local loop refers to the circuit between the customer premises and the
central office (CO). This may include an access layer, referred to as an RT,
which permits digital connections to be greater distances from the central
office. The local loop interconnects the ISDN device to an ISDN switch--a
DMS-100, for example. Note that all digital services are sensitive to the dis-
tance between the switch and end device.
The Physical Layer
In order to properly troubleshoot ISDN, it is very important that you have
a good understanding of its technology, terminology, architecture, and
functionality. Here is a list of some terms used with ISDN that you should
familiarize yourself with:
LT/ET
The line termination and exchange termination points are called
LT and ET, respectively. They handle the termination of the local loop
and switching functions.
NT1
The NT1 is the network termination point. It is often the demarc.
TE1
A device with a four-wire, twisted-pair digital interface is referred to as
terminal equipment type one. Most modern ISDN devices are of this type.
TE2
Terminal equipment type two devices do not contain ISDN
interfaces. A terminal adapter (TA) is required.
R reference point Devices without internal ISDN functions are called
TE2s, and require a connection to a TA for operation in ISDN networks.
There is no standard connection between these devices, however--the
connection is referred to as the R reference point.
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