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Common ISDN Problems

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ISDN Fundamentals

SDN was developed in large part from the phone company's conver-

sion to digital networks from analog switches.This conversion, which
started in the 1960s, resulted in the following features:

Clearer, cleaner signals

Compressible voice, resulting in better trunking utilization

Longer distances between switching devices

Value-added features, including caller ID and three-way calling

Greater bandwidth--a single connection to the phone company can
service more than one phone number

Elimination of load coils and amplifiers in the network

The concept of ISDN was originally conceived as a means to move the digital

network into the home, where a single ISDN connection would provide two stan-
dard phone lines and digital services for data. This migration from the analog
phone would continue to use the existing copper wire plant, while adding services
that would ultimately increase revenues.

Unfortunately, users failed to accept ISDN in the numbers desired. This

was especially true in the United States, where installation problems, service
availability, and pricing all conspired to hinder acceptance.

In the late 1990s, ISDN was finding a new marketplace. Always On ISDN

uses the D channel to replace legacy X.25 networks, especially in point-of-
sale transactions. (A description of the B and D channels is included later in
this chapter.) Standard ISDN service is popular for videoconferencing and as
a residential connection to the Internet. Cable modems and xDSL technolo-
gies will probably replace this market in the 21st century, however.

Common ISDN Problems

s with problems affecting other protocols and networking devices,

ISDN problems appear in certain common areas. Some frequently encoun-
tered problems are presented in this section for administrators to consider
when evaluating real-world issues. Later in this chapter, the commands that
are appropriate for troubleshooting these problems with Cisco routers will
be presented.

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