System information

CHAPTER
Troubleshooting ISDN Connections 17-373
17
Troubleshooting ISDN Connections
Integrated Services Digital Network (ISDN) refers to a set of digital services that are becoming
available to end users. ISDN involves the digitization of the telephone network so that voice, data,
text, graphics, music, video, and other source material can be provided to end users from a single
end-user terminal over existing telephone wiring. Proponents of ISDN imagine a worldwide network
much like the present telephone network, but with digital transmission and a variety of new services.
ISDN is an effort to standardize subscriber services, user/network interfaces, and network and
internetwork capabilities. Standardizing subscriber services attempts to ensure a level of
international compatibility. Standardizing the user/network interface stimulates development and
marketing of these interfaces by third-party manufacturers. Standardizing network and internetwork
capabilities helps achieve the goal of worldwide connectivity by ensuring that ISDN networks easily
communicate with one another.
ISDN applications include high-speed image applications (such as Group IV facsimile), additional
telephone lines in homes to serve the telecommuting industry, high-speed file transfer, and video
conferencing. Voice, of course, will also be a popular application for ISDN.
Many carriers are beginning to offer ISDN under tariff. In North America, large local-exchange
carriers (LECs) are beginning to provide ISDN service as an alternative to the T1 connections
(digital carrier facilities provided by telephone companies) that currently carry bulk wide-area
telephone service (WATS) services.
ISDN Components
ISDN components include terminals, terminal adapters (TAs), network-termination devices,
line-termination equipment, and exchange-termination equipment. ISDN terminals come in two
types. Specialized ISDN terminals are referred to as terminal equipment type 1 (TE1). Non-ISDN
terminals such as DTE that predate the ISDN standards are referred to as terminal equipment type 2
(TE2). TE1s connect to the ISDN network through a four-wire, twisted-pair digital link. TE2s
connect to the ISDN network through a terminal adapter. The ISDN TA can either be a standalone
device or a board inside the TE2. If the TE2 is implemented as a standalone device, it connects to
the TA via a standard physical-layer interface. Examples include EIA/TIA-232-C (formerly
RS-232-C), V.24, and V.35.
Beyond the TE1 and TE2 devices, the next connection point in the ISDN network is the network
termination type 1 (NT1) or network termination type 2 (NT2) device. These are network-termination
devices that connect the four-wire subscriber wiring to the conventional two-wire local loop. In
North America, the NT1 is a customer premises equipment (CPE) device. In most other parts of the
world, the NT1 is part of the network provided by the carrier. The NT2 is a more complicated device,