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Chapter 10

Wide Area Networking Protocols

and it was designed to achieve this over the telephone systems already in
place. ISDN is referenced by a suite of ITU-T standards that encompass the
OSI model's Physical, Data Link, and Network layers.

The ISDN standards define the hardware and call-setup schemes for end-to-

end digital connectivity.

PPP is typically used with ISDN to provide data encapsulation, link integ-

rity, and authentication.

These are the benefits of ISDN:

It can carry voice, video, and data simultaneously.

Call setup is faster than with a modem.

Data rates are faster than on a modem connection.

Full-time connectivity across the ISDN is spoofed by the Cisco IOS
routers using dial-on-demand (DDR) routing.

Small office and home office sites can be economically supported with
ISDN BRI services.

ISDN can be used as a backup service for a leased line connection
between the remote and central offices.

Modem racking and cabling can be eliminated by integration of dig-
ital modem cards on Cisco IOS Network Access Server (NAS).

ISDN Components

The components used with ISDN include functions and reference points.
Figure 10.6 shows how the different types of terminal and reference points
can be used in an ISDN network.

In North America, ISDN uses a two-wire connection, called a U reference

point, into a home or office. The NT1 device is used to convert the typical
four-wire connection to a two-wire connection that is used by ISDN phones
and terminal adapters (TAs). Most routers can now be purchased with a
built-in NT1 (U) interface.

Figure 10.7 shows the different reference points and terminal equipment

that can be used with Cisco ISDN BRI interfaces.

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