3Com DIGITAL MODEM , I

The Integrated Services Digital Network (ISDN) is a digital

communications standard for supporting voice, data, and

video applications through conventional telephone wires.

Speciﬁcally, ISDN is a method of connecting to the public

telephone network using digital technology. ISDN uses the

existing copper wire and public telephone network

infrastructure. All existing voice services provided by the

telephone companies are accessible using ISDN, and for the

ﬁrst time, high-speed dial-up data can be provided

cost-effectively to the mass market.

Two electrical interfaces are deﬁned as part of ISDN. The ﬁrst

and less expensive is Basic Rate ISDN, or 2B+D. A Basic Rate

Interface (BRI) has two 56 or 64 Kbps (kilobits per second)

B (

earer) channels for user data, and one 16 Kbps signaling

D (

ata) channel, used for packet communications with the

public network and providing status and control information.

In most regions, the ISDN BRI is priced at 1.5 to 1.8 times the

monthly cost of an analog POTS (plain old telephone service)

telephone line. You may dial voice or data connections on

either or both of the B channels simultaneously. Calls placed

from an ISDN line may connect to remote sites that have

analog POTS, Switched 56, ISDN BRI, or T1 telephone lines.

There is full compatibility with existing services, and no need

for ISDN BRI on both ends.

The other electrical interface, which is somewhat more

expensive, is the ISDN Primary Rate Interface (PRI). PRI is

based on the more expensive and higher-speed T1 networks

that are typically deployed at larger sites. PRI consists of 23

B channels and one 64 Kbps D channel, providing large

businesses with a greater degree of ﬂexibility.