3Com DIGITAL MODEM manual Introduction

1 INTRODUCTION

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(Bearer) channels for user data, and one 16 Kbps signaling D (Data) 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.