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ISDN Overview

117353-B Rev. 00

4-3

Primary Rate Interface

PRI is the other ISDN service. In the United States, Canada, and Japan, PRI

transmits data at a rate of 64 Kb/s over 23 B channels, and sends signaling

information at a rate of 64 Kb/s over one D channel (23B + D). In Europe, PRI

transmits data at the same rates with 30 B channels and one D channel (30B + D).

PRI channels combine into two physical, twisted-pair lines.

There are two types of PRI service because the United States, Canada, and Japan

derive the PRI transmission rates from the T1 standard, which operates at line

speeds of 1.544 Mb/s. Europe and Australia, however, use the E1 standard, which

operates at line speeds of 2.048 Mb/s.

The D Channel

The D channel is integral to ISDN services. It provides out-of-band signaling,

which means that the line that carries signaling information is separate from the

line that carries data. Using out-of-band signaling, call setup information does not

interfere with the data you send, and the result is more accurate communication.

The D channel also provides what the ITU-T (formerly CCITT) defines as

supplementary services. These are extra services that you can purchase from your

ISDN provider, for example, calling line ID, which enables you to determine the

phone number of the incoming call.

Link Access Procedure-D

To perform call setup and signaling functions between your equipment and the

ISDN network, the ITU-T defined recommendation Q.921, which outlines Link

Access Procedure-D (LAPD), the data link layer protocol for the D channel. ISDN

uses LAPD to establish a switched connection on a particular B channel. The B

channel then allows data to travel between the two end points of that connection.

LAPD transmits information in frames. The fields in a LAPD frame contain

addresses, control, and frame-check sequence information. The address and

control information comprise the LAPD header, which sequences the packets and

keeps them in the right order. The frame-check sequence allows the receiver of a

frame to detect any errors that may have occurred during transmission.

Unlike other data link layer protocols, LAPD can handle multiple users on the

same multiaccess interface. Figure 4-3

shows a LAPD frame.

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Avaya Configuring Dial Services Bedienungsanleitung Seite 109