4000 Router Series Layer 2 - WAN Configuration Guide

Configuring ISDN

Integrated Services Digital Network (ISDN) evolved from IDN. It provides end-to-end digital connectivity

and supports an extensive range of services, covering both voice and data services.

This feature is supported only on routers installed with BS, BU, E1, or T1 interface modules. For more

information about interface modules, see HP MSR Router Series Interface Module Guide.

Overview

ISDN is a circuit-switched telephone network system and allows digital transmission of voice and data

over ordinary telephone wires.

ISDN interfaces

ISDN provides a finite set of standard multi-purpose user-network interfaces (UNIs), where:

• Basic rate interface (BRI)—Breaks the 144 kbps payload rate down into two 64 kbps bearer

channels (B channels) and one 16 kbps data channel (D channel). This is sometimes referred to as

2B+D.

• Primary rate interface (PRI)—Referred to as 30B + D or 23B + D, where:

{ CE1 PRI—Carried over an E1 (about 2 Mbps) with 30 B channels of 64 kbps and 1 D channel

of 64 kbps in most parts of the world. An E1 provides 32 timeslots numbered 0 through 31,

where timeslot 0 transmits synchronization information, timeslot 16 is used as a D channel, and

the rest of the timeslots are used as B channels.

{ CT1 PRI—Delivered on one or more T1 carriers of nearly 1.5 Mbps (divided into 24 timeslots)

mainly in North America. A PRI has 23 B channels (timeslots 1 through 23) and 1 D channel for

signaling (timeslot 24).

ANSI recommends the T1 system, and ITU-T recommends the E1 system.

ISDN uses TDM to share the same frequency channel by dividing the signal into different timeslots. ISDN

provides two types of channels:

• B channel—A user channel that transmits user information such as voice and data at 64 kbps.

• D channel—A control channel that transmits the public channel signaling. These signals control

calls on B channels of the same interface. The rate of a D channel is 16 kbps (BRI) or 64 kbps (PRI).

ISDN protocol stack

ISDN provides a connection for voice and data transmission upon requirements. ISDN may belong to the

physical layer from the point of view of the OSI architecture. However, it implements two protocol stacks

below Layer 4:

• B channel protocol stack—Used over the B channel for transmitting voice and data. It supports

protocols such as PPP at Layer 2 of the B channel.

• D channel protocol stack—Used over the ISDN D channel for controlling calls on the B-channel. It

supports protocols of the D channel.