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Device Addressing

In this section you will learn:

• Why a device needs a unique address

• What the address is used for

• Forms of address management

• The difference in address management between PRI and BRI lines

• Practical implications of mixing address types

Why does ISDN need addresses?

A Basic Rate Interface (BRI) line may have as many as eight devices attached.

All these devices may use the common D channel at the same time. For example, one

device may be trying to make a call at the same time as a call is being received and

answered by another device.

The ISDN network therefore needs to be able to conduct 'conversations' with each device

independently.

To do this successfully, each device requires its own address, called a Terminal Endpoint

Identifier or TEI. This address is not the telephone number

. Telephone numbers belong

to the line, not to any device attached to the line.

Fixed and dynamic addressing

A PRI line can have only one device connected. This device will have a single, predefined

address which, in practice, is always zero. When a ISPBX is attached to the PRI line, it will

perform its own call management and call routing functions for the devices attached to it.

A BRI line can have up to eight devices connected to it. Device addresses may be pre-

configured or dynamically allocated.

• In dynamic addressing, each device has to request an address from the network

before it can perform any signalling activity.

• With fixed addressing, the address of each device has to be configured to match

the addresses pre-configured for this line.

ISDN service suppliers do not allow both fixed and dynamic addressing on the same line.

In practice, an ISDN line where the device addresses are fixed will only permit a single

device. The is no convention to describe this type of line configuration, but ISDN service

providers sometimes give this a name that reflects this type of configuration.

Dynamic address assignment

Dynamic address assignment means that the device must ask the network for an address

before it can perform any signalling.

The Q.921 standard (also called LAPD) that defines the link level protocol used on the D-

channel incorporates a mechanism by which the device can request an address.

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Eicon Networks ISDN NT1 Spécifications Page 30