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Managing inbound calls

In this section you will learn:

• How to manage more than one device on a single ISDN line

• How to ensure that the appropriate call types are directed to the correct type of

device.

• What call filters are and how to set them

Handling incoming calls

Consider the example in the diagram. An ISDN line has a telephone, a fax machine, and a

PC attached to it. The objective is to achieve the following:

• When someone rings the phone, only the phone rings.

• When someone rings the fax machine, only the fax machine answers.

• The PC never answers any calls.

How might this be achieved?

When a device receives a SETUP message, it can apply a filter to that message. This filter

examines the information that arrives in the SETUP message so that it may process or

ignore the message. Only messages which pass the filter are acted upon.

In practice, equipment is only able to apply one test to each parameter. The kind of

information which can be filtered in a SETUP message includes:

• Called party number (destination number)

• Calling party number (origin number)

• Call type (Bearer Capabilities)

ISDN calls can be divided into two main categories: analogue data and unstructured data.

A SETUP must include the call type (bearer capabilities) of the call. Without this

information, the SETUP is not valid and the network will instantly disconnect any such call

request, citing "mandatory information message missing" as the cause. When a SETUP is

sent by the network to the destination, the call type is also mandatory.

Handling incoming analogue calls

Within the analogue data call type, you can also distinguish between Group 3 fax and

Voice.

So, the following filters could be set:

• Telephone only answers Voice calls, as indicated by the Bearer Capabilities

• Fax only answers Fax Group 3 calls, as indicated by the Bearer Capabilities

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Eicon Networks ISDN NT1 Especificaciones Pagina 40