Eicon Networks ISDN NT1 Spécifications télécharger pdf (Page 42)

In this case, the destination fax machine will not answer the call as it is configured to

accept only analogue calls.

In practice, a device can only apply one filter to each parameter of the incoming SETUP. A

filter such as "CallType=G3Fax OR CallType=Analogue" is not normally possible.

For these reasons, this solution cannot be recommended. It would work very reliably if all

networks were ISDN; and all Fax machines placed G3 Fax type calls and they were

configured to answer such calls.

Fortunately, there is alternative method for directing incoming calls to the correct device -

Multiple Subscriber Numbering, or MSN.

Using MSN for incoming calls

In this scenario we need a line with at least two subscriber numbers. This example has

three numbers; the third number may be used by the PC to identify itself for outgoing calls.

The number belongs to the line, but is used by the device for filtering purposes.

This example also demonstrates a technique to stop a device from answering incoming

calls. In this example, the PC is not running any programs that will answer incoming calls.

Let us assume now that the PC is

running a program that answers incoming calls. We

have no means of stopping the program, but we don't want it to answer any incoming calls.

If we configure the PC to answer calls to 9999, the PC will never answer a call since this

number will never be offered.

In practise, if you don't configure a Called Party Number filter, a device will not apply any

filter on this information and answer the call provided it passes any other configured filters.

Summary

In this section you learned:

• SETUP messages can have a filter applied to them

• The Call Type can be used to filter calls

• The restriction to only one filter makes using Call Types impractical

• The most useful call filter is Called Party Number

• To make use of Called Party Number requires that a single ISDN line be assigned

several different numbers

• This system is known as Multiple Subscriber Numbering or MSN

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Eicon Networks ISDN NT1 Spécifications Page 42