Eicon Networks ISDN NT1 Specifikace stáhnout pdf (Strana 15)

Bearer Capabilities

In this section you will learn:

• Bearer Capabilities in depth

• The significance of Bearer Capabilities in diagnosing problems

• A practical example of using Bearer Capabilities to resolve a problem

Bearer Capabilities - What are they?

Although the ISDN doesn't always need to know what protocol is in use in the B channel,

there are circumstances where this information is useful.

For instance, if you make a telephone call over ISDN, it is useful to tell the network that

this is a voice call so that it can connect your call with an analogue telephone in the PSTN.

If the ISDN network thinks that your B channel contains a protocol unknown to it, then it

can only connect your call directly to another ISDN line.

You have the opportunity to tell the ISDN about the protocol that you're using in the B

channel when you request the call. This information is added to the signalling information

sent on the D channel when the call is requested. This call information is often called

Bearer Capabilities.

The receiver of the call can also see the bearer capabilities when a call is offered to him by

the network.

Generally, there is no possibility to negotiate bearer capabilities. You must decide what

bearer capabilities you want to use before placing the call. Bearer capabilities are fixed for

the lifetime of the call and cannot be dynamically changed. If you attempt to place a call

without specifying the bearer capabilities, the request will be rejected by the ISDN.

Using Bearer Capabilities

Understanding Bearer Capabilities and their implications becomes significant when

diagnosing problems, particularly when using diagnostic traces.

An example of the problems that can occur involves fax calls not being received correctly

because there are two possible types of Bearer Capability for a fax call.

• 3.1kHz analogue call

This often happens where the call originated inside the PSTN. The ISDN has no

way of knowing exactly what type of equipment (telephone, fax, modem, etc.)

placed the call, so it uses this "catch-all" bearer capability.

• Fax Group 3 call

This is often used where the originates with a fax machine that is directly connected

to an ISDN.

For the call to be successful, the chosen Bearer Capabilities must be supported by all

equipment throughout the path taken by this call across the network. If any single piece of

equipment does not support the requested Bearer Capabilities, the call will not be

connected. In general, there is no support for negotiating Bearer Capabilities.

We therefore need to ask the questions:

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Eicon Networks ISDN NT1 Specifikace Strana 15