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Notice that there is one timeslot which has no channel assigned to it. This is reserved for

your network supplier to use for diagnostic purposes. The D channel appears between B

channel 15 and B channel 16.

Fractional PRI

ISDN service suppliers have the opportunity to supply interfaces where not all the

channels are active. In most countries, when you take a Primary Rate Interface, you are

charged a rental per channel. If you don't need all the channels that are available, you can

ask to have some of these channels deactivated. This is known as fractional Primary

Rate. The number of channels that you may request will vary according to the marketing

policy of your service supplier.

What happens if you try to use more channels than your subscription permits? In ISDN,

the network is the arbitrator of everything. When you want to place a call through an ISDN

network, you send a request in your D channel to the network. The network will either

attempt to satisfy your request or it will refuse it.

This user has only one B channel active in his subscription. He will be successful in

making the first call. If he requests a second call while the first is still active, the network

will reject the request.

Fractional BRI also exists, but is less common.

Protocols on B and D channels

The B channel is a neutral conduit for bits; so the meaning of the bits flowing in the

channel must be understood by the users at each end.

If the two users are not using the same "language", they won't understand each other and

so there is no meaningful communication. Imagine making a telephone call where the

person who answers doesn't speak the same language as you. You can't even apologise

for disturbing them!

The same is equally true of D channels. There are various "dialects" of the signalling

protocol. You must be using the same dialect as your network in order to successfully

communicate with it.

B and D channel protocols

You must use a protocol to establish meaningful communication across a channel. It is

important that both parties to the communication use the same protocol.

This is particularly important for the D channel. Your signalling requests and responses

must be understandable by the network. Even if your ISDN device and ISDN line are both

functioning correctly, you might not be able to make successful calls if you're using a D

channel protocol that isn't the same as the network's.

ISDN requires that you use a protocol defined by the ITU-T called Q.931 for signalling in

the D channel. However, there are several signalling protocols based on Q.931 in use

round the world. For instance, NI-1 and 5ESS are used in North America while much of

the rest of the world is now using EuroISDN (also called ETSI or DSS1).

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Eicon Networks ISDN NT1 Specifiche Pagina 7