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B & D channels

In this section you will learn

• What the B and D channels do

• How the B and D channels share the line

• How the structures of BRI and PRI compare

• What Bearer Capabilities are and on which channel they reside

• How to vary the the amount of bandwidth you have available to you

What does a B channel do?

The B channel carries ISDN Bearer Services across the network and so carries the

content of call (the voice, fax or data) between users.

The B channel is a neutral conduit for bits and carries data at 64 000 bits per second (56

000 bits per second in some North American networks).

The ISDN does not need to know what the bits represent. The job of the network is to

accept a stream of bits supplied by one user at one end of the B channel and to deliver

them to the other user at the opposite end of the channel.

Within an interface, the B channels are numbered. In a Basic Rate Interface they are

numbered 1 & 2; in a Primary Rate Interface, they are numbered 1 to 30 (or 23 in North

America). When two users are connected, there is no relationship between the channel

numbers used at each end. You might have one user's B channel number 17 connected

with the other user's B channel number 2. The ISDN is responsible for managing this

relationship.

Notice that channel number 17 would only be possible on a PRI, while channel number 2

is possible on both a BRI and a PRI. ISDN does not restrict the interconnection of B

channels between the two kinds of interface.

What does the D channel do?

The D channel carries the ISDN Network Services between the user and the network. It

maintains the user's relationship with the network.

This includes:

• the requests and responses used when you make or receive a call

• call progress messages

• messages informing you that the called party has closed the call

• error messages telling you why a call has not been established for you

The D channel operates at 16 000 bits per second in a BRI and at 64 000 bits per second

in a PRI.

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Eicon Networks ISDN NT1 Specifiche Pagina 5