Eicon Networks ISDN NT1 Specifiche Scaricare il pdf (Pagina 6)

B & D channel characteristics

An ISDN channel has two and only two ends. B channels terminate at a user. A B channel

can therefore connect two and only two users. A B channel cannot be Y-shaped. B

channels are therefore described as end-to-end.

In the case of the D channel one end is with the user. The other end is in the network.

A D channel is not end-to-end. You cannot normally, therefore, use a D channel to carry

data between two users.

Notice how the D channels (the red lines) do not pass through the network. Notice also

how each user has only one D channel and it is not connected in any way with the D

channel of the other user.

The B channel (the blue line) passes directly across the network.

How B & D channels share the line: Basic rate

The two B channels and the one D channel that make up a Basic Rate ISDN line are

assembled together within the interface using a technique called Time Division

Multiplexing.

It works like this. Imagine a clock with the second hand spinning. For a portion of the arc

described by the second hand, the interface is carrying data for the D channel; for another

portion of the arc, the interface is carrying data for a B channel.

How B & D channels share the line: Primary rate

A EuroISDN Primary Rate Interface contains 30 B Channels and one D channel. In

North America and some other countries a PRI contains 23 B channels and one D

channel.

A B channel operates at 64 000 bits per second. (56 000 bits per second in some parts

of North America and other countries.)

A D channel operates at 64 000 bits per second in a Primary Rate Interface. Note that

this is different to the BRI where the D channel runs at 16 000 bits per second.

The animation below shows the multiplexing in a European Primary Rate Interface.

The Primary Rate Interface spends equal amounts of time transmitting data for each of the

B channels and

for the D channel because they all operate at the same speed.

1 2 3 4 5 6 7 8 9 10 11 ... 55 56

Nessun commento

Eicon Networks ISDN NT1 Specifiche Pagina 6