Eicon Networks ISDN NT1 Spécifications télécharger pdf (Page 38)

Connection collision

What happens if more than one device attempts to answer the call at the same moment?

The CONN messages collide in the D channel.

If you're wondering why we're not worried about the ALERT messages colliding (which

could happen), this is because these messages are not critical to what happens next.

Don't forget that there is only one D channel and all devices that want to transmit have to

contend for it. This is where the NT1 helps. It is able to signal to one or other device (or to

both) that a collision has occurred.

It is not predictable which device will be informed of the collision, or whether they both are.

The device which is not informed of the collision will not be aware that this happened and

continues to transmit on the D channel successfully. The other device stops transmitting. It

then waits for the D channel to become free and tries again. By this time, it's too late

because the network has awarded the call to the device which won the collision.

Summary

In this section you learned:

• All calls are managed by a set of procedural messages between the network and

the user

• These are all carried via the D channels at either end

• The end-to-end connection for the call itself is set up on the B Channel

• The most important messages are SETUP and DISC.

• Disconnections are accompanied by a cause code which will reveal why the call

terminated

• Procedures also exist to manage multiple devices attempting to answer incoming

calls

• Devices sharing a BRI do not cooperate with each other, they communicate with the

network

Q/A

1. What channel do the call management messages use?

All call management messages are carried on the D Channel.

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Eicon Networks ISDN NT1 Spécifications Page 38