Eicon Networks ISDN NT1 Spécifications télécharger pdf (Page 35)

Frequently, delivery of the calling (origin) number is an extra service from your service

provider that you have to pay for. This service is often called CLIP (Caller Line

Identification Presentation) or sometimes just CLI. See the section Managing Inbound

Calls for more information on this subject.

At the destination, an ISDN device capable of receiving the incoming call responds with an

ALERT message. An ALERT is sent to the originator of the call. This is just like the 'the

remote phone is ringing' tone in a conventional telephone call.

In the case of an ISDN telephone, an ALERT means that the device has started to ring to

alert the user to the incoming call.

When the device answers the call, it sends a CONN (connect) message to the network. A

CONN is then sent to the originator of the call who then acknowledges it with CONN_ACK.

At the end of this process, a B channel has been established between the two users and

data (which may, of course, represent voice or fax; or any other information) can flow

freely between them.

The picture below summarises the exchange of messages during call set up.

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Eicon Networks ISDN NT1 Spécifications Page 35