Eicon Networks ISDN NT1 Spezifikationen PDF herunterladen (Seite 34)

It is mandatory to specify the Bearer Capabilities in a SETUP.

Although an initial SETUP would normally specify the destination number for the call, this

is not mandatory. A SETUP is quite valid even if there is no called party number specified.

This is how the Eicon DIVA LineCheck utility works. Where the destination phone number

is available before the call is placed, for example, with Dial-Up networking, it is normal that

the SETUP would contain this information.

In the case of a telephone, however, none of the digits of the destination number are

known at the moment that the receiver is lifted. In this case the telephone sends a SETUP

with any called party number information. The network may take this opportunity to attach

a dial tone generator to the B channel. Alternatively, the telephone itself may generate a

dial tone. This makes the ISDN telephone behave like a conventional analogue telephone.

As each number button on the telephone is pressed, an additional message called INFO is

sent, which will report the number keyed. It is then the responsibility of the network to

accumulate dialled digits until it has enough information to make a routing decision for the

call. Naturally there has to be a limit to the length of time that the network will wait for the

next digit. If this time limit is exceeded, the network will disconnect the (potential) call. This

mode of operation is known as overlapped sending.

Where the complete called party number is known before sending the SETUP, the SETUP

will also contain an indicator called Sending Complete. This tells the network that it

should attempt the connection using only the digits supplied because no further digits will

be sent by the TE. If the number is incomplete, this should be reported as an unknown

destination error.

Is the SETUP message sent to User B the same one that User A sent to the

network?

Clue.

Is it the same D channel?

Is the SETUP message sent to User B the same one that User A sent to the network?

No, the SETUP message does not pass through the network. The network generates a

new SETUP message to send to the User B. It may have copied some of the

information that USER A provided.

How many D channels are shown in the diagram?

The answer is 2. The D channel is not end-to-end. It only connects a user to the

network.

More about the incoming SETUP

The incoming SETUP that indicates that a call is available will contain the call type (Bearer

Capabilities). The called number (destination) and calling number (origin of call) are

optional and may not be supplied.

If the called (destination) number is not supplied in the SETUP message then the

implication is that the call can be answered by any device on the line.

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Eicon Networks ISDN NT1 Spezifikationen Seite 34