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5.1.3 Overlap sending

If overlap sending is used, the SETUP message contains either:

a) no called number information; or

b) incomplete called number information;or

c) called number information which the network cannot determine to be complete.

On receipt of such a SETUP message, the network starts timer T302 (the value of timer T302 is specified in 9.1), sends a SETUP ACKNOWLEDGE message to the user, and enters the overlap sending state.In case a), the network will return dial tone, if required by the tone option.In this case it may include progress indicator No. 8, in-band information or appropriate pattern is now available in the SETUP ACKNOWLEDGE message.

Note 1 - Some networks which systematically provide the conventional telephone dial tone will not generate the progress indicator when providing the dial tone.

When the SETUP ACKNOWLEDGE message is received, the user enters the overlap sending state and optionally starts timerT304 (the value of timer T304 is specified in 9.2).

After receiving the SETUP ACKNOWLEDGE message, the user sends the remainder of the call information (if any) in one or more INFORMATION messages.

The called party number information may be provided by the user as follows:

a) in the called party number information element; or

b) in the keypad facility information element, exclusively.

The called party number must be sent in unique way.

Note 2 - The support of a) is mandatory in all networks. Whether the support of b) is mandatory or optional requires further study.

Note 3 - Besides the possible called party number [conveyed by method a) or b) as described above], the INFORMATION messages may contain additional call information (i.e. for supplementary services). The interpretation of the contents of keypad facility information elements is network-specific, and in accordance with the dialling plan provided to that user. It should be noted that the user shall transfer all the additional call information (contained within the keypad facility information element) before the network determines that the called party number (contained within the called party number information element or the keypad facility information element) is complete, and terminates the overlap sending procedure using the CALL PROCEEDING message as recommended in 5.1.5.2.

If, for symmetry purposes, the user employs timer T304, the user restarts timer T304 when each INFORMATION message is sent.

The call information in the message which completes the information sending may contain a sending complete indication, (e.g. the # character or, as a network option, the sending complete information element) appropriate to the dialling plan being used. The network shall restart timer T302 on the receipt of every INFORMATION message not containing a sending complete indication.