A user initiates call establishment by transferring a SETUP message across the user-network interface. Following the transmission of the SETUP message, the call shall be considered by the user to be in the call initiated state. The message shall always contain a call reference, selected according to the procedures given in 4.3. In selecting acall reference, the dummy call reference value shall not be used in association with the basic call. The bearer capability information element is mandatory in the SETUP message, even in the case of overlap sending.
If the user knows all appropriate channels controlled by the D-channel are in use, it shall not transfer a SETUP message across the user-network interface. If the user does not monitor the status of channels in use, it may send a SETUP during an all channels busy condition. In this case the network returns a RELEASE COMPLETE message with cause No. 34,no circuit/channel available.
Furthermore the SETUP message may also contain all or part of the call information (i.e. address and facility requests)necessary for call establishment depending on whether en-bloc or overlap procedures are being used respectively (see 5.1.3).
If en-bloc sending is used, the SETUP message shall contain all the information required by the network to process the call, and, in particular, the called party address information if present, is contained as follows:
a) in the called party number information element possibly completed by the called party subaddress information element; or
b) the keypad facility information element which may also be used to convey other call information.
Note - The support of a) is mandatory in all networks. Whether the support of b) is mandatory or optional requires further study.
If en-bloc sending is used, the SETUP message may contain the sending complete indication (i.e. either the Sending complete information element or the "#" character within the Called party number information element). It is mandatory for the network to recognize at least one of the sending complete indications. It is for further study which sending complete indication is preferred.
For overlap sending, see 5.1.3.
Called party subaddress information, if present, shall be given in the Called party subaddress information element and,in the case of overlap sending, shall be sent only in the SETUP message.