The network will indicate the arrival of acall at the user-network interface by transferring a SETUP message across the interface. This message is sent if the network can select an idle B-channel. In some circumstances (e.g. provision of other bearer services see 6.1) the SETUP message may also be sent when no B-channel is idle. The number of calls presented in these circumstances may be limited.
In addition to the mandatory information elements, the SETUP message may include, as required, the information elements described in 3.1.14 (e.g. Display, Low layer compatibility).
If a multipoint terminal configuration exists at the user-network interface, this message shall be sent using a broadcast capability at the data link layer. In this case, the SETUP message should contain the appropriate part of the called party number as required and/or sub-address if provided. However, if the network has knowledge that a single-point configuration exists at the interface, a point-to-point data link shall be used to carry the SETUP message. The knowledge that a point-to-point configuration exists may be based on information entered at the time of configuration of the access. After sending the SETUP message, the network starts timer T303. If the SETUP message is sent via a broadcast data link,timer T312 shall also be started. (The values of timers T303 and T312 are specified in 9.1.) The network then enters the call present state.
Note 1 - Timer T312 is used to supervise the retention of the call reference when the SETUP message was transmitted by a broadcast data link. The value of timer T312 is such that if a network disconnect indication is received during the call establishment phase, it maximizes the probability that all responding users will be released prior to release of the call reference. Refer to 5.3.2 e) and 5.2.5.3 (Case 1) for procedures to be followed on expiry of timer T312.
If en-bloc receiving is used, the SETUP message shall contain all the information required by the called user to process the call. In this case, the SETUP message may contain the Sending complete information element.
Upon receipt of a SETUP message, the user will enter the Call present state.
Depending on the contents of the received message, either en-bloc receiving procedure (see 5.2.5.1) or overlap receiving procedure (see 5.2.4) follows. However, if the SETUP message includes the Sending complete information element, en.bloc receiving procedure shall follow. Therefore, those users who support overlap receiving procedure shall recognize the Sending complete information element.
Note 2 - Users supporting only the en-bloc receiving procedure need not recognize the Sending complete information element and may directly analyze the received SETUP message on the assumption that all the call information is contained in the message.
If no response to the SETUP message is received by the network before the first expiry of timer T303, the SETUP message will be retransmitted and timers T303 and T312 restarted.
Note 3 - In the case of overlap sending within the network, the appropriate part of the called party number as required(e.g. for supplementary services) may also be conveyed by means of INFORMATION messages to the called user on a point-to-point data link (see 5.2.4).