Procedures for call establishment are as described in 5.1 and 5.2 with the following modifications.
On call request, the calling user sends a SETUP message identifying, within the Bearer capability and Channel identification information elements, a temporary signalling connection to be established on SAPI = 0. The SETUP message is encoded to indicate:
i) Bearer capability information element
- Unrestricted digital information in the information transfer capability field;
- Packet mode in the transfer mode field;
- User information layer 2 protocol is Recommendation Q.921 and user information layer 3 protocol is this Recommendation in the layer and protocol identification field.
ii) Channel identification information element
- Exclusive in the preferred/exclusive field;
- D-channel in the D-channel indicator field;
- No channel in the channel selection field.
If the network determines that the requested temporary signalling connection service is not authorized or is not available, the network shall initiate call clearing in accordance with 5.3.2 a) or 5.3.2 c) with one of the following causes:
a) No. 57 - Bearer capability not authorized;
b) No. 58 - Bearer capability not presently available;
c) No. 63 - Service or option not available, unspecified; or
d) No. 65 - Bearer service not implemented.
The called user accepts the temporary signalling connection request by sending a CONNECT message towards the calling user. After the called user has received a CONNECT ACKNOWLEDGE message, it may begin sending USER INFORMATION messages. Once the calling user receives a CONNECT message, it can begin sending USER INFORMATION messages.