In symmetric arrangements, call collision scan occur when both sides simultaneously transfer a SETUP message indicating the same channel. In the absence of administrative procedures for assignment of channels to each side of the interface, the following procedure is employed.
First, one side of the interface will be designated the network and the other side of the interface will be designated the user. Second, for the three possible scenarios where the same channel is indicated by combinations of preferred and exclusive from the user and network sides, the following procedure is used:
a) Network preferred, user preferred
The network preferred channel is awarded and an alternate channel is indicated in the first response to the user SETUP message.
b) Network exclusive, user exclusive
The network exclusive channel is awarded and the user SETUP message is cleared with a RELEASE COMPLETE message with cause No. 34, no circuit/channel available.
c) Network preferred, user exclusive,or network exclusive, user preferred
The side of the interface with an exclusive indicator in a SETUP message is awarded the channel and an alternate channel is indicated in the first response to The side using a preferred indicator in the SETUP message.
Channel identification is allowed in both directions for ALERTING and CONNECT.