If the user selects an already established channel for the outgoing X.25 virtual call, then the procedures described in 6.3 apply. If the selected channel is not established to the AU/PH, then the procedures for activating a channel described in the following subsections are to be used before establishing the virtual call using the procedures of 6.3.
For outgoing X.25 data calls, the user first must decide whether circuit-switched (Case A) or packet-switched services (Case B) are desired from the network. For outgoing circuit calls, the user follows the procedures of 6.1.1. For outgoing packet calls, the user decides whether the B-channel or D-channel is to be used for the packet call. If the user decides to use the B-channel, then the procedures described in 6.1.2.1 are used. If the user decides to use the D-channel,then the procedures described in 6.1.2.2 are used.
Note - Some networks may not support every type of access. In the case of B-channel access, the network will clear a request for unsupported services by sending a RELEASE COMPLETE message with cause No.65, bearer capability not implemented. In the case of a request for D-channel access (an SABME with SAPI=16), on a network port which does not support the service, no response is required of the network.