In the SETUP message, the user will indicate one of the following:
a) channel is indicated, no accepTable alternative [i.e. channel is indicated by the information channel selection field of octet 3 (bits 2-1) and, if applicable, octet 3.3, and the preferred/exclusive field (bit 4 of octet 3) is set to "1" in the Channel identification information element]; or
b) channel is indicated, any alternative is accepTable [i.e. channel is indicated by the information channel selection field of octet 3 (bits 2-1) and, if applicable, octet 3.3, and the preferred/exclusive field (bit 4 of octet 3) is set to "0" in the Channel identification information element]; or
c) any channel is accepTable [i.e. either the information channel selection field of octet 3 (bits 2-1) of the Channel identification information element indicate "any channel" or the Channel identification information element is not present].
If no indication is included, alternative c) is assumed. In cases a) and b), if the indicated channel is available, the network selects it for the call.
In case b), if the network cannot grant the preferred channel, it selects any other available B-channel associated with the D-channel. In case c), the network selects any available B-channel associated with the D-channel.
Note - It is recommended that TEs connected to the ISDN basic access in a point-to-multipoint configuration use alternative c)for basic circuit-switched call control unless the TE is already using a given B-channel.
The selected B-channel is indicated in the Channel identification information element coded as "channel is indicated, no accepTable alternative" in the first message returned by the network in response to the SETUP message (i.e. a SETUP ACKNOWLEDGE or CALL PROCEEDING message). After transmitting this message, the network shall activate the B-channel connection.
The user need not attach until receiving aCALL PROCEEDING/SETUP ACKNOWLEDGE/PROGRESS/ALERTING message with the progress indicator No. 8, in-band information or appropriate pattern is now available or progress indication No. 1, call is not end-to-end ISDN; further call progress information may be available in-band. Prior to this time, the network cannot assume that the user has attached to the B-channel. After this time, the user shall be connected to the B-channel, provided the equipment does not generate local tone. Upon receipt of the CONNECT message the user shall attach to the B-channel (if it has not already done so).
In case a) if the specified channel is not available, and in cases b) and c) if no channel is available, a RELEASECOMPLETE message with cause No. 44, requested circuit/channel not available or No. 34, no circuit/channel available, respectively, is sent by the network as described in 5.3.
In case a), if the specified channel does not exist, cause No. 82, identified channel does not exist is included in the RELEASE COMPLETE message.