When the SETUP message is delivered by a point-to-point data link, negotiation for the selection of a B-channel will be permitted between the network and the user. Only B-channels controlled by the same D-channel will be the subject of the selection procedure. The selection procedure is as follows:
a) In the SETUP message, the network will indicate one of the following:
1) 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
2) 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
3) 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]; or
4) no B-channel available [i.e. the information channel field (Bits 2 and 1 of octet 3) of the Channel identification information element set to "00"].
Note - Not all networks will support the no B-channel available condition.
b) In cases 1) and 2), if the indicated channel is accepTable and available, the user selects it for the call.
In case 2), if the user cannot grant the indicated channel, it selects any other available B-channel associated with the D-channel, and identifies that channel in the Channel identification information element as "channel is indicated, no accepTable alternative" in the first message sent in response to the SETUP message.
In case 3), the user selects any available B-channel associated with the D-channel, and identifies that channel in the first message sent in response to the SETUP message.
If in case 1) the B-channel indicated in the first response message is not the channel offered by the network, or in cases 2) and 3) the B-channel indicated in the first response message is unaccepTable to the network, it will clear the call by sending a RELEASE message with cause No. 6, channel unacceptable.
In case 4), the user rejects the call by sending RELEASE COMPLETE message with cause No. 34 no circuit/channel available unless it is able to proceed with the call. Unless the procedures of the Call Waiting supplementary service (see Recommendation Q.953 [84]) are followed, the user wishing to re.use aB-channel it has already allocated to another call (e.g. by multiplexing packet calls) shall send the appropriate message containing the channel identification information element, coded as channel is indicated, no alternative acceptable.
c) If no Channel identification information element is present in the first response message, the B-channel indicated in the SETUP message will be assumed.
d) When a B-channel has been selected by the user, that channel may be connected by the user.
e) In case 1) if the indicated B-channel is not available, or in cases 2), 3), and 4) if no B-channel is available and the user cannot proceed with the offered call, the user returns a RELEASECOMPLETE message with cause No. 44 requested circuit/channel not available or No. 34 no circuit/channel available, respectively, and returns to the Null state.
See 5.2.4 and 5.2.5 for the appropriate first response to the SETUP message.