The channels selected for the multirate call shall be on one interface and shall be indicated in the SETUP message. The procedures in 5.1.2 and 5.2.3.1 shall be followed to complete the channel selection.
The Channel identification information element is coded as per 4.5.13.
The number of channels identified shall provide the information transfer rate identified in the Bearer capability information element. If the information transfer rate implied by the channel(s)or interface indicated in the Channel identification information element does not match the information transfer rate in the Bearer capability information element the procedures in 5.8.6.2 shall apply.
Channel selection conflict occurs when the channels selected for an incoming and outgoing call do not constitute two disjoint sets of time slots. When channel selection conflict occurs, The procedures in 5.7 shall apply.
Some networks may offer on access:
1) contiguous channel assignment(channels must be adjacent within a single interface); and/or
Note 1 - On a 2048 kbit/s interface(containing a D-channel) channels 15 and 17 shall be considered contiguous.
2) non-contiguous channel assignment(channels may be either adjacent or non-adjacent within a single interface).
Some networks may require that 384 kbit/sand/or 1536 kbit/s (in a 2048 kbit/s interface) occupy specified contiguous time slots (see Annex A/I.431).
If the entire interface of a primary rate interface is used (i.e. 24 B-channels on a 1544 kbit/s interface or 30 B-channels on a 2048 kbit/s interface) octets 3.2 and 3.3 of the Channel identification information element shall not be included.
If the entire interface of a basic access interface is used (i.e. 2 B-channels) octets 3.2 and 3.3 of the Channel identification information element shall not be included and the "information channel selection" shall be coded "11", any channel.
In cases a) and b) of 5.1.2, if all the indicated B-channels are available, the network shall select them for the call.
In case b), if the network cannot grant any referred B-channel, it shall select any other available B-channels associated with the D-channel and on the same access, to replace the unavailable preferred-channel, or select all the B-channels on another interface controlled by the D-channel.
Note 2 - Whether only the B-channels that cannot be provided should be changed, or if all the B-channels can then be changed is for further study.
In case c), the network shall select any available suiTable B-channels.
In case a), if any specified B-channel is not available, and in cases b) and c) if insufficient B-channels are available,the network shall send a RELEASE COMPLETE message with cause No. 44, requested circuit/channel not available or No. 34, no circuit/channel available, respectively, as described in 5.3.
The following recommendations are made on the use of cause values.
1) When the calling user or called user is not an authorized subscriber of the multirate circuit-mode bearer capability, cause No. 57 bearer capability not authorized shall be returned to the calling user.
2) When a network (public or private)cannot support the specified transfer rate or bearer capability, cause No. 65 bearer capability not implemented shall be returned to the calling user.
3) When there are insufficient channels on a single interface to support the information transfer rate requested, cause No. 34 no circuit/channel available or cause No. 17 user busy shall be returned to the calling user (see Recommendation Q.850 which is also reproduced in Appendix I).