When all of the alternative bearers offered by the calling side use the same capacity on the ISDN interface channel, selection procedures in 5.1.2 and 5.2.3 shall apply.
When a preferred bearer requires a larger bearer than an allowed alternate fallback, e.g. 6 × 64 kbit/s to 64 kbit/s, channel selection procedures will follow 5.1.2 and 5.2.3 for the most preferred bearer, i.e. with the largest capacity requirement. If fallback occurs, the fallback bearer will use the lowest available time slot(s) corresponding to the channel selected for the preferred bearer. The remaining time slots will be available for further use after indication of fallback on the interface. For example, if time slots 7 through 12 were selected for a preferred 6 × 64 kbit/s bearer and later the call falls back to 64 kbit/s or speech, time slot 7 will be used for the call and time slots 8 through 12 will be available for further use after the CONNECT message has been received for the call. If early backward cut through is applicable to any of the bearer capabilities indicated, it will be provided on the lowest numbered time slot within the channel selected.
NOTE – These fallback procedures are not permitted if the user explicitly requests H0, H11 or H12.
Optional procedures in Annex N may be used to provide for flexible channel negotiation in conjunction with bearer fallback procedures.