For some bearer services or teleservices,the originating user can indicate that
- fallback to an alternative bearer capability is allowed; or
- fallback to an alternative bearer capability is not allowed.
If the calling user allows fallback to occur to an alternative bearer capability, then the user shall indicate this to the network by means of repeated Bearer capability information elements within the SETUP message sent to indicate the presence of a call request. This procedure allows a maximum of two Bearer capability information elements in the SETUP message.
The order of the information elements shall indicate the priority of the bearer capabilities. Bearer capability information elements shall be in ascending order of priority, i.e. a subsequent Bearer capability information element shall indicate a bearer capability with higher priority.
If fallback allowed is indicated in the SETUP message as described above, and fallback occurs at the destination user,or fallback does not occur, the originating network shall include in the CONNECT message sent to the calling user the Bearer capability information element of the resultant bearer service or teleservice.
If fallback allowed is indicated in the SETUP message as described above, and fallback occurs within the ISDN (e.g. bearer capability selection is not supported or the selected route does not support the preferred bearer capability), the originating network shall include in a PROGRESS message or other appropriate call control message sent to the calling user a Progress indicator information element with the progress description No. 5, interworking has occurred and has resulted in a telecommunication service change. The originating network shall include the Bearer capability information element of the resultant bearer service or teleservice.
When a PROGRESS message is sent containing a Progress indicator information element with progress description No. 5, interworking has occurred and has resulted in a telecommunication service change, neither the user nor the network shall stop timers described in 5.1.6 as a result of this action.