In some networks, the originating user can indicate that:
- fallback to an alternative high layer compatibility is allowed; or
- fallback to an alternative high layer compatibility is not allowed.
If the calling user allows fallback to occur to an alternative high layer compatibility, then the user shall indicate this to the network by means of repeated High layer compatibility information elements within the SETUP message sent to indicate the presence of a call request. This procedure allows a maximum of two High layer compatibility information elements in the SETUP message.
The order of the information elements shall be in ascending order of priority, i.e. a subsequent High layer compatibility information element shall indicate a high layer compatibility 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 High layer compatibility information element of the resultant high layer compatibility.
If fallback allowed is indicated in the SETUP message as described above, and fallback occurs within the ISDN, 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 High layer compatibility information element of the resultant high layer compatibility.
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.