If the calling user and the network operator allow high layer compatibility selection, then the destination network shall indicate this to the destination user by means of repeated High layer compatibility information elements within the SETUP message sent to indicate the presence of a call request.
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 the user wishes to accept the call without having fallback occur, the user shall include in the CONNECT message sent to the network the High layer compatibility information element of the requested high layer compatibility.
If fallback allowed is indicated in the SETUP message as described above, and the user wishes to accept the call with having fallback occur to the lowest priority alternative high layer compatibility,the user may, but need not, include in the CONNECT message sent to the network the High layer compatibility information element of the alternative high layer compatibility.
If no High layer compatibility information element is indicated by the called user, the network shall assume that the lowest priority high layer compatibility is selected.
If fallback allowed is indicated in the call request, and no interworking has been encountered (i.e. a progress description No.1, call is not end-to-end ISDN, further call progress information may be available in-band or progress description No. 2, destination address is non-ISDN has not been sent), then the destination network shall indicate the resultant high layer compatibility to the originating network at the time the bearer is established, even if no High layer compatibility information element is received from the destination user.