Procedures for bearer capability selection are described in subclauses 5.11.1 and 5.11.2. Procedures for high layer compatibility selection are described in subclauses 5.12.1 and 5.12.2.
Each basic telecommunications service has the required bearer capability information element encodings, and if applicable the required High layer compatibility information element encodings, defined for that service (e.g. see Recommendation Q.939).
The destination user shall identify the requested teleservice by taking the presented Bearer capability and High layer compatibility information elements in all combinations. Where a permutation is not identified as a defined teleservice, that combination shall be ignored. Where a combination is identified as a defined teleservice, that combination may be considered for the purposes of service provision. If there are no valid combinations, the presented Bearer capability information elements shall be considered in order to identify a bearer service.
The destination user shall identify the requested bearer services from the values of the presented Bearer capability information elements.
NOTE – These requirements do not preclude the user performing compatibility checking on all compatibility information according to Annex B.
The originating network shall optionally perform subscription checks for all valid combinations in the order defined for the particular service. If the user has not subscribed to the prime service, the network shall check for the next following basic service and so on. If the user has not subscribed to any of the basic services, the call shall be released with cause No. 57, Bearer capability not authorized. If a fallback occurs as a result of these checks, the procedures of 5.11.1 and 5.12.1 shall apply.
The destination network shall optionally perform subscription checks for all valid combinations in the order defined for the particular service. If the user has not subscribed to the prime service, the network shall check for the next following basic service and so on. The subscription check may then result in one of the following four possibilities:
a) The user has subscribed to the prime service
The call shall be offered to the called user without any modification following the procedures in 5.11.2 or 5.11.3 "bearer capability selection", and 5.12.2 or 5.12.3 "high layer compatibility selection".
b) The user has not subscribed to the prime service, but to one of the valid combinations, different to the lowest basic service
The call shall be offered to the called user, with the highest subscribed basic service including the fallback possibility. The procedures in 5.11.2 or 5.11.3 "bearer capability selection", and 5.12.2 or 5.12.3 "high layer compatibility selection" will then apply. No indication of the fallback will be sent towards the calling user before a bearer has been established, unless the called user indicates fallback prior to the bearer establishment.
c) The user has subscribed to the lowest basic service among the valid combinations
The call shall be offered to the called user containing the subscribed basic service and an indication of fallback shall be sent towards the calling user in the next message to be sent.
d) The user has not subscribed to any service
The call shall be cleared.