(This Annex forms an integral part of this Recommendation)
The procedure for bearer service change may not be provided on all networks. On those networks that support it,a user may use this procedure after making a suiTable subscription-time arrangement.
When a bearer service requested in an originator's SETUP message cannot be provided by the network, the network would reject the call or, under some circumstances, the network may change the bearer service and provide bearer service change notification. These procedures are currently applicable only to a change from 64 kbit/s unrestricted to 64 kbit/s restricted, and from 64 kbit/s restricted to 64 kbit/s restricted with rate adaption.
Note - During an interim period some networks may only support restricted 64 kbit/s digital information transfer capability,i.e. information transfer capability solely restricted by the requirement that the all-zero octet is not allowed. For interworking the values given in Appendix I/I.340 should apply. The interworking functions have to be provided in the network restricted capability. The ISDN with 64 kbit/s transfer capabilities will not be offered by this interworking, other than by conveying the appropriate signalling message to or from the ISDN terminal.
Up to two Bearer capability information elements may be present in the SETUP message from the originating user,corresponding to the allowed bearer service modifications given above. The bearer capability information element shall be immediately preceded by the Repeat indicator information element with the meaning field specifying Prioritized list for selecting one possibility. Hence, the order of Bearer capability information elements would indicate order of bearer service preference.
If the SETUP message contains Bearer capability information elements not agreeing with any of the permissible ordered combinations listed above, the network will reject the call attempt.
After sending a CALL PROCEEDING message,when the originating network or terminating premises equipment determines that the preferred bearer service cannot be provided, it sends a NOTIFY message toward the call originator. The NOTIFY message contains a Notification indicator information element with a coding which indicates to the originating party the change in bearer service and also contains a Bearer capability information element specifying the attributes of the new bearer service.
Receipt of the NOTIFY message is not acknowledged. The call originator may allow the call to continue or may initiate call clearing in accordance with clause 5.