This clause specifies the functional signalling procedures for the control of supplementary services at the radio interface. The Functional protocol utilises functions and services provided by TS 24.008 basic call control procedures and the functions of the data link layer as defined in TS 44.005. In UMTS only, integrity protected signalling (see TS 24.008, subclause 'Integrity Protection of Signalling Messages,' and in general, see TS 33.102) is mandatory. In UMTS only, all protocols shall use integrity protected signalling. Integrity protection of all SS signalling messages is the responsibility of lower layers. It is the network which activates integrity protection. This is done using the security mode control procedure (TS 25.331). The defined procedures specify the basic methodology for the control (e.g. registration, erasure, invocation, etc.) of supplementary services. The first category, called the Separate Message Category utilises separate message types to indicate a desired function. The hold and retrieve families of messages are identified for this category. The second category called the Common Information Element Category utilises the Facility information element to transport the protocol defined in TS 24.080. The use of the Facility information element is common to many services, and its contents indicates what type of procedure is being requested. This category can be signalled both in the mobile to network and the network to mobile directions. The control of supplementary services includes the following cases:

1. the request of supplementary service procedures during the establishment of a call;
2. the request of supplementary service procedures during the clearing of a call;
3. the request of call related supplementary service procedures during the active state of a call;
4. the request of supplementary service procedures independent from an active call;
5. the request of multiple, different supplementary service procedures within a single message;
6. the request of supplementary service procedures related to different calls.

The correlation of a call related supplementary service operation and the call which it modifies is provided by use of the transaction identifier (cases a, b, c, e and f). The correlation of supplementary service operations and their responses, is provided by the combination of the transaction identifier of the messages containing the Facility information element and the Invoke identifier present within the Facility information element itself (cases a, b, c, d, e and f). The identification of different supplementary service operations within one single message is provided by the Invoke identifier present within the Facility information element itself (case e). The identification of supplementary service related operations to different calls is provided by using different messages with the corresponding transaction identifier of the appropriate call (case f), i.e. different transaction identifier values are used to identify each call individually.

The messages defined in this clause are specified as separate functional messages for the request, acknowledgement and rejection of specific procedures. These procedures can only be performed during the active phase of a call. The functions of these messages are not to be duplicated or overlapped by the ones of the Common Information Element Category. The following separate messages are defined:

For detailed description of the Hold and Retrieve functions see TS 24.083.

The Common Information Element Category uses operations defined in TS 24.080 for supplementary services signalling. Procedures are initiated by sending an operation including an invoke component. The invoke component may yield a Return Error, Return Result or Reject component (also included in an operation) depending on the outcome of the procedure. The operation state machines, and procedures for management of Invoke IDs specified in CCITT Recommendation Q.774 White Book are used. A REGISTER message, a FACILITY message or certain existing TS 24.008 Call Control message is used to carry the Facility information element which includes these operations. These operations request, acknowledge or reject the desired supplementary service procedure.

If (according to the rules specified in TS 29.002) a supplementary service operation is to be rejected the operation will be denied, and provided the transaction is still in progress, an appropriate reject component will be returned in a Facility Information Element. The handling of the transaction depends on whether the operation is call related or call independent.