Section: 5.1 Overview 5.1.1 General
Content: This subclause describes the call control (CC) protocol, which is one of the protocols of the Connection Management (CM) sublayer (see 3GPP TS 24.007[ Mobile radio interface signalling layer 3; General Aspects ]  [20]). Every mobile station must support the call control protocol. If a mobile station does not support any bearer capability at all then it shall respond to a SETUP message with a RELEASE COMPLETE message as specified in subclause 5.2.2.2. In Iu mode only, integrity protected signalling (see subclause 4.1.1.1.1 of the present document and in general, see 3GPP TS 33.102[ 3G security; Security architecture ]  [5a]) is mandatory. In Iu mode only, all protocols shall use integrity protected signalling. Integrity protection of all CC 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 (3GPP TS 25.331[ None ]  [23c] and 3GPP TS 44.118[ None ]  [111]). In the call control protocol, more than one CC entity are defined. Each CC entity is independent from each other and shall communicate with the correspondent peer entity using its own MM connection. Different CC entities use different transaction identifiers. With a few exceptions the present document describes the call control protocol only with regard to two peer entities. The call control entities are described as communicating finite state machines which exchange messages across the radio interface and communicate internally with other protocol (sub)layers. This description is only normative as far as the consequential externally observable behaviour is concerned. Certain sequences of actions of the two peer entities compose "elementary procedures" which are used as a basis for the description in this subclause. These elementary procedures may be grouped into the following classes: - call establishment procedures; - call clearing procedures; - call information phase procedures; - miscellaneous procedures. The terms "mobile originating" or "mobile originated" (MO) are used to describe a call initiated by the mobile station. The terms "mobile terminating" or "mobile terminated" (MT) are used to describe a call initiated by the network. Figure 5.1a/3GPP TS 24.008[ Mobile radio interface Layer 3 specification; Core network protocols; Stage 3 ]  gives an overview of the main states and transitions on the mobile station side. The MS side extension figure 5.1a.1/3GPP TS 24.008[ Mobile radio interface Layer 3 specification; Core network protocols; Stage 3 ]  shows how for the Network Initiated MO call the MS reaches state U1.0 from state U0 $(CCBS)$. Figure 5.1a.2/3GPP TS 24.008[ Mobile radio interface Layer 3 specification; Core network protocols; Stage 3 ]  illustrates the additional state transitions possible in the MS due to SRVCC handovers from PS to CS or 5G-SRVCC handover from NG-RAN to UTRAN. Figure 5.1a.3/3GPP TS 24.008[ Mobile radio interface Layer 3 specification; Core network protocols; Stage 3 ]  illustrates the additional state transitions possible in the MS due to SRVCC handovers from CS to PS. Figure 5.1b/3GPP TS 24.008[ Mobile radio interface Layer 3 specification; Core network protocols; Stage 3 ]  gives an overview of the main states and transitions on the network side. The Network side extension figure 5.1b.1/3GPP TS 24.008[ Mobile radio interface Layer 3 specification; Core network protocols; Stage 3 ]  shows for Network Initiated MO Calls the Network reaches state N1.0 from state N0 $(CCBS)$.Figure 5.1b.2/3GPP TS 24.008[ Mobile radio interface Layer 3 specification; Core network protocols; Stage 3 ]  illustrates the additional state transitions possible in the network due to SRVCC handovers from PS to CS or 5G-SRVCC handover from NG-RAN to UTRAN. Figure 5.1b.3/3GPP TS 24.008[ Mobile radio interface Layer 3 specification; Core network protocols; Stage 3 ]  illustrates the additional state transitions possible in the network due to SRVCC handovers from CS to PS. Figure 5.1a/3GPP TS 24.008[ Mobile radio interface Layer 3 specification; Core network protocols; Stage 3 ] : Overview call control protocol/MS side Figure 5.1a.1/3GPP TS 24.008[ Mobile radio interface Layer 3 specification; Core network protocols; Stage 3 ] : Overview call control protocol/MS side, extension Figure 5.1a.2/3GPP TS 24.008[ Mobile radio interface Layer 3 specification; Core network protocols; Stage 3 ] : Overview call control protocol/MS side, extension for SRVCC from PS to CS or 5G-SRVCC handover from NG-RAN to UTRAN Figure 5.1a.3/3GPP TS 24.008[ Mobile radio interface Layer 3 specification; Core network protocols; Stage 3 ] : Overview call control protocol/MS side, extension for SRVCC from CS to PS Figure 5.1b/3GPP TS 24.008[ Mobile radio interface Layer 3 specification; Core network protocols; Stage 3 ]  Overview call control protocol/Network side Figure 5.1b.1/3GPP TS 24.008[ Mobile radio interface Layer 3 specification; Core network protocols; Stage 3 ]  Overview call control protocol/Network side, extension Figure 5.1b.2/3GPP TS 24.008[ Mobile radio interface Layer 3 specification; Core network protocols; Stage 3 ] : Overview call control protocol/Network side, extension for SRVCC from PS to CS or 5G-SRVCC handover from NG-RAN to UTRAN Figure 5.1b.3/3GPP TS 24.008[ Mobile radio interface Layer 3 specification; Core network protocols; Stage 3 ] : Overview call control protocol/Network side, extension for SRVCC from CS to PS