4.0 General

23.2283GPPIP Multimedia Subsystem (IMS)Release 18Stage 2TS

The IP Multimedia CN subsystem comprises all CN elements for provision of multimedia services. This includes the collection of signalling and media related network elements as defined in TS 23.002 [1]. IP multimedia services are based on an IETF defined session control capability which, along with multimedia transport capabilities, utilises the IP-Connectivity Access Network (this may include an equivalent set of services to the relevant subset of CS Services).

In order to achieve access independence and to maintain a smooth interoperation with wireline terminals across the Internet, the IP multimedia subsystem attempts to be conformant to IETF "Internet standards". Therefore, the interfaces specified conform as far as possible to IETF "Internet standards" for the cases where an IETF protocol has been selected, e.g. SIP and RTP.

The IP multimedia core network (IM CN) subsystem enables operators to offer their subscribers multimedia services. The IM CN subsystem should enable the convergence of, and access to, voice, video, messaging, data and web-based technologies for the wireless and wireline user.

The complete solution for the support of IP multimedia applications consists of terminals, IP-Connectivity Access Networks (IP‑CAN), and the specific functional elements of the IM CN subsystem described in this technical specification. Examples of IP-Connectivity Access Network are:

– the GPRS core network with GERAN and/or UTRAN radio access networks; and

– EPC core network and E-UTRAN radio access network; and

– 5GS access network.

Figure 4.0 below represents the IMS reference architecture including interfaces towards CS network and other IP based multimedia Networks. Details of the roles of these nodes are described in clauses 4.6, 4.7 and 4.7a.

NOTE 1: Some entities defined as part of the IMS Subsystem can also be used by other subsystems.

NOTE 2: The Ici and Izi reference points are only applicable for IP Multimedia Networks that are IMS subsystems.

NOTE 3: In certain configuration, two entities can exchange SIP messages directly with each other without a reference point being defined between them (e.g., intermediate entity(ies) not record-routed).

Figure 4.0: Reference Architecture of the IP Multimedia Core Network Subsystem

A description of the functional entities can be found in TS 23.002 [1].