An IBCF provides application specific functions at the SIP/SDP protocol layer in order to perform interconnection between two operator domains. It enables communication between IPv6 and IPv4 SIP applications. network topology hiding, controlling transport plane functions, screening of SIP signalling information, selecting the appropriate signalling interconnect and generation of charging data records. Details are described in TS 23.228.

Interrogating-CSCF (I-CSCF) is the contact point within an operator's network for all connections destined to a user of that network operator, or a roaming user currently located within that network operator's service area.

An IMS capable UE with additional ICS-specific functionality.

This entity is used as serving GSN towards home network and relay some PDU notification messages between serving GSN and Gateway GSN. This entity can be only located in a visited PLMN.

This entity is used as serving MSC towards home network and relay some messages between home network and serving MSC. This entity can be only located in a visited PLMN.

The IM subsystem comprises all CN elements for provision of IP multimedia services comprising audio, video, text, chat, etc. and a combination of them delivered over the PS domain. The entities related to IMS are CSCF, MGCF, MRF, etc. as defined in the stage 2 of the IM subsystem TS 23.228. See TS 22.228 for some service examples of IMS.

The IMS-AGW is involved when when NAT (Network Address Translation) is invoked between the UE and the IMS domain.

An IMS-MGW may terminate bearer channels from a switched circuit network and media streams from a packet network (e.g., RTP streams in an IP network). The IMS-MGW may support media conversion, bearer control and payload processing (e.g. codec, echo canceller, conference bridge), it: The IMS-MGW will be provisioned with the necessary resources for supporting UMTS/GSM transport media. Further tailoring (i.e. packages) of the H.248 may be required to support additional Codecs and framing protocols, etc

The IP Short Message Gateway function is used for two functions: to deliver SMS messages over the IP network and to provide interworking between SMS users and Instant Messaging users. The interworking function translates between MAP or Diameter based signalling and SIP signalling to convey messages and responses between the two systems. Both functions are described in TS 23.204.

IM Services Identity Module (ISIM) application residing on the UICC.

If the Terminal wants to engage in IMS operation, then after UICC activation, the Terminal shall select an ISIM application, if an ISIM application is listed in the EF_DIR file, using the SELECT by DF name as defined in TS 31.101.

After a successful ISIM application selection, the selected ISIM (AID) is stored on the UICC. This application is referred to as the last selected ISIM application.

The Interworking Function (IWF) is a functional entity associated with the MSC. The IWF provides the functionality necessary to allow interworking between a PLMN and the fixed networks (ISDN, PSTN and PDNs). The functions of the IWF depend on the services and the type of fixed network.

The IWF is required to convert the protocols used in the PLMN to those used in the appropriate fixed network. The IWF may have no functionality where the service implementation in the PLMN is directly compatible with that at the fixed network. The interworking functions are described in TS 29.007.