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In this diagram, only 3 layers are shown, by eliminating the transport layer. Due to IMS is an access independent core network, the UE would represent any terminal from any access network. Therefore, we standardizing all the terminals as UE, defined as User Equipment, the 3GPP way to name IMS/SIP UA (User Agent). UE allows a user access to network services. Any types of transport layer must provide an IP packet-based channel routing between UE and IMS Session Control elements.

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Its address is discovered by UEs using the mechanism described in the clause "Procedures related to Local CSCF Discovery“;

The P-CSCF shall not modify the Request URI in the SIP INVITE message;

Generation of CDRs,

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Generation of CDRs

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Within an operator's network, different S-CSCFs may have different functionalities.

It shall reject IMS communication to/from Public User Identity(s) that are barred for IMS communications after completion of registration.

Generation of CDRs.

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A Home Network may contain one or several HSSs: it depends on the number of mobile subscribers, on the capacity of the equipment and on the organisation of the network;

The HSS also generates User Security information for mutual authentication, communication integrity check and ciphering.

The HSS consists of the following functionalities:• IP multimedia functionality to provide support to control functions of the IM

subsystem such as the CSCF;• The subset of the HLR/AUC functionality required by the PS Domain (GPRS and EPC;.• The subset of the HLR/AUC functionality required by the CS Domain.

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The OSA Application Server does not directly interact with the IMS network entities but through the OSA Service Capability Servers (OSA SCS-s). Further information on OSA is provided in TS 22.127 [4].

A SIP Application Server specified within 3GPP that supports IMS reference points (e.g. ISC, Sh, Ut), in support of an application, is considered as part of the IM CN subsystem. Examples of such ASs are SCC AS and TAS.

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