Content for  TS 23.002  Word version:  18.0.0

The Cx reference point supports information transfer between CSCF and HSS. The main procedures that require information transfer between CSCF and HSS are

1. Procedures related to Serving CSCF assignment
2. Procedures related to routing information retrieval from HSS to CSCF
3. Procedures related to authorisation (e.g., checking of roaming agreement)
4. Procedures related to authentication: transfer of security parameters of the subscriber between HSS and CSCF
5. Procedures related to filter control: transfer of filter parameters of the subscriber from HSS to CSCF

Further information on the Cx reference point is provided in TS 23.228.

The Gm reference point supports the communication between UE and IM CN subsystem, e.g. related to registration and session control. The protocol used for the Gm reference point is SIP (as defined by RFC 3261, other relevant RFC's, and additional enhancements introduced to support 3GPP's needs).

The Mn reference point describes the interfaces between the MGCF and IMS-MGW in the IMS. It has the following properties:

• full compliance with the H.248 standard functions for IMS - PSTN/PLMN interworking.
• flexible connection handling which allows support of different call models and different media processing purposes not restricted to H.323 [54] usage.
• open architecture where extensions/Packages definition work on the interface may be carried out.
• dynamic sharing of IMS-MGW physical node resources. A physical IMS-MGW can be partitioned into logically separate virtual MGWs/domains consisting of a set of statically allocated Terminations.
• dynamic sharing of transmission resources between the domains as the IMS- MGW controls bearers and manage resources according to the H.248 [52] protocols and functions for IMS.

The Mg reference point allows the MGCF to forward incoming session signalling (from the circuit switched network) to the CSCF for the purpose of interworking with circuit switched networks. The protocol used for the Mg reference point is SIP (as defined by RFC 3261, other relevant RFC's, and additional enhancements introduced to support 3GPP's needs).

The Cr reference point allows interaction between an Application Server and an MRFC for media control. The Cr reference point enables the MRFC to fetch and cache documents and resources from an Application Server and to return data to an Application Server. The Cr reference point enables media control protocol requests, responses and notifications to be sent between the MRFC and an Application Server. Further information on the Cr reference point is provided in TS 23.218.

The Mr' reference point allows interaction between an Application Server and an MRFC for session control without passing through an S-CSCF. Further information on the Mr' reference point is provided in TS 23.218.

The Mr reference point allows interaction between an S CSCF and an MRFC. The protocol used for the Mr reference point is SIP (as defined by RFC 3261, other relevant RFC's, and additional enhancements introduced to support 3GPP's needs).

The Mp reference point allows an MRFC to control media stream resources provided by an MRF. The Mp reference point has the following properties:

• Full compliance with the H.248 [52] standard.
• Open architecture where extensions (packages) definition work on the interface may be carried out.

The Mw reference point allows the communication and forwarding of signalling messaging between CSCFs, e.g. during registration and session control.

This interface is used with an MRB operating in In-Line mode and is described further in TS 23.218). The S CSCF receives a session initiation request from an MRB that originated from an Application Server requesting MRF resources and whose destination is an MRF resource identified by the MRB. The S CSCF routes the message to the identified MRFC. Subsequent messages in the same session dialog between the AS and MRFC traverse the MRB as well as the S CSCF.

The Rc reference point is used by the AS to request that media resources be assigned to a call when utilizing an MRB in both Query and In-Line modes (as described in TS 23.218).

This reference point allows the Serving CSCF to forward the session signalling to the Breakout Gateway Control Function for the purpose of interworking to the circuit switched networks. The Mi reference point is based on external specifications i.e. SIP [61].

This reference point allows the Breakout Gateway Control Function to exchange session signalling message with the Media Gateway Control Function for the purpose of interworking to the circuit switched networks, or for transit scenarios. The Mj reference point is based on external specifications i.e. SIP [61].

This reference point allows the Breakout Gateway Control Function/IBCF to forward the session signalling to another Breakout Gateway Control Function. The Mk reference point is based on external specifications i.e. SIP [61].

This interface between CSCF and SLF is used to retrieve the address of the HSS which holds the subscription for a given user or information related to a service. This interface is not required in a single HSS environment. An example for a single HSS environment is a server farm architecture. Details are described in clause 5.8.1 of TS 23.228.

The Mb reference point is used for IMS media transport between relevant IMS functions. IMS media is defined in some of the 26-series (e.g. TS 26.114, TS 26.141).

This interface between Serving CSCF and the Application Servers (i.e., SIP Application Server, OSA Service Capability Server, or CAMEL IM-SSF) is used to provide services for the IMS. Details are described in clause 4.2.4 of TS 23.228.

This is the interface between the Emergency CSCF and the EATF in serving network (i.e. EATF as defined in TS 23.167), it is used to provide services for emergency sessions.

This is the interface between the Interrogating CSCF and the EATF in serving network (i.e. EATF as defined in TS 23.167), it is used to provide services for emergency sessions.

The Application Server (SIP Application Server and/or the OSA Service Capability Server) may communicate to the HSS. The Sh interface is used for this purpose. Details are described in clause 4.2.4 of TS 23.228. This reference point is also used to exchange signalling between IP-SM-GW and the HSS for SMSIP delivery.

The CAMEL Application Server (IM-SSF) may communicate to the HSS. The Si interface is used for this purpose. Details are described in clause 4.2.4 of TS 23.228.

The Ut interface resides between the UE and the SIP Application Server. The Ut interface enables the user to manage information related to his services. Such as creation and assignment of Public Service Identities, management of authorization policies that are used e.g. by Presence service, conference policy management, etc. The AS may need to exhibit security related functions for the Ut interface, the details of these security functions are described in TS 33.222. For the protocol at the Ut reference point HTTP shall be supported.

This interface between AS and SLF is used to retrieve the address of the HSS which holds the subscription for a given user. This interface is not required in a single HSS environment. An example for a single HSS environment is a server farm architecture. Details are described in clause 5.8.1 of TS 23.228.

The Mx reference point allows the communication and forwarding of signalling messages between a CSCF/BGCF and an IBCF, e.g. during session establishment.

This interface is used by the IBCF to control the TrGW, e.g. to request network address translation binding.

This interface between Interrogating CSCF and the Application Servers (i.e. SIP Application Server, OSA Service Capability Server, or CAMEL IM-SSF) is used to:

• forward SIP requests destined to a Public Service Identity hosted by an Application Server directly to the Application Server. Details are described in clause 5.4.12 of TS 23.228; and
• initiate a request on behalf of a user or Public Service Identity, if the AS has no knowledge of the serving S CSCF for that user or Public Service Identity. Details are described in clause 5.6.5.3 of TS 23.228.

The Iq reference point is between the Proxy CSCF and the IMS Access Gateway. It conveys the necessary information that is needed to allocate and release transport addresses. Details are described in TS 23.228, Annex G.

The Ml reference point allows the E CSCF to request the LRF to validate the location information received from the UE, to determine or query the LRF for routing information to emergency centres. Location information may be received from LRF and/or reference key to allow the PSAP to retrieve location at a later stage.

The Ici reference point allows two IBCFs to communicate with each other in order to provide the communication and forwarding of signalling messaging between IM CN subsystem networks.

The Izi reference point allows two TrGWs to forward media streams between IM CN subsystem networks.

The I2 reference point is used to route session control signalling between the MSC Server and the IMS, as defined in TS 23.292 and in TS 23.237. This can be a roaming and inter-operator reference point. I2 is used to support one or more of the following features:

• An MSC Server enhanced for ICS, as defined in TS 23.292.
• An MSC Server enhanced for ICS and SRVCC, as defined in TS 23.292, TS 23.216 and TS 23.237.
• An MSC Server enhanced for ICS and SRVCC with the MSC Server assisted mid-call feature as defined in TS 23.237.
• An MSC Server enhanced for ICS and SRVCC with the SRVCC for call in alerting phase feature as defined in TS 23.237.

The I3 reference point is used between the MSC Server enhanced for ICS and the TAS to interwork CS signalling and communication service setting procedures, as defined in TS 23.292.

The I1 reference point is used between the ICS UE and the SCC AS for service control signalling over CS access, as defined in TS 23.292.

The Ms reference point is used between an IBCF and an AS for identity attestation, identity signing, and signature verification, as defined in TS 23.228.