5 Architecture model and reference points

23.2923GPPIP Multimedia Subsystem (IMS) centralized servicesRelease 17Stage 2TS

Tools: ARFCN - Frequency Conversion for 5G NR/LTE/UMTS/GSM

5.1 Overview

ICS enables IMS services when using CS access for media transport. Users are provided with a consistent experience of services.

When using PS access networks which support the full duplex speech media component of an IMS service, procedures defined in TS 23.228 [2] are used to provide IMS services.

For other access networks, media established via the CS domain is used in conjunction with IMS service control. When using a UE that has been enhanced for ICS, IMS service control is enabled by a transparent control channel (the Service Control Signalling Path) between the UE and IMS. When using a UE which has not been enhanced for ICS, IMS service control can be enabled by an MSC Server enhanced for ICS.

For systems with a CS domain access based on TS 24.008 [6], CAMEL shall be used for implementing the IN triggers.

5.2 Reference architecture

Figure 5.2-1 provides the reference architecture for IMS sessions established using CS bearers.

Figure 5.2-1: IMS Service Centralization and Continuity Reference Architecture

The architecture introduces the following:

– The SCC AS, which provides functions specific to IMS Service Centralization and Continuity.

– Enhancements to the MSC Server for ICS.

– Enhancements to the UE for ICS

– Enhancements to the MSC Server for ICS to interwork with IMS emergency architecture as defined in TS 23.167 [25]

Not all of the above are required in a network implementing ICS.

5.3 Functional Entities

5.3.1 SCC AS

The SCC AS is a home network based IMS Application that provides functionality required to enable IMS Centralized Services. The SCC AS is inserted in the session path using originating and terminating iFCs; it is configured as the first AS in the originating iFC and as the last AS in the terminating iFC chain. The SCC AS may also be invoked through the use of PSI termination procedures when using CS access.

The SCC AS implements one or more of the following functionalities:

– ICS User Agent (IUA): The ICS User Agent (IUA) function provides SIP UA behaviour on behalf of the UE for setup and control of IMS sessions using CS bearers that are established with or without the use of Gm or I1 between the UE and the SCC AS. For sessions established using Gm or I1 between the ICS UE and the SCC AS the IUA combines the service control signalling with the description of the bearer, e.g. SDP, established via the CS access to present a standard IMS session on behalf of the UE. An SCC AS may support I1 reference point. If supporting the I1 reference point, and before initiating an I1 Service Control Signalling Path flow, the SCC AS shall ensure that the ICS UE supports I1 and the PLMN the UE is in supports transport of network initiated I1 signalling.

– CS Access Adaptation (CAA): The CS Access Adaptation (CAA) is an adaptation function for the service control signalling communicated transparently via the CS domain between the UE and the SCC AS. The CAA processes the service control signalling received via the CS access for interworking with other IMS functional elements. The CAA is only used when using the CS network for communication of service control signalling.

– Terminating Access Domain Selection (T-ADS): Terminating Access Domain Selection (T-ADS) provides:

– Directs an incoming session to an ICS User;

– For one or more UEs of an ICS User:

– Influences the selection of one or more contacts amongst the registered contacts and;

– Influences the selection of an access network for delivery of the incoming session to the selected contact, or;

– Performs breakout to the CS Domain by fetching the CSRN.

T-ADS shall take into account the access network’s capabilities, UE capabilities, IMS registration status, CS status, existing active sessions, user preferences, UE 3GPP PS Data Off status (active/inactive), 3GPP PS Data Off Exempted Service configuration setting, operator policies such as access network specific voice domain preferences and the media component types, and whether there is an ongoing IMS voice over PS session. In addition, T-ADS may take into account the access network information provided directly by the UE.

The UE may assist the T-ADS. The UE assisted T-ADS (UE T-ADS), based on configuration that takes into account operator policy, user preferences and local access network capabilities, performs the following:

– Detect media that is candidate for delivery over the CS domain associated with an incoming session.

– Identify the domain in which the session is to be established (CS or PS).

– Determines the mechanism to complete the establishment (origination or termination) – applicable to sessions to be established in the CS domain only.

– Notifies the SCC AS of the mechanism the establishment will be completed on – if possible.

For delivery of an incoming session to non ICS UE, the SCC AS may deliver the session in other domain if call delivery in the originally selected domain fails, e.g. delivers the call in CS domain when session establishment in the PS domain does not succeed.

For delivery of an incoming session to an ICS User, the T-ADS shall perform the following:

1. Assists in delivery of an incoming session, whether to:

– Deliver all media via PS.

– Deliver speech media via CS and use Gm for service control.

– Deliver speech media via CS and use I1 for service control.

– Deliver speech media via MSC Server enhanced for ICS.

– Deliver speech media via MSC Server not enhanced for ICS.

2. It assists in selection of an access network for delivery of the incoming session to the ICS User contact address(es). Selection criteria as specified in TS 23.221 [10] clause 7.2b, Access Domain Selection for terminating sessions are used for access network selection.

For incoming sessions with bi-directional speech media destined to a selected contact:

– For 3GPP access, the SCC AS shall retrieve from the serving PLMN via the HSS (+UDM in case of 5GC as specified in TS 23.501 [55]) the most recent access network capabilities ("IMS voice over PS Session Supported Indication", current RAT type) unless there is, for the same UE, an existing IMS voice over PS session or an existing speech session using the CS bearer that is not using the Gm service control signalling path.

– For networks which support WLAN access, and if WLAN access is allowed to support IMS voice based on operator policy, the SCC AS shall additionally take the time of the most recent IMS 3rd party registration or re-registration via WLAN access into account.

If the most recent access network capabilities can be retrieved, then:

– If the most recently used PS access network (including WLAN access) indicates that IMS voice over PS is supported then bi-directional speech media can be delivered on this access.

– If the most recently used PS access network indicates that IMS voice over PS is not supported, then bi-directional speech media shall not be delivered on this access.

– If the current RAT type indicates that simultaneous transfer on PS and CS domains is supported, then speech media can be delivered via CS and Gm can be used for service control.

– If the current RAT type indicates that simultaneous transfer on PS and CS domains is not supported, then Gm shall not be used for service control.

If there is an existing speech session using the CS bearer that is not using the Gm service control signalling path then Gm shall not be used for service control.

3. For delivery of incoming sessions to the UE registered in CS domain via standard MSC Server, it fetches the CSRN for breakout to the CS domain.

If a UE has an ongoing IMS voice over PS session, the T-ADS shall select the PS domain when delivering an incoming session to the same UE.

When using Gm reference point, T-ADS for ICS UE sessions may be executed in the ICS UE in conjunction with the network, based on user preferences, operator policy and taking into account its own capabilities and those of the access network.

5.3.2 UE enhanced for ICS

The ICS UE is an IMS UE that has additional ICS-related capability, which includes the following functions:

– Communication with the SCC AS for service control signalling.

– Establishment of the Bearer Control Signalling Path to setup the media through the CS domain.

– Execution of ADS for originating sessions as specified in TS 23.221 [10] clause 7.2a.

– Assisting the SCC AS in the execution of T-ADS when Gm is used.

The ICS UE supports the Gm reference point (as defined in TS 23.228 [2]) and shall use the Gm reference point when using the PS network for service control signalling.

NOTE 1: Since an ICS UE is based on an IMS UE, the Gm reference point is always supported on an ICS UE.

The ICS UE may support the I1 reference point (as defined in clause 5.4.1). If the ICS UE supports the I1 reference point, it shall be used only when:

– the home operator has preconfigured it to be used; and

– use of the Gm reference point is not possible due to the PS network not being available (e.g. due to the ICS UE camping on a GERAN with no DTM available).

Otherwise, the Gm reference point or standard CS procedures shall be used.

If the ICS UE supports Gm ICS and has a single call for which CS procedures are being used and Gm is not being used for service control then it may add the Gm service control signalling path, as described in clause 7.5a. If the ICS UE that supports Gm ICS does not add the Gm service control signalling path then it shall continue to use CS procedures for service control and for establishing additional calls until all ongoing calls end.

NOTE 2: An ICS UE that supports Gm ICS may have a call for which Gm is not being used for service control if, for example, a handover has occurred from an access network that doesn’t support Gm, or after CSFB, as described in TS 23.272 [44].

5.3.3 MSC Server enhancements for ICS

The MSC Server (e.g. as described in TS 23.002 [3]) may be enhanced for the support of ICS.

In addition to the standard MSC Server behaviour, an MSC Server that has been enhanced for ICS shall provide the following for an identified ICS user:

– Processes the user-network signalling received over the CS access (e.g. A/Iu and E interface) for interworking with IMS SIP and vice versa.

– Controls the MGW functions described in TS 23.002 [3] to enable the interworking between CS access and RTP bearers.

– Performs the interworking to support multimedia call and emergency call in ICS.

– May implement a communication service setting conversion function between CS signalling (e.g. as described in TS 24.010 [30] for systems based on TS 24.008 [6]) and communication service setting procedures (as defined in TS 24.173 [8]).

– Disregard any supplementary services or IN (e.g. CAMEL) triggers that may be present in the user profile.

NOTE 1: These can be present in the user profile for the case where the user is served by an MSC Server not enhanced for ICS.

– Adds the User Location Information (e.g. CGI or SAI) and/or UE Time Zone Information to IMS signalling.

For subscribers not identified as ICS users and for subscribers identified as ICS users but using a media type not listed in clause 22.4 of TS 22.101 [9] and TS 22.173 [4], the MSC Server functionality is unchanged.

In addition to the standard MSC Server behaviour, an MSC Server that has been enhanced for ICS may support the Combined CS Access Authentication procedure as specified in Annex G.

NOTE 2: If the Combined CS Access Authentication prodecure is used all users of this MSC Server shall be authenticated by this procedure only.

If users are authenticated utilizing the Combined CS Access Authentication procedure, the MSC Server enhanced for ICS shall verify that 3G CS authentication parameters that are retrieved from the S-CSCF.

NOTE 3: 3G authentication can be performed also over 2G radio (GERAN).

MSC Server enhancements for ICS are not required for the support of an ICS UE.

5.3.4 Emergency CSCF (E-CSCF)

E-CSCF is defined in TS 23.167 [25].

5.4 Reference points

5.4.1 Reference Point UE – SCC AS (I1)

The I1 reference point is used between the UE and the SCC AS for service control signalling over CS access. All I1 Service Control Signalling Path flows shall be uniquely identifiable from other signalling flows on the same bearer.

The I1 reference point performs the following functions:

– optional session set up via CS access for mobile originating and terminating sessions;

– optional signalling for additional IMS parameter exchange during session setup;

– IMS services control via CS access.

5.4.2 Reference Point MSC Server – CSCF (I2)

The I2 reference point shall be used to route service control signalling between the MSC Server enhanced for ICS and the home IMS. The Mw reference point specified in TS 23.228 [2] together with ICS specific extensions shall be used over the I2 reference point.

5.4.3 Reference Point MSC server – CS-MGW (Mc)

The Mc reference point, as defined in TS 23.002 [3], is established between the MSC Server enhanced for ICS and the CS-MGW.

5.4.4 Reference Point MSC Server ‑ TAS (I3)

The I3 reference point shall be used between the MSC Server enhanced for ICS and the TAS to interwork CS signalling (e.g. as described in TS 24.010 [30] for systems based on TS 24.008 [6]) and communication service setting procedures (as defined in TS 24.173 [8]).

5.4.5 Reference Point MSC Server – E-CSCF (I6)

The I6 reference point shall be used to route emergency service control signalling between the MSC Server enhanced for ICS and the E-CSCF. The Mw reference point specified in TS 23.228 [2] together with ICS specific extensions shall be used over the I6 reference point.