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Content for  TS 23.221  Word version:  17.0.0

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7  Call controlp. 36

7.1  General Aspectsp. 36

The following technical requirements are applied to support multimedia in GSM/UMTS.
  1. GSM/UMTS shall enable the provisioning of multimedia services and multivendor interworking between UE and network.
  2. Handover and roaming to and from GSM shall be supported provided GSM is capable of supporting the ongoing media service.
  3. For multimedia services the standardized multimedia protocol shall be run transparently via a PDP-context or a CS connection established using GSM SM/CC . This allows transparent hand-over and roaming between GSM and UMTS provided that GSM supports the QoS requirements.
  4. SIP from the IETF shall be the multimedia call control supported over the PS domain, where the network functional entities for multimedia support are within the PLMN.
  5. H.324M shall be supported within the CS domain.
Figure 7.1 illustrates the realisation of the multimedia service based on requirement 3. 'Multimedia Protocol' indicates the functionality either inside the communicating user's terminal or a server (e.g. SIP server). It is essentially a control function both for user plane and control plane for the multimedia communication.
Copy of original 3GPP image for 3GPP TS 23.221, Fig. 7.1: Support of multimedia making use of GSM SM/CC
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7.2  Domain selection for mobile terminated calls from the PSTN/CS domain |R5|p. 37

7.2.1  Introductionp. 37

It is an operator's decision whether mobile terminated calls from the PSTN/CS domain are routed first to the CS domain, or to the IM subsystem. Both options may co-exist within one operator's network.
The ability to select whether the CS domain or the IMS will provide the services is termed the Service Domain Selection (SDS) (see clause 7.2.2) and whether the CS domain or the IMS/IP CAN is used to communicate call termination from the network and the UE is termed access domain selection (ADS) (see clause 7.2a and clause 7.2b).
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7.2.2  Calls directed to the CS domainp. 37

When the mobile terminated call set-up arrives at a G-MSC server or G-MSC, then that call may be subject to service domain selection to determine whether the terminating call is to be delivered to the CS domain or the IMS is used to provide the services for the user. The G-MSC Server or G-MSC interrogates the HSS for routing information. The HSS decides on the way the call shall be treated next (e.g. IM CN subsystem, CS domain (e.g. the subscriber is roaming in a legacy network), service platform involvement). According to the decision, the HSS returns information that will make the G-MSC progress the call towards an MGCF (for onward handling in the IMS), a VMSC or to provide further processing (e.g. invoke CAMEL G-MSC processing).
SDS may take into account for terminating calls whether the user is roaming or not, user preferences, service subscription and operator policy. In addition, for a mobile terminated call set-up towards a non IMS Centralized Services user (see TS 23.292), SDS may also take the following factors (but not limited to) into account for the selection decision:
  • The state of the UE in the circuit switched domain. The state information includes: Detached, Attached;
  • The state of the UE in the IMS. The state information includes: Registered, Unregistered;
  • The access network capabilities, including the information available in the HSS on whether IMS voice over PS Session is supported (as specified in TS 23.060 and TS 23.401);
  • The UE capabilities (e.g. SRVCC capability as defined in TS 23.216).
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7.2.3  Calls directed to the IMSp. 37

When the mobile terminated call set-up arrives at a MGCF, then the MGCF passes the session to an I-CSCF which interrogates the HSS for routing information. The HSS returns information that will enable the I-CSCF to progress the call towards an S-CSCF. From the S-CSCF the session may be re-directed to the CS domain or may continue in the IMS upon instruction from the access domain selection functionality (see clause 7.2b).
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7.2a  Domain selection for UE originating sessions / calls |R7|p. 37

For UE originating calls, the UE performs access domain selection. The UE shall be able to take following factors into account for access domain selection decision:
  • The state of the UE in the circuit switched domain. This state information shall be included: Detached, Attached.
  • The state of the UE in the IMS. The state information shall include: Registered, Unregistered.
  • The domain used by an existing session / call for the same service (see the clause 7.2d).
  • User preferences and any available operator policy.
  • The "IMS voice over PS session supported indication" as defined in TS 23.060 and TS 23.401.
  • Whether the UE is expected to behave in a "Voice centric" or "Data Centric" way.
  • UE capability of supporting IMS PS voice.
  • Indications the network may have provided to the UE in combined EPS/IMSI attach/TAU, e.g. "SMS-only" indication or "CSFB Not Preferred" indication as defined in TS 23.272.
  • Whether 3GPP PS Data Off is active or not and whether IMS voice is included in 3GPP PS Data Off Exempt Services or not.
The service domain selection functionality will decide whether the call is serviced in the CS domain or the IMS. Service domain selection functionality may take into account for originating calls whether the user is roaming or not, user preferences, service subscription and operator policy. If the UE is configured for Voice over IMS, the service domain selection functionality should take the "IMS voice over PS session supported indication" into account and should only initiate IMS voice calls (with the voice bearer in the PS domain) using the RAT where the "IMS voice over PS session supported indication" applies and indicates support. The "IMS voice over PS session supported indication" applies to E-UTRAN when received in E-UTRAN, and applies to UTRAN when either received in GERAN or UTRAN.
To allow for appropriate domain selection for originating voice calls, a CSFB and/or IMS/CS-voice capable UE is set to behave as "Voice centric" or "Data centric" in E-UTRAN:
  • A UE set to "Voice centric" shall always try to ensure that Voice service is possible. A CSFB and an IMS/CS-voice capable UE set to "Voice centric" unable to obtain voice service in E-UTRAN (e.g. CSFB and IMS voice are not supported or the configured preferences on how to handle voice services prevent usage of any available voice services or 3GPP PS Data Off activated and IMS voice is not included in 3GPP PS Data Off Exempt Services), shall disable the E-UTRAN capability, which results in re-selecting GERAN or UTRAN. The E-UTRAN capability is re-enabled by the UE under the conditions described in TS 24.301. A voice centric CSFB capable UE that receives "CSFB Not Preferred" or "SMS-only" indication as a result of combined EPS/IMSI attach or combined TA/LA Update procedures and unable to obtain voice services over IMS shall disable the E-UTRAN capability, which results in re-selecting GERAN or UTRAN.
  • A UE set to "Data centric" does not disable the E-UTRAN capability if voice services cannot be obtained. Upon receiving combined EPS/IMSI attach accept or combined TA/LA Update accept with "SMS-only" indication, a data centric UE stays in the current RAT and is not allowed to use CSFB. Upon receiving combined EPS/IMSI attach accept or combined TA/LA Update accept with "CSFB Not Preferred" indication, a data centric UE stays in the current RAT and is allowed to use CSFB.
As specified in TS 23.401 and TS 23.060, a CSFB and/or IMS capable UE indicates in Attach/RAU/TAU Request whether it is set to "Voice centric" or "Data centric" and it indicates its configured preferences on how the UE is supposed to handle voice services in E-UTRAN. Depending on operator's configuration, this information can be used by the network to choose the RFSP Index in use, see clause 4.3.6 of TS 23.401 and clause 5.3.5 of TS 23.060 for examples of how RFSP Index can be used for selective idle mode camping.
To allow for appropriate domain selection for originating voice calls, the following applies for a CSFB and IMS capable UE in E-UTRAN:
  • It should be possible to provision UEs with the following HPLMN operator preferences on how a UE that is both CSFB and IMS capable is supposed to handle voice services (See Annex A for examples on UE behaviour for each configuration):
    • CS Voice only: the UE will use only the CS domain (possibly using IMS signalling, see TS 23.292) to originate voice calls and it will not attempt to initiate voice sessions over IMS using a PS bearer. The UE attempts combined EPS/IMSI attach or combined TA/LA Update procedures. If combined EPS/IMSI attach or combined TA/LA Update fail to register the IMSI in the CS domain or succeed with an "SMS-only" indication or succeed with a "CSFB Not Preferred" indication, the UE behaves as described for "voice centric" or "data centric" according to its setting.
    • CS Voice preferred, IMS PS Voice as secondary: if CS voice is available the UE will use the CS domain (possibly using IMS signalling, see TS 23.292) to originate and terminate voice calls. The UE attempts combined EPS/IMSI attach or combined TA/LA Update procedures (as defined in TS 23.272). If combined EPS/IMSI attach or combined TA/LA Update procedures fail to register the IMSI in the CS domain or succeed with an "SMS-only" indication or succeed with a "CSFB Not Preferred" indication, the UE attempts to use voice over IMS to originate voice calls. If the UE fails to use IMS for voice e.g. due to "IMS voice over PS session supported indication" indicates voice is not supported, the UE behaves as described for "voice centric" or "data centric" according to its setting.
    • IMS PS Voice preferred, CS Voice as secondary: if IMS voice is available the UE will use IMS to originate and terminate voice sessions. The UE may attempt combined EPS/IMSI attach with or without "SMS only", combined TA/LA Update with or without "SMS only" or EPS attach/TAU procedures. If the UE fails to use IMS for voice, e.g. due to "IMS voice over PS session supported indication" indicates voice is not supported (in Attach accept or subsequent TAU accept), then the UE uses the CS domain (as defined in TS 23.272), possibly using IMS signalling (see TS 23.292). If not already performed, the UE attempts combined TA/LA Update procedures. If the combined TA/LA Update procedures fail to register the IMSI in the CS domain or succeed with an "SMS-only" indication or succeed with a "CSFB Not Preferred" indication and the UE has failed to use IMS for voice, the UE behaves as described for "voice centric" or "data centric" according to its setting.
    • IMS PS Voice only: the UE will use IMS to originate voice sessions and will not attempt CS Voice. The UE may attempt combined EPS/IMSI attach with "SMS only", combined TA/LA Update with "SMS only" or EPS attach/TAU. If the UE fails to use IMS for voice, e.g. due to "IMS voice over PS session supported indication" indicates voice is not supported on E-UTRAN, the UE behaves as described for "voice centric" or "data centric" according to its setting.
To allow for appropriate domain selection for originating voice calls, the following CS/IMS voice preference setting applies for a CS and IMS capable UE in UTRAN:
  • It should be possible to provision UEs with the following HPLMN operator preferences on how a CS/IMS enabled UE is supposed to handle voice services:
    • CS Voice only: the UE will not attempt to initiate voice sessions over IMS using a PS bearer. This does not preclude the use of IMS signalling to control a CS bearer (see TS 23.292);
    • CS Voice preferred, IMS PS Voice as secondary: the UE will try to use the CS domain (possibly using IMS signalling, see TS 23.292) to originate voice calls;
    • IMS PS Voice preferred, CS Voice as secondary: the UE will try to use IMS to originate voice sessions. If the UE fails to use IMS for voice e.g. due to "IMS voice over PS session supported indication" indicates voice is not supported (in Attach accept or subsequent RAU accept), then the UE will use the CS domain to originate voice calls, possibly using IMS signalling (see TS 23.292).
In limited service state, a UE should always camp on a RAT which is likely to support emergency calls.
The TS 23.167 describes when an emergency call is to be initiated in the CS domain or using IMS. When the CS domain is to be used the following applies:
  • A CSFB/IMS enabled UE which is registered in the CS domain, e.g. using CSFB procedures or using above-mentioned selection principles, shall initiate emergency call attempts in the CS domain by using available CS domain procedures.
  • A CS Voice capable UE which is not registered in the CS domain shall initiate emergency calls in the CS domain by autonomously switching to a RAT which supports the CS domain (e.g. GERAN or UTRAN).
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7.2b  Access Domain Selection for terminating sessions |R7|p. 40

Terminating Access Domain Selection (T-ADS) selects CS access and/or one or more PS access network(s) to be used to deliver a terminating session to the UE.
T-ADS functionality for call termination is performed in the terminating network, and can optionally be assisted by the UE procedures.
For IMS Service Centralization and Continuity enabled networks, T-ADS is a functionality located in the IMS. The UE enhanced for IMS Service Centralization and Continuity is able to assist the T-ADS.
For IMS Service Centralization and Continuity users, T-ADS is performed as specified in TS 23.292 and TS 23.237.
If the UE assisted T-ADS is used, then prior to responding to a session request requesting one or more media (i.e. bi-directional speech media) bearers in the PS domain, the UE shall determine, based on the PS domain's access network capabilities including the information whether IMS voice over PS Session is supported (as specified in TS 23.060 and TS 23.401) (if indicated), user preferences and operator policy, whether the media bearers can be accepted on the PS domain. If the media bearers cannot be accepted, then the UE shall perform SDS.
For terminating sessions:
  • T-ADS is always performed after the terminating services.
The T-ADS may take the following factors (but not limited to) into account for the selection decision:
  • The service requested (e.g. voice call, file transfer, image share, etc.) in the incoming session.
  • The state of the UE in the circuit switched domain. This state information shall be included: Detached, Attached.
  • The state of the UE in the IMS. The state information shall include: Registered, Unregistered.
  • The UE capabilities.
  • The time of the most recent IMS registration or re-registration via WLAN access.
  • The time when 3GPP access network had the most recent radio contact with the UE.
  • The access network capabilities including the information whether IMS voice over PS Session is supported (as specified in TS 23.060 and TS 23.401) by the 3GPP access network that had the most recent radio contact with the UE.
  • The media components included in the incoming session.
  • User preferences and operator policy.
  • UE 3GPP PS Data Off status (active/inactive).
  • Configuration related to the list of applicable 3GPP PS Data Off Exempt Services.
The T-ADS shall take into account the domains / access types used by an existing session.
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7.2c  Domain Selection for UE originating SMS |R8|p. 40

To allow for appropriate domain selection for SMS delivery, the following applies for an IMS capable UE which supports SMS over IP networks:
  • It should be possible to provision UEs with the following HPLMN operator preferences on how an IMS enabled UE is supposed to handle SMS services:
    • SMS is not to be invoked over IP networks: the UE does not attempt to deliver SMS over IP networks. The UE attempts to deliver SMS over NAS signalling.
    • SMS is preferred to be invoked over IP networks: the UE attempts to deliver SMS over IP networks. If delivery of SMS over IP networks is not available, the UE attempts to deliver SMS over NAS signalling.
  • The UE shall not attempt to deliver SMS over IP networks over 3GPP access, if 3GPP PS Data Off as defined in TS 22.011 is activated and SMS over IP is not in the list of 3GPP PS Data Off Exempt Services.
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7.2d  Domain selection decisions during ongoing sessions |R10|p. 41

Ongoing IMS voice sessions are not impacted by a change of the IMS voice over PS session indicator (i.e. the UE performs a TAU or Combined TA/LA update in ECM-CONNECTED mode and receives IMS voice over PS session indicator indicating that IMS voice over PS sessions are not supported).
If the UE has an ongoing IMS voice over PS session, then regardless of the value of the IMS voice over PS session indicator, the UE shall originate new calls in the same access domain as the ongoing IMS voice over PS session until the release of the ongoing IMS voice over PS session.
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7.2e  Selection of method for UE originating USSD requests |R12|p. 41

To allow for selection of appropriate method for UE originating USSD request delivery, the following applies for a UE when the UE supports both the USSD using IMS and the USSD using CS domain.
The UE shall first attempt USSD requests in the domain used for originating voice sessions. If an attempt to deliver the USSD request using IMS fails due to missing network support and the UE is registered in the CS domain, the UE shall attempt to deliver the USSD request using the CS domain.
To handle the case when the UE can use IMS for originating voice call but the network does not support USSD request using IMS, the UE may be provisioned with the following HPLMN operator preferences to instruct the UE to invoke USSD requests via the CS domain even when the UE can use IMS for originating voice calls:
  • USSD requests are to be invoked via the CS domain: the UE shall not attempt to deliver the USSD request using IMS even when the UE would use IMS for originating voice calls.
The UE shall not attempt to deliver USSD using IMS over 3GPP access, if 3GPP PS Data Off as defined in TS 22.011 is activated and USSD over IMS is not in the list of 3GPP PS Data Off Exempt Services.
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7.3  Routing sessions from the IMS to the CS Domain |R5|p. 41

If the IMS Service Centralization and Continuity is not used and if a subscriber is subscribed to both the CS domain and the IMS, then there may arise the need to route sessions that arrived in the IMS to the CS domain using a number related to the subscriber's MS-ISDN. All sessions which are forwarded from the IMS to the CS domain shall enter the CS domain through a G-MSC or G-MSC Server. The G-MSC or G-MSC Server will handle the session as a mobile terminating call.
If the IMS Service Centralization and Continuity is used, the routing of the session from IMS to CS shall be done according to TS 23.292.
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