Content for  TS 23.216  Word version:  18.0.0

3GPP2 1xCS IWS uses the S102 reference point to communicate with the MME and to transport 3GPP2 1xCS signalling messages to the SRVCC UE. The role of the 3GPP2 1xCS IWS is:

• To be a signalling tunnelling end point towards the MME for receiving/sending encapsulated 3GPP2 1xCS signalling messages to/from the UE; and
• To emulate a 1xRTT BSS towards the 1xRTT MSC (reference point A1 as defined in 3GPP2 A.S0014 [20] between 1xBS and MSC).

In addition to the standard MSC Server behavior defined in TS 23.002, an MSC Server which has been enhanced for SRVCC provides the following functions as needed for support of SRVCC:

• Handling the Relocation Preparation procedure requested for the voice component from MME/SGSN and with priority if priority indication is received via the Sv reference point;
• Behaving as MSC Server enhanced for ICS as defined in TS 23.292 if supported and when ICS flag is received via the Sv reference point is set to true and optionally if the MSC Server is configured to know that the VPLMN has a suitable roaming agreement with the HPLMN of the UE;
• Invoking the session transfer procedure or emergency session transfer procedure from IMS to CS as according to TS 23.237 and with priority if priority indication is received via the Sv reference point;
• Coordinating the CS Handover and session transfer procedures;
• In the case of a non-emergency session, handling the MAP_Update_Location procedure without it being triggered from the UE;
• In the case of an emergency session, conditionally sending a MAP Subscriber Location Report to a GMLC to support location continuity according to TS 23.271.

An MSC Server enhanced for SRVCC may be further enhanced for vSRVCC by providing, in addition to the functions specified in clause 5.3.2, the following functions as needed for the support of vSRVCC:

• Initiating the handover towards the target system for BS30 when it receives the Sv request with a vSRVCC indication from the MME and with priority if priority indication is received via the Sv reference point.
• Negotiating with the SCC AS for the last active session to determine if it should perform SRVCC or vSRVCC procedure.

In addition to the standard MSC Server behaviour defined in TS 23.002 and TS 23.292 for MSC Server enhanced for ICS, an MSC Server which has been enhanced for SRVCC from CS to PS provides the following functions as needed:

• The MSC Server informs the RAN about the possibility to perform CS to PS SRVCC by sending a "CS to PS SRVCC operation possible" to the RNC/BSC only for normal TS 11 call. The RAN uses that information for deciding the cells for which the UE reports measurements that lead to handover request to the core network.
• The MSC Server determines the possibility of performing CS to PS SRVCC based on:
  • The CS to PS SRVCC capability indication of the UE received from IMS as defined in TS 23.237.
  • The presence of the CS to PS SRVCC allowed indication in the subscription data of the user.
  • The IMS registration status of the UE.
• The MSC Server informs RAN that CS to PS SRVCC is not possible by sending a "CS to PS SRVCC operation possible" set to "false" to the RNC/BSC on reception of the UE's IMS registration expiration notification from SCC AS during an ongoing CS call.
• The target MME/SGSN selection for supporting CS to PS SRVCC is based on Target ID contained in the HO required and can be performed using the SGSN/MME selection procedures specified in TS 29.303 or using local configuration in the MSC Server.
• The call is a normal TS 11 call (i.e. not an emergency call - TS12, TS 22.003).
• If roaming restriction to E-UTRAN access needs to be enforced, an MSC server connected to RNCs that may handover or invoke release with redirection to E-UTRAN is configured with a list of HPLMN IDs that are permitted to access E-UTRAN unless restricted by the UE individual access restriction information received from HLR/HSS (see TS 29.002). Also, the MSC server shall set the E-UTRAN Service Handover IE (in Iu mode) for CS bearers to an appropriate value to inform the RNC about connected-mode mobility restriction to E-UTRAN.

3GPP vSRVCC UE signals its vSRVCC capability to the network. If the UE supports vSRVCC capability, it shall also support SRVCC capability. If needed, the vSRVCC UE initiates the multimedia codec negotiation on the CS domain after the vSRVCC handover is completed.

If the UE supports the 5G-SRVCC from NG-RAN to 3GPP UTRAN, the UE shall follow the rules and procedures described in TS 23.502 with the following additions and clarifications:

• Supports UTRAN cell measurement procedure.
• Supports 5G-SRVCC from 5GS to 3GPP UTRAN in NAS and AS.
• Derives CS security context from EPS security context which are derived from 5GS security context.

The PDN-GW shall support the PS-to-CS handover indicator as notified by the MME/SGSN (see detailed procedures in clause 6.2.2).

When HSPA capable UTRAN selects a target cell for SRVCC handover, it needs to send an indication to SGSN that this handover procedure requires SRVCC. UTRAN may be capable of determining the neighbour cell list based on the "SRVCC operation possible" indication, the UE's Radio Access Capabilities for SRVCC from UTRAN (HSPA) to UTRAN/GERAN and/or presence of an established voice bearer (i.e. bearer with Traffic Class = Conversational and Source Statistic Descriptor = 'speech') for a specific UE. An example algorithm is provided in clause A.3. The procedure for UTRAN Radio Access Network sharing is described in TS 23.251.

When GERAN/UTRAN selects a target cell for CS to PS SRVCC handover, it shall send an indication to MSC Server enhanced for CS to PS SRVCC that this handover procedure requires CS to PS SRVCC. GERAN/UTRAN may be capable of determining the neighbour cell list based on the "CS to PS SRVCC operation possible" indication received from MSC Server enhanced for CS to PS SRVCC, the UE's Radio Access Capabilities for CS to PS SRVCC over GERAN/UTRAN, and the existence of CS voice call. An example algorithm is provided in Annex A, clause A.4.

The PCRF enforces the architecture principle to use QCI=1 (and traffic-class conversational with source statistics descriptor ="speech") for voice bearer with IMS sessions anchored in the SCC AS, based on the service the session relates to. This may be achieved by deploying S9 reference point, or configuration and roaming agreements. The PCEF should enforce to multiplex the media streams from multiple concurrent normal voice sessions. For vSRVCC, the PCRF enforces that the video bearer of IMS sessions anchored in the SCC AS, is marked with the "PS to CS session continuity indicator", see TS 23.203, and hence can be identified as vSRVCC marked video PS bearer in clause 5.3.3.2.2.

UDM/HSS includes STN-SR and C-MSISDN as part of the subscription data sent to the AMF. If the STN-SR is present, this indicates the UE is 5G-SRVCC subscribed. The UDM/HSS also informs the AMF when STN-SR is modified or removed from the subscription.

If the AMF supports the 5G-SRVCC from NG-RAN to 3GPP UTRAN, the AMF shall follow the rules and procedures described in TS 23.502 with the following additions and clarifications:

• Handles UE 5G-SRVCC capability in NAS message.
• Indicates 5G-SRVCC possible to NG-RAN.
• Selects the MME_SRVCC according to the Target ID and 5G-SRVCC HO Indication.
• Supports N26 interface.
• Receives STN-SR and C-MSISDN as part of the subscription data sent by UDM/HSS.
• Indicates 5G-SRVCC HO Indication in Forward Relocation Request message, with either:
  • STN-SR and C-MSISDN during the handover procedure of a non-emergency session; or
  • the Emergency Indication and the equipment identifier during the handover procedure of an emergency session.
• Skips Nsmf_PDUSession_Context procedure for all the PDU sessions.
• Shall not include any UE EPS PDN Connection IE into Forward Relocation Request message.
• When 5G-SRVCC is performed for an emergency session, perform "Location continuity for Handover of an Emergency session from NG-RAN" as specified in TS 23.273.

If the NG-RAN supports 5G-SRVCC from NG-RAN to 3GPP UTRAN, the NG-RAN performs the HO trigger, follow the rules and procedure described in TS 38.300 with the following additions and clarifications:

• Receives the 5G-SRVCC possible indication from AMF.
• Sends the 5G-SRVCC possible indication to Target RAN during Handover procedure.
• Supports the UTRAN cell measurement procedure.
• Determines based on the radio conditions and on UE capability that 5G-SRVCC is to be initiated.
• Indicates 5G-SRVCC HO Indication in HO Required message if the 5G-SRVCC from 5G to 3G is initiated.

The S102 reference point provides a tunnel between MME and 3GPP2 1xCS IWS to relay 3GPP2 1xCS signalling messages. 1x CS signalling messages are those messages that are defined for A21 interface as described in 3GPP2 A.S0008-C [8].

The Sv reference point provides (v)SRVCC support from 3GPP E-UTRAN/UTRAN (HSPA) to 3GPP UTRAN/GERAN. MME/SGSN includes the optional ICS flag if received from the HSS. If (v)SRVCC with priority is supported, MME includes the priority indication if the SRVCC is for IMS-based MPS session. The Sv reference point provides additionally SRVCC support from CS to PS.

The Sv reference point provides 5G-SRVCC support from 3GPP NG-RAN to 3GPP UTRAN.

For 3GPP2 1xCS SRVCC, the S1-MME reference point provides S1 Information Transfer message (TS 36.413) between UE and MME to relay the 3GPP2 1xCS signalling messages. For 3GPP (v)SRVCC, the S1-MME reference point allows handover signalling between E-UTRAN and MME. It is defined in TS 36.300

For 3GPP HSPA SRVCC, the Iu-ps reference point allows handover signalling between UTRAN and SGSN. It is defined in TS 25.413.

For 3GPP (v)SRVCC, the S6a is used to download SRVCC related information to MME during E-UTRAN attach procedure or to inform MME that STN-SR information in the HSS has changed. For 3GPP vSRVCC, the S6a is used to download vSRVCC flag to MME during E-UTRAN attach procedure or to inform MME that vSRVCC flag in the HSS has changed.

For 3GPP SRVCC, either the Gr or the S6d is used to download SRVCC related information to SGSN during UTRAN (HSPA) attach procedure or to inform SGSN that STN-SR information in the HSS has changed.

If the subscriber is allowed to have CS to PS SRVCC, the HSS shall include the "CS to PS SRVCC allowed" indication in the Insert Subscriber Data sent to the MSC Server. The "CS to PS SRVCC allowed" indication may be specific for each VPLMN.

For 5G-SRVCC, the N26 reference point which is defined in TS 23.501 allows handover signalling between AMF and MME_SRVCC with the following additional parameters:

• For a non-emergency session:
  • 5G-SRVCC HO Indication;
  • STN-SR;
  • C-MSISDN;
  • SupportedCodec IE; and
  • MS ClassMark 2
• For an emergency session:
  • Emergency indication; and
  • Equipment identifier.

For 5G-SRVCC, the N2 reference point which is defined in TS 38.300 allows handover signalling between NG-RAN and AMF with the following additional parameter:

• 5G-SRVCC HO Indication.
• 5G-SRVCC possible indication.