12.1 Introduction

24.2373GPPIP Multimedia (IM) Core Network (CN) subsystem IP Multimedia Subsystem (IMS) service continuityRelease 17Stage 3TS

This clause specifies the procedures for PS-CS SRVCC and vSRVCC access transfer. Procedures are specified for the SC UE, the SCC AS, the EATF, the MSC server enhanced for ICS, the MSC server enhanced for SRVCC and the ATCF.

For SC UE or SCC AS not supporting ICS procedures, PS-CS SR-VCC access transfer enables transfer of

– single session with active speech media component; and

– up to one session with active speech media component and up to one session with inactive speech media component when the MSC Server assisted mid-call feature is supported.

For SC UE or SCC AS, PS-CS access transfer in vSRVCC enables the transfer of a single session with active speech and video media components.

In order to fulfil the requirements for the PS to CS SRVCC for calls in alerting phase or vSRVCC access transfer for calls in alerting phase, the SC UE needs to be:

– engaged in a session with speech media component in early dialog state before PS to CS SRVCC access transfer is performed; or

– engaged in a session with active speech media component and active video media component in early dialog state before vSRVCC access transfer is performed

according to the following conditions:

– a SIP 180 (Ringing) response for the initial SIP INVITE request to establish this session has been sent or received; and

– a SIP final response for the initial SIP INVITE request to establish this session has not been sent or received.

If one of the dialogs meets the above conditions then after successful completion of the PS to CS SRVCC procedures:

– Subclauses 12.2.2, 12.2.3, 12.2.3A and 12.2.4 shall be followed for a SC UE engaged in one or more ongoing sessions.

– Subclauses 12.2.3B and 12.2.4 shall be followed for a SC UE that is engaged in a session in early dialog state.

If one of the dialogs meets the above conditions then after successful completion of the vSRVCC procedures:

– Subclauses 12.2A.2, 12.2A.3, 12.2A.4 and 12.2A.6 shall be followed for a SC UE engaged in one or more ongoing sessions.

– Subclauses 12.2A.5 and 12.2A.6 shall be followed for a SC UE that is engaged in a session in early dialog state.

NOTE: The UE determines from the handover command sent by the eNodeB as specified in 3GPP TS 36.331 [62] that the network intends to perform SRVCC handover or vSRVCC handover, based upon whether the radio resources allocated are for a TS11 bearer or a BS30 bearer.