Along with LTE introduction, 3GPP also standardized Single Radio Voice Call Continuity (SRVCC) in Release 8 specifications to provide seamless continuity when an UE handovers from LTE coverage (E-UTRAN) to UMTS/GSM coverage (UTRAN/GERAN).

Along with LTE introduction, 3GPP also standardized Single Radio Voice Call Continuity (SRVCC) in Release 8 specifications to provide seamless continuity when an UE handovers from LTE coverage (E-UTRAN) to UMTS/GSM coverage (UTRAN/GERAN). With SRVCC, calls are anchored in IMS network while UE is capable of transmitting/receiving on only one of those access networks at a given time.

With SRVCC, a call anchored in IMS core can continue in to be served by UMTS and GSM networks outside of LTE coverage area.

Since its introduction in Release 8, SRVCC has evolved with each new release, a brief summary of SRVCC capability and enhancements are noted below.

3GPP Release 8
Introduces SRVCC for voice calls that are anchored in the IMS core:

• From E UTRAN access to 3GPP2 1xCS
• From E UTRAN/UTRAN (HSPA) access to 3GPP UTRAN/GERAN CS accesses

To support this functionality 3GPP introduced new protocol interface and procedures between MME and MSC (Sv Interface) for SRVCC from E-UTRAN to 3GPP UTRAN/GERAN, between SGSN and MSC (Sv Interface) for SRVCC from UTRAN (HSPA) to 3GPP UTRAN/GERAN and between the MME and the 1x CS IWS (S102 Interface) for SRVCC from E-UTRAN to CDMA 2000 1xRTT.

3GPP Release 9
Introduces the SRVCC support for emergency calls that are anchored in IMS core.  IMS emergency calls, placed via LTE access, need to continue when SRVCC handover occurs from LTE network to GSM/UMTS/CDMA networks. This evolution resolves a key regulatory exception.

This enhancement supports IMS emergency call continuity:

• From E-UTRAN to CDMA2000 1x CS
• From E UTRAN/UTRAN (HSPA) to 3GPP UTRAN/GERAN CS

Functional and interface evolution of EPS entities are needed to support IMS emergency call with SRVCC.

3GPP Release 10
Introduces procedures of enhanced SRVCC :

• Support of mid-call feature during SRVCC handover (eSRVCC)
• Support of SRVCC PS-CS transfer of a call in alerting phase (aSRVCC)

MSC Server assisted mid-call feature enables packet switched (PS) – circuit switched (CS) Access Transfer for UEs not using IMS Centralized Service (ICS) capabilities, while preserving the provision of mid-call services (inactive sessions or sessions using the Conference service).
SRVCC in alerting phase feature adds the ability to perform access transfer of media of an IM session in PS to CS direction in alerting phase for access transfers.

3GPP Release 11
Introduces two new capabilities:

• Single Radio Video Call Continuity for 3G-CS (vSRVCC)
• Single Radio Voice Call Continuity from UTRAN/GERAN to E-UTRAN/HSPA (rSRVCC)

vSRVCC feature provides support of video call handover from E-UTRAN to UTRAN-CS for service continuity when video call is anchored in IMS and UE is capable of transmitting/receiving on only one of those access networks at a given time.

Service continuity from UTRAN/GERAN CS access to E-UTRAN/HSPA was not specified in 3GPP Releases 8, 9 and 10. To overcome it, Release 11 provides support of Voice Call Continuity from UTRAN/GERAN to E-UTRAN/HSPA.

To enable video call transfer from E-UTRAN to UTRAN-CS, IMS/EPC is evolved to pass relevant information to the EPC side and S5/S11/Sv/Gx/Gxx interfaces are enhanced for video bearer related information transfer.

To support the Single Radio Voice Call Continuity from GERAN to E-UTRAN/HSPA, GERAN specifications are evolved to enable MS & BSS to support seamless service continuity when MS handovers from GERAN CS access to E-UTRAN/HSPA for voice call.

To support Single Radio Voice Call Continuity from UTRAN to E-UTRAN/HSPA, UTRAN specifications are evolved to enable the RNC to perform rSRVCC HO and provide relative UE capability information to the RNC.