Hello All,
In this blog, I am going to explain voice call continuity in LTE network. There are number of proposals given for voice call continuity during early stage of VOLTE. Single radio voice call continuity is standardized by 3GPPP.
In this blog, I am going to explain voice call continuity in LTE network. There are number of proposals given for voice call continuity during early stage of VOLTE. Single radio voice call continuity is standardized by 3GPPP.
What is Single Radio Voice Call Continuity ?
SRVCC stands for Single Radio Voice Call Continuity. Putting it simple, it is a Handover technology between "VoIP over IMS in LTE" and Voice Call (CS) in a legacy system (e.g, WCDMA). It means it is for Handover between a Packet call in LTE and a Circuit Call in a legacy system (WCDMA).
3GPP 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).
Below example diagram shows SRVCC from LTE network to UMTS network.
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.
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.
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.
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.
Hope you guys enjoyed this blog. Please provide feedback so I can further incorporate that in next blogs.
Below example diagram shows SRVCC from LTE network to UMTS network.
SRVCC Evolution
Right from the introduction of basic SRVCC in rel 8, there have been many feature additions. Followings are some of the features used nowadays but list will go longer in coming days.| Name | 3GPP Release | Description |
| Basic SRVCC | Rel 8 | Call Continuity from E-UTRAN to UTRAN/GRAN |
| aSRVCC | Rel 10 | Packet switched to CS call transfer during Alerting Phase |
| eSRVCC | Rel 10 | Enhanced SRVCC (MSC Server assisted Mid Call Feature) |
| vSRVCC | Rel 11 | Video SRVCC |
| rSRVCC | Rel 11 | reverse SRVCC from UTRAN/GRAN to E-UTRAN |
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.
Hope you guys enjoyed this blog. Please provide feedback so I can further incorporate that in next blogs.
