Understanding SRVCC – Part 1 (Updated)

It’s time to set aside the voice issues and embrace LTE!

SRVCC stands for Single Radio Voice Call Continuity. This is the official 3GPP solution for voice over LTE. There is CSFB too, but it’s pretty lousy and supposed to be an intermittent solution. SRVCC is the long term solution proposed by keeping IMS network in view. It will be a perfect solution when the whole work has IMS deployed and still have 3G/2G networks running. There was a significant debate about VoLTE solution but seems like world is moving towards IMS, atleast in my little mind. :-)

SRVCC is supposed to continue, an IMS call on LTE network, seamlessly over a 3G/2G network. 3GPP TS 23.216 deals with this solution. Major requirement is NOT to have UE connect to 2 different RAT’s simultaneously. This means more battery life on handsets! 

For SRVCC to work there is a need for Sv interface between MME/SGSN and MSC. MSC needs to communicate with IMS network over ISUP interface? This means a software upgrade on MME/SGSN/MSC is needed. No special changes are need on SGW/PGW/GGSN. No additional functionality is required on EUTRAN, other than it has to detect UTRAN and GERAN. HSS needs to send couple of additional flags. PCRF is required to enforce a bearer over QCI 1, dedicated voice bearer? UE is expected to send few SRVCC IEs during attach procedure and is expected to be IMS capable. 

Assuming all the above is done and IMS network is in place, a UE will now make and receive voice calls over LTE via IMS network. While on a voice call over LTE, if EUTRAN detects that UE is moving towards a UTRAN/GERAN, it will initiate a handover procedure for UE towards the SGSN. A PS handover will be initiated for all bearers except for the one with QCI=1. MME will then initiate a PS-CS handover for QCI =1 bearer towards MSC. At this point MME is dealing with CS handover and PS handover simultaneously. Assuming everything went right, all PS bearers are successfully handed over to SGSN and CS bearer is handed over to MSC for continuation of 3G. The voice call now continues via MSC -> RAN -> UE. While in 3G if UE makes a call, the call lands on MSC then on IMS? 

This is a very brief description will post detailed call flows soon, refer to 3GPP TS 23.216 for more details.

More Links

Qualcomm Tests SRVCC
http://www.engadget.com/2012/02/02/qualcomm-chips-complete-first-successful-voip-over-lte-to-wcdma/
http://gigaom.com/broadband/qualcomm-ericsson-just-brought-mobile-calls-into-the-ip-age/