Single Radio Voice Call Continuity (SRVCC)
Hello Folks, Welcome to Learnizo Global. This article will introduce you to a high-level view of SRVCC. Let us imagine a subscriber, roaming in a 4G LTE mobile network (E-UTRAN ”Evolved UMTS Terrestrial Radio Access Network”) that is in its early deployment phase over a legacy mobile network (GERAN/UTRAN” GSM EDGE Radio Access Network/ UMTS Terrestrial Radio Access Network”). In this phase, E-UTRAN covers only hot spots and is limited in continuous coverage. When this subscriber, in a call moves out of the LTE coverage, what will happen to the call? Obviously, the call will drop. This is applicable to all such subscribers. More the number of dropped calls, the network loses more subscribers. This would fail the basic idea of serving customers with the new technology being deployed. In order to reduce the number of dropped calls, 3GPP proposed SRVCC handovers.
Single Radio Voice Call Continuity (SRVCC) allows an ongoing Mobile to Mobile voice call over LTE to be handed over to the circuit-switched domain in GSM or WCDMA when losing LTE coverage, in order to reduce the number of dropped calls.
SRVCC allows subscribers to continue an ongoing call when roaming from an E-UTRAN to a UTRAN or GERAN. Putting it simply, it is a Handover technology between “VoLTE” and Voice Call (Circuit Switched) in a legacy system (GERAN/UTRAN). It means it is for Handover between a Packet call in LTE and a Circuit Call in a legacy system.
SRVCC Architecture
SRVCC Explained
Based on measurements provided by the UE (User Equipment), the E-UTRAN determines that SRVCC handover is required and sends a handover request to the MME (Mobility Management Entity), identifying a target cell in the 2G/3G network. The MME then triggers the SRVCC procedure in the MSC (Mobile Switching Center) server by sending a handover request over the Sv interface. The MSC coordinates the handover of the UE to the legacy network with the transfer of the voice call session from packet switching to circuit switching in the IMS (IP Multimedia Subsystem). The MSC creates the necessary circuit-switched bearers and then sends a handover response message to the MME, containing the handover command message required for the UE to access the 2G/3G network. Throughout the process, overall control of the call session is maintained by the IMS. If the 2G/3G network and handset are able to carry packet-switched data simultaneously with circuit-switched voice, a user’s data sessions can be handed over at the same time as the voice call. The data sessions may be handed back to LTE when the mobile moves back into LTE coverage and concludes its voice call.
SRVCC Evolution
The initial SRVCC mechanism is not so well optimized especially in terms of the core network process. Evolution of SRVCC technology over a period of time includes the below features. The list still grows longer.
| Common Name | 3GPP Release | Features |
| Basic SRVCC | Release 8 | Call Continuity from E-UTRAN to UTRAN/GRAN |
| aSRVCC | Release 10 | Packet switched to Circuit Switched call transfer during Alerting Phase |
| eSRVCC | Release 10 | Enhanced SRVCC (Support for MSC Server assisted Mid Call Feature) |
| bSRVCC | Release 1999 | SRVCC at Pre-Alerting(Before Ringing) Phase |
| vSRVCC | Release 11 | Video SRVCC |
| rSRVCC | Release 11 | SRVCC from UTRAN/GRAN to E-UTRAN |
SRVCC is very similar to general Handover. But going into detail, you may have some questions.
How does the network know whether it has to initiate SRVCC or general PS handover? Or
How UE knows whether it should convert its IMS call to CS (AMR) call?
What would happen to the IMS/SIP call after SRVCC?
This all depends on the “Decision Making” process in UE and Network.
The answer to these questions may not be clearly stated in the 3GPP specification and there might be some variations depending on Network Operator’s requirement. We shall discuss more different SRVCC decision-making scenarios in our further articles. We will also discuss the difference between CSFB (Circuit Switched Fall Back) and SRVCC. Till then, stay safe and happy learning with Learnizo Global.