SRVCC in LTE/EPS/4G
The Evolved Packet System (EPS) – or simply LTE or 4G – does not allow the Circuit Switched (CS) domain, making the traditional CS connections (i.e. voice calls) unavailable over E-UTRAN (i.e. LTE radio). The voice connections over LTE radio access are either over-the-top (OTT) services (like WhatsApp, Skype, Messenger and many others) over the regular mobile internet data connection, or with IP Multimedia Subsystem (IMS) as a control platform. The latter solution gives the mobile network operator control over connection quality (i.e. guaranteed bit rate bearers may be allocated), and also over the pricing. This solution is often referred to as VoLTE, Voice over LTE.
There are cases when a VoLTE connection was initiated with IMS control, but the user device moves away from the cells providing IMS voice support over PS, for example, losing LTE coverage and being left only with 2G or 3G cells available for connections. In order not to drop the call, the Single-Radio Voice Call Continuity (SRVCC) service was introduced. It allows a PS-to-CS domain switch of an ongoing IMS call, i.e. IMS call that started in an LTE cell with Evolved Packet Core (EPC) may be handed over to 2G or 3G cell with classic CS domain nodes, like MSC servers, handling and controlling the ongoing call.
The original SRVCC was subsequently enhanced with options for CS-to-PS transfer, for moving calls in early setup phases, and also allowing video calls to be moved between the domains.
3GPP Release 15 is the first phase of 5G system standardisation. Apart from a New Radio (NR), also the new 5G Core network (5GC) architecture was specified. The interworking scenarios only include interoperability with 4G LTE with EPC, and also WiFi access. As with EPC, the new 5GC only supports PS services, making IMS-based voice (sometimes referred to as Vo5G) the only option for the operator-controlled voice service.
When a User Equipment (UE) moves outside the 5G coverage the ongoing IMS call can be handed over only to 4G LTE cells, using regular PS handover procedures, and will continue as an IMS call over the PS domain. From 4G the good old SRVCC to 2G or 3G is available, but the UE will be in trouble when the 5G signal is getting worse and worse and there are no suitable 4G LTE cells in the area.
NG-RAN to UTRAN SRVCC
Release 16 will add a new option to avoid dropping calls in some scenarios: the SRVCC will be possible from 5G Next Generation Radio Access Network (NG-RAN) to 3G Universal Terrestrial Radio Access Network (UTRAN).
In order to avoid adding new interfaces and protocols, the procedure is in some sense a combination of two existing ones: there is a PS handover from 5G to 4G (on the core network side only) happening together with PS-to-CS SRVCC towards an MSC Server enhanced for SRVCC. The element that sits in-between the 5G and 3G worlds is an MME_SRVCC (i.e. MME Supporting 5G-SRVCC). It needs to support handovers from NG-RAN to E-UTRAN (i.e. 4G RAN) and SRVCC from E-UTRAN to UTRAN. The second part of the procedure is executed without PS handover to 3G which means that, after the 5G SRVCC to UTRAN, all the PDU sessions of the UE are released and will need to be re-established.
This new SRVCC option may prevent some call drops, especially in the 5G stand-alone deployments, where 5G base stations (gNB) are not associated with any 4G base stations (eNB).
Release 16 work is going on with a closure date planned for 2020 and will also bring a number of other enhancements to the constantly evolving 3GPP system.