It’s a bit sad but unfortunately still true, in some places there is still better GSM coverage than LTE. Network operators using VoLTE thus need to be able to hand-over an ongoing VoLTE call to a GSM circuit switched channel when the user leaves the LTE coverage area. The method to do this is referred to as Single Radio Voice Call Continuity (SR-VCC) and the basic flavor of it has probably been deployed by all VoLTE operators that have a legacy radio network out there. When taking a closer look one notices that there is a surprising variety of states a voice call can be in and the basic flavor can’t handle all of them. Let’s have a closer look.
The simplest case is that the call is already established, i.e. the terminating subscriber has accepted the call and a speech path is present. That’s what the default SR-VCC implementation can handle.
Another state a voice call can be in is the alerting phase. Depending on how quickly the terminating subscriber accepts the call, this phase can take many seconds which increases the probability that the subscriber runs out of LTE coverage while the call is being alerted. The initial 3GPP specifications only defined the SR-VCC procedure for calls that have been fully established but not for the alerting phase. If the subscriber is running out of LTE coverage during the alerting phase the call would drop. As a consequence 3GPP later specified an extension referred to as Alerting-SR-VCC (aSRVCC) to close this gap. Both the network and the mobile device have to support the extension and mobile devices signal support to the IMS network with a ‘+g.3gpp.srvcc-alerting’ tag in the SIP ‘Contact’ header.
Other states a call can be in when an SR-VCC is required are for example established conference calls, call hold, or the user being active in one call while a second call is currently incoming (call waiting). To allow for these special call states to be handed over into a circuit switched connection as well, 3GPP has additionally specified SRVCC for mid-call services. Like above, both the network and the device have to support the functionality and the mobile device announces support with a ‘+g.3gpp.mid-call’ tag in the SIP ‘Contact’ header.
One of the latest SR-VCC enhancements in 3GPP TS 24.237 is to support the handover from IMS to a circuit switched channel before the alerting phase (bSRVCC), i.e. the few seconds between the initial SIP ‘Invite’ message and the SIP ‘180 Ringing’ message. And if you are really a daring operator you can try yourself at Reverse-SRVCC (rSRVCC) which hands over an ongoing GSM or UMTS circuit switched call to VoLTE when LTE coverage is regained.
In practice there are few if any network operators with legacy radio technologies who do not support at least the basic form of SR-VCC for established one to one voice calls. I strongly suspect that support of the more advanced features is less wide spread and it will depend on how fast LTE networks reach similar coverage level as legacy technologies if the more advanced features which bring significant complexity in the core network will be widely deployed in practice.