There are two big enemies of mobile communication. Weak signal and battery drain. With the LTE technology in place we need to deal with the situation when our signal gets too weak. The higher frequencies have a problem to penetrate walls, currently the radio network is not covering whole country yet, etc.. these problems seems to be only temporary (as the LTE is moving forward) but at this point of time we can’t rely on the LTE network only. When we are out of 4G coverage while we are not calling we’ll simply fallback to CS network and VoLTE supports CS breakout or CS retry so our voice service is still working.
But what if it happens during an ongoing call? Can we handover and still maintain the session? It is possible but not easy as because of the battery drain we can access only one radio network at the time. What we are looking for is called enhanced Single Radio Voice Call Continuity (eSRVCC) and it is described in the GSMA IR.64 and ETSI TS 124 237. (SRVCC is applied during PS to CS access transfer in a single radio system from LTE to 3G/2G. For devices that support active WiFi, and 3G/2G and LTE dual radio, the enhanced Dual Radio Voice Call Continuity (eDRVCC) is applied.)
eSRVCC call flow is probably one of the most complex flows you can encounter in VoLTE. Read firstly about the basic flows VoLTE in IMS. In case of doubts or when more scenarios/details needed please refer to the spec.
To allow this functionality we need to add some more IMS elements in the network:
Access Transfer Control Function (ATCF)
ATCF acts as SIP signalling anchor and is located in the SIP signalling path between P-SCSF and S-CSCF. Very often it is part of the SBC. ATCF controls the ATGW, where the media plane is anchored. During the session transfer, the ATCF establishes a new session with the SCC AS. This new session substitutes the old session between the ATCF and the SCC AS.
Access Transfer Gateway (ATGW)
The ATGW anchors the media session.
Service Centralization and Continuity Application Server (SCC AS)
The SCC AS anchors originated and terminated sessions when using the PS or CS access. It is also responsible for the Terminating Access Domain Selection (T-ADS).