VoLTE coverage will be limited for long time and it is clear that initially, without a solution, many Drop Calls could happens. Therefore, in case of poor VoLTE coverage, the UE will need to be able to drop to a better network coverage.
To do this 3GPP defined a procedure called SRVCC. This procedure was originally introduced in 3GPP Release 8, but by Rel 10 onwards has been implemented since the process ensures a considerably lower level of voice interruption and drops calls.
In Rel 11 have been also defined the SRVCC for video calls (vSRVCC) and reverse SRVCC Handover (CS call to IMS PS). Basically, the SRVCC is a vertical handover mechanism from an IMS-based LTE Network to a legacy one (2G/3G).
The procedure must to ensure that the voice service be always available, whatever the coverage technology. This is very important if you look at the phenomenon also in User Experience perspective.
Unlike the CSFB process, that passes from LTE to 2G/3G before, or during, the setup of a voice call, the SRVCC involves the transfer of a call when it is already active (in LTE).
Note: Single Radio refers to the capacity of the terminal (UE) to transmit/receive at any given time on only one of the access networks available. This eliminates the need for the UE to have multiple radio access technologies and, therefore, the consequent advantage of a lesser complexity of the terminal.
Single Radio Voice Call Continuity – SRVCC
The SRVCC is an LTE functionality that allows a VoIP/IMS call in the LTE packet domain to be moved to a legacy voice domain (GSM/UMTS or CDMA 1x).
How Does It Works?
Handover from LTE to the legacy network is required when the user moves out of the LTE coverage area.
Using SRVCC, the handover is undertaken in two stages:
- Radio Access Technology transfer: The handover for the radio access network and this is a well-established protocol that is in use for transfers from 3G to 2G for example.
- Session Transfer: The session transfer is the new element that is required for SRVCC. It is required to move the access control and voice media anchoring from the EPC (Evolved Packet Core) of the packet switched LTE network, to the legacy network.
During the handover process the CSCF within the IMS architecture maintains the control of the whole operation. Furthermore, if the legacy 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 (MultiThread). The data sessions may be handed back to LTE when the mobile moves back into LTE coverage and concludes its voice call.
The SRVCC handover process takes place in a number of steps. Let’s describe the flow:
- The overall procedure starts with a MEASUREMENT REPORT by the UE
- The E_UTRAN recognizes that there are the conditions to perform the SRVCC procedure, and then sends a HANDOVER REQUEST to the MME, also specifying a target cell in the 2G/3G network
- Through the Sv interface, the MME triggers the SRVCC procedure by sending to the MSC server the HANDOVER REQUEST
- The MSC then ALLOCATES the CS RESOURCES and then initiates the IMS SESSION TRANSFER PROCEDURES to allow service continuity
- During the SWITCH TO CS domain in the IMS core, the MSC, through the Sv interface, sends the SRVCC HANDOVER RESPONSE to the MME informing it of the network’s switching
- The MME sends the HANDOVER COMMAND to the E-UTRAN
- The E-UTRAN orders to the UE MOVE into 2G/3G network – HANDOVER COMMAND
- The UE SWITCHES to legacy network (CS)
- The GERAN/UTRAN DETECTS the HANDOVER and informs the MSC that the HANDOVER is COMPLETED
- The MSC, in turn, always through the Sv interface, sends a message to the MME informing it that the SRVVC PROCESS is COMPLETE
- The MME informs the E-UTRAN by sending the RELEASE PS RESOURCES message