What is SRVCC: Single Radio Voice Call Continuity

SRVCC, Single Radio-Voice Call Continuity is used with VoLTE and it enables voice calls to be transferred to and from legacy 2G & 3G services.

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SRVCC - Single Radio Voice Call Continuity is required within LTE as voice calls often need to be transferred between LTE and legacy circuit switched services like 2G GSM or 3G UMTS as coverage of LTE may not be complete.

As voice calls in LTE are packet data based within the IMS environment, call continuity with legacy circuit switched services is not straightforward and can be handled in various ways. SRVCC, single radio voice call continuity provides a standardised means by which these transfers can be made in a seamless manner with the minimum of dropped calls.

What is SRVCC?

SRVCC, Single radio Voice Call Continuity, is a scheme that enables Inter Radio Access Technology, Inter RAT handover as well as a handover from packet data to circuit switched data voice calls.

By using SRVCC operators are able to make the handovers while maintaining existing quality of service, QoS and also ensuring that call continuity meets the critical requirements for emergency calls.

Some ideas for handover require that the handset has two active radios to facilitate handover. This is not ideal because it requires additional circuitry to enable the two radios to be active simultaneously and it also adds considerably to battery drain.

The SRVCC requires only a single active radio in the handset and requires some upgrades to the supporting network infrastructure.

SRVCC network architecture

The concept for SRVCC was originally included in the 3GPP specification Release 8. Since then it has evolved to take account of the various issues and changing requirements. As a result GSMA recommends that 3GPP Rel 10 or later is implemented as this ensures a considerably lower level of voice interruption and dropped calls.

The network upgrades required to the cellular network are needed in both the LTE network and that of the legacy network or networks. SRVCC requires that software upgrades are required to the MSS - Mobile SoftSwitch subsystem in the legacy MSC - Mobile Switching Centre, the IMS subsystem and the LTE/EPC subsystem. No upgrades are required for the radio access network of the legacy system, meaning that the majority of the legacy system remains unaffected.

The upgrades required for the MSC are normally relatively easy to manage. The MSC is normally centrally located and not dispersed around the network, and this makes upgrades easier to manage. If they are not easily accessible then a new dedicated MSC can be used that has been upgraded to handles the SRVCC requirements.

How SRVCC works

The SRVCC implementation controls the transfer of calls in both directions.

LTE to legacy network handover
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 Evolved Packet Core, EPC of the packet switched LTE network to the legacy circuit switched network.

During the handover process the CSCF within the IMS architecture maintains the control of the whole operation.

SRVCC single radio voice call continuity block diagram showing how calls are transferred
Voice handover using SRVCC on LTE

The SRVCC handover process takes place in a number of steps:

  1. The handover process is initiated by a request for session transfer from the IMS CSCF.
  2. The IMS CSCF responds simultaneously with two commands, one to the LTE network, and the other to the legacy network.
  3. the LTE network receives a radio Access Network handover execution command through the MME and LTE RAN. This instructs the user device to prepare to move to a circuit switched network for the voice call.
  4. The destination legacy circuit switched network receives a session transfer response preparing it to accept the call from the LTE network.
  5. After all the commands have been executed and acknowledged the call is switched to the legacy network with the IMS CSCF still in control of the call.

Legacy network to LTE
When returning a call to the LTE network much of the same functionality is again used.

To ensure the VoLTE device is able to return to the LTE RAN from the legacy RAN, there are two options the legacy RAN can implement to provide a swift and effective return:

  • Allow LTE information to be broadcast on the legacy RAN so the LTE device is able to perform the cell reselection more easily.
  • Simultaneously release the connection to the user device and redirect it to the LTE RAN.

SRVCC interruption performance

One of the key issues with VoLTE and SRVCC is the interruption time when handing over from an LTE RAN to a legacy RAN.

The key methodology behind reducing he time is to simultaneous perform the redirections of RAN and session. In this way the user experience is maintained and the actual interruption time is not unduly noticeable.

It has been found that the session redirection is the faster of the two handovers, and therefore it is necessary for the overall handover methodology to accommodate the fact that there are difference between the two.

SRVCC has been implemented along with VoLTE. In this way calls transferring to and from legacy circuit switched services can managed in a seamless manner without the user realising any difference.

SRVCC is one of the many elements that has been added to the LTE network, and although it does require some upgrades, these are generally relatively straightforward.

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