Most of the mobile operators tries for cost saving due to several reasons. If an operator recently deployed an LTE network, then it should provide voice service over the new built network. But, how? Of course there is an excellent solution for that, it is Voice over LTE. But it is not so easy and too expensive because of an extra network structure must be added, which is IMS (IP Multimedia Subsystem) for Voice over LTE.
But, there is another cheaper solution, but with a little bit disadvantage.
Circuit Switched Fall Back feature or with a shortened version CSFallback (or CSFB). If we check the feature name “fall back”, so we need to go back to the legacy networks to make voice call.
What is disadvantage which we mentioned above? Answer is “call setup delay”.
Imagine, if you want to make a voice call, the UE should leave the LTE network and find suitable cell in GSM or WCDMA networks and camp on it.
Next step is to start a voice call which normally we do directly from GSM or WCDMA networks. This transition period causes the setup delay from LTE to the legacy networks.
CSFB can be redirected to GSM, CDMA and WCDMA networks.
CSFB is applicable for ongoing PS service if UE needs to make CS session. The PS session will moved to the target network with handover or redirection.
CSFB can be redirected to legacy networks with measurement based handover, blind based handover or redirection.
If a UE initiates a CS service, the MME instructs the UE to fall back to a legacy network before the UE performs the service and it stays in that network during the CS session.
There are some important points in CSFB call flow which need to remind, only these points will be highlighted on the following image not to confuse in messaging swamp.
Major requirements:
SGs interface:
Need to configure the interface between MME and MSC.
Mobility management and paging between the LTE and the target CS network.
Transmits combined EPS/IMSI attach/detach and combined TAU/LAU related messages.
UE:
Need to support CSFB feature.
Need to support combined EPS/IMSI attach/detach and combined TAU/LAU.
MME:
Need support SGs interface to the MSC/VLR.
Need to support SMS over SGs.
Need to support combined EPS/IMSI attach/detach and combined TAU/LAU.
MSC:
Need to supports combined EPS/IMSI attach /detach.
Need to supports SMS over SGs.
Forwards paging messages transmitted through the SGs interface.
EUTRAN/UTRAN/GERAN:
Need to be configured CSFB related functions