CSFB process

When the terminal is turned on, it is given priority to stay in the LTE network, and the voice and other services fall back to the circuit domain through CSFB technology, and then return to the LTE network after the service is over.

Data services do not need to fall back, and are directly carried by LTE.

Short message service does not need to be returned, and short messages are routed to LTE network through SGs interface (the interface between MME and MSC).

CSFB Boot Joint Accessories and Location Area Update Process

When UE attaches to and stays in LTE network, in order to receive the called party and use the short message service, it is necessary to perform joint attachment and joint location update through SGs interface, and the registration status and location information are stored in CS domain.

CSFB call signaling flow

In LTE network, after the user presses the key, UE initiates a fallback request, which is guided by LTE network without going through SGs interface.

After falling back to 2/3G network, the call flow is the same as that in traditional circuit domain.

CSFB calls it signaling flow.

The call is routed to SGs MSC with joint location update, and then transferred to LTE network for paging through SGs interface. After the UE responds to the page, the LTE network guides it to measure and search for a suitable 2G cell.

After the UE retreats to the 2G network, the called process is the same as the traditional circuit domain process.