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Whats is Circuit Switched Fallback?

What is a Circuit Switched Fallback (CSFB)?

Circuit Switched Fallback (CSFB) is a technology used in mobile networks to enable voice calls for devices that are connected to a 4G LTE (Long-Term Evolution) network but do not support native voice over LTE (VoLTE). CSFB allows these devices to “fallback” to a circuit-switched 2G or 3G network for voice calls, as LTE is primarily designed for high-speed data transmission.

How A Circuit Switched Fallback works
  • LTE Data Connection: Devices connected to an LTE network primarily use it for high-speed data services, such as internet browsing, video streaming, and app downloads. Voice calls, however, are not natively supported over LTE.
  • Voice Call Initiation: When a CSFB-capable device initiates a voice call, the LTE network sends a signal to the device indicating that it needs to switch to a circuit-switched network for the call.
  • Network Handover: The device then transitions from the LTE network to a circuit-switched 2G or 3G network for the duration of the voice call. This handover typically occurs seamlessly and transparently to the user.
  • Voice Call Establishment: Once the device is connected to the circuit-switched network, it establishes the voice call using the traditional circuit-switched technology employed by 2G or 3G networks.
  • Data Connection Restoration: After the voice call is completed, the device switches back to the LTE network to resume high-speed data services.

Circuit Switched Fallback ensures that devices without VoLTE support can still make and receive voice calls, even when connected to an LTE network. It utilizes the existing 2G or 3G infrastructure to provide voice call capabilities for such devices, leveraging the widespread coverage and reliability of circuit-switched networks.

It’s important to note that Circuit Switched Fallback is an interim solution as the industry transitions to fully voice-capable LTE networks through technologies like Voice over LTE (VoLTE) and Voice over New Radio (VoNR). These technologies enable voice calls to be transmitted as data packets over LTE or 5G networks, providing higher call quality and improved efficiency.