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circuit switch fallback

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Circuit-switched fallback (CSFB) is a technology used in mobile communication networks to enable voice services for Long-Term Evolution (LTE) networks. LTE is a packet-switched technology designed for high-speed data transmission, and it doesn't natively support circuit-switched voice calls, which are traditionally handled by circuit-switched technologies like Global System for Mobile Communications (GSM) or Code Division Multiple Access (CDMA).

  1. LTE and CS Domain Separation:
    • LTE networks use packet-switched technology for data services, while GSM or CDMA networks use circuit-switched technology for voice services.
    • The LTE and CS domains are separate in terms of network architecture.
  2. Initiating a Voice Call:
    • When a user initiates a voice call on an LTE-capable device, the LTE network needs to find a suitable circuit-switched network to handle the voice call.
  3. Tracking Area Update (TAU) or Routing Area Update (RAU):
    • The mobile device initiates a Tracking Area Update (for LTE) or Routing Area Update (for GSM) procedure to inform the network about its current location.
  4. Network Decides on the Appropriate Network:
    • The LTE network determines whether there is a need for a voice call and, if so, it decides to hand over the call to a circuit-switched network.
  5. Redirecting the Call:
    • The LTE network sends a redirection command to the mobile device, instructing it to camp on a circuit-switched network (GSM or CDMA).
  6. Temporary Handover to Circuit-Switched Network:
    • The mobile device releases its LTE connection and establishes a connection with the circuit-switched network for the duration of the voice call.
  7. Voice Call Establishment:
    • The voice call is established using the circuit-switched technology.
  8. End of Voice Call:
    • Once the voice call is completed, the mobile device releases the circuit-switched connection and re-establishes the LTE connection for data services.
  9. Seamless Transition:
    • The transition between LTE and the circuit-switched network is designed to be as seamless as possible to ensure a smooth user experience.

CSFB is considered a temporary solution, as mobile networks transition to more advanced technologies like Voice over LTE (VoLTE), which enables voice calls to be carried over the LTE packet-switched network directly. VoLTE provides better efficiency, improved call quality, and faster call setup times compared to CSFB.