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eps fallback 3gpp

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When a mobile device is connected to an LTE network, it communicates with the evolved NodeB (eNB) and the Evolved Packet Core (EPC). However, there are situations where the LTE connection may be lost, such as when the device moves out of LTE coverage or encounters other network issues. In such cases, the device may need to fall back to a 2G or 3G network to maintain connectivity.

The EPS fallback procedure involves the following key components:

  1. Tracking Area Update (TAU): In LTE, a mobile device periodically performs a Tracking Area Update to inform the network about its location. If the device moves to an area where LTE coverage is not available, it may trigger a TAU procedure to notify the network of its new location.
  2. Fallback Decision: When the network detects that the device is no longer reachable via LTE, it makes a decision to initiate the fallback procedure. This decision can be based on factors such as signal strength, available networks, and network policies.
  3. CSFB (Circuit-Switched Fallback): The Circuit-Switched Fallback procedure is part of the fallback mechanism. It involves switching the device from the packet-switched LTE mode to the circuit-switched mode of 2G (GSM) or 3G (UMTS) networks. CSFB ensures that voice and circuit-switched services can be maintained even if the primary LTE connection is lost.
  4. EPS Bearer Modification: The EPS bearer is a logical connection between the mobile device and the LTE network for data communication. When falling back to a legacy network, the EPS bearer may need to be modified to adapt to the capabilities of the 2G or 3G network.
  5. Handover or Redirection: The network may perform a handover or redirection procedure to guide the device from LTE to the selected 2G or 3G network.