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eps fallback 5g call flow

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The EPS (Evolved Packet System) fallback in the context of 5G refers to a scenario where a 5G device, for some reason, needs to fall back to the LTE (Long-Term Evolution) network, which is part of the 4G infrastructure. This might happen due to several reasons, such as the unavailability of 5G coverage or issues during the initial access to the 5G network. Let's go through the technical details of the EPS fallback call flow:

  1. Initial Access Attempt in 5G:
    • The 5G device will first attempt to attach to the 5G network by sending an Initial Registration Request (RRC Connection Request) to the 5G gNodeB (gNB), which is the base station in the 5G network.
  2. Failed 5G Access:
    • If the 5G device fails to get the required resources or encounters issues during the initial access procedure in the 5G network, it will initiate the fallback process.
  3. Initiation of EPS Fallback:
    • After detecting the need for fallback, the 5G device will send a specific request to the 5G gNB indicating its intention to fallback to LTE. This request is typically an RRC message known as a "RRC Connection Release" with the cause indicating the need for fallback.
  4. Release of 5G Resources:
    • Upon receiving the fallback request, the 5G gNB will release the resources allocated for the 5G connection. This ensures that the resources can be used by other devices or for other purposes in the 5G network.
  5. Selection of Suitable LTE Cell:
    • After initiating the fallback, the 5G device will search for available LTE cells and select a suitable LTE eNB (eNodeB), which is the base station in the LTE network.
  6. Attach Procedure in LTE:
    • The 5G device will then initiate the attach procedure in the LTE network by sending an Initial Attach Request to the selected LTE eNB. This request will contain necessary information like the device's identity, capabilities, and other relevant parameters.
  7. Authentication and Authorization:
    • The LTE core network (EPC - Evolved Packet Core) will authenticate and authorize the 5G device based on the provided information. This involves validating the device's credentials and determining its access rights.
  8. Establishment of LTE Connection:
    • Once the authentication and authorization are successful, the LTE eNB will allocate resources for the device, and a connection will be established between the 5G device and the LTE network.
  9. Data Transfer:
    • With the LTE connection established, the 5G device can now transfer data over the LTE network. This allows the device to maintain connectivity and continue its communication services, albeit at LTE speeds and capabilities.
  10. Fallback Recovery and Monitoring:
  • Operators and network elements will monitor the network conditions to identify the reasons for fallback and take necessary actions to ensure a smooth transition back to 5G when feasible. The 5G device will periodically attempt to re-establish a 5G connection based on the network's availability and conditions.

The EPS fallback in 5G ensures continuity of service for devices by allowing them to switch to the LTE network when encountering issues in the 5G network. This fallback mechanism leverages the existing LTE infrastructure to provide connectivity and maintain service availability for users.