Question: What action does the MME take if an eNB's S1 SETUP REQUEST includes a PLMN Identity that the MME cannot identify?
Option 1: The MME requests additional PLMN information from the eNB.
Option 2: The MME rejects the eNB's S1 Setup Request with a cause value (e.g., “Unknown PLMN”)
Option 3: The MME proceeds with the setup, ignoring the unrecognized PLMN Identity.
Option 4: The MME sends a query to nearby eNBs for PLMN verification.
Explanation: In E-UTRAN, maintaining accurate and recognized Public Land Mobile Network (PLMN) identities is crucial for successful S1 setup between the evolved Node Base station (eNB) and the Mobility Management Entity (MME). If the MME cannot identify any of the PLMNs listed in the eNB's S1 SETUP REQUEST, it must reject the request to prevent potential security breaches or misconfigurations. The rejection is accompanied by a specific cause value, such as “Unknown PLMN”, to inform the eNB of the reason for the failure. This ensures that only recognized and authorized networks can establish connections, upholding the integrity of the cellular network infrastructure.