Question: In E-UTRAN/EPC architecture, how does the NAS Node Selection Function (NNSF) in eNB or HeNB GW typically determine a UE's MME association during an attach or tracking area update in a non-registered TA?
Option 1: Based solely on the UE's Permanent Identifier (e.g., IMSI)
Option 2: Based on the UE's S-TMSI and selected PLMN
Option 3: Based on its GUMMEI and selected PLMN
Option 4: Randomly among available MMEs for load balancing
Explanation: During initial attach or when a UE updates its tracking area in a non-registered state, the network lacks an existing temporary identifier context. Thus, the NNSF relies on more broadly identifiable parameters. The GUMMEI (Globally Unique MME Identifier) paired with the selected PLMN (Public Land Mobile Network) provides a unique network identifier and the UE's chosen network, respectively, allowing for accurate MME association without relying on temporary identifiers that may not yet be assigned or recognized in this context. This approach ensures proper routing via the S1 interface from the outset of the UE's connection setup.