Question: What happens to a User Equipment's (UE) existing Temporary Mobile Subscriber Identity (TMSI) when a new identity is received through the MME-VLR SGs interface, but neither a new TMSI nor an International Mobile Subscriber Identity (IMSI) is included in the information element?
Option 1: The UE automatically generates a new TMSI to replace the old one.
Option 2: The UE's existing TMSI is immediately deleted, and it operates without a TMSI until further update.
Option 3: The UE keeps its old TMSI if one was available before receiving the update.
Option 4: The UE reverts to using its IMSI as the primary identity for all subsequent communications.
Explanation: According to the specifications, if neither a new TMSI nor an IMSI is included in the received information element, the UE will retain its existing TMSI (if any) and continue using it. This ensures continuity of service without interrupting the UE's connectivity due to identity updates.