Question: What action does the MME take when it receives a detach request from a UE in a CSG cell for which the UE has no valid CSG subscription, and the detach type is "IMSI detach" with an active PDN connection for emergency bearer services?
Option 1: The MME initiates the detach procedure without sending any message to the UE.
Option 2: The MME sends a DETACH ACCEPT message and deactivates all non-emergency EPS bearers by initiating an EPS bearer context deactivation procedure, but only if the UE has no active PDN connections.
Option 3: The MME sends a DETACH REQUEST message including EMM cause #25 "not authorized for this CSG" without taking any further action.
Option 4: The MME sends a DETACH ACCEPT message and deactivates all non-emergency EPS bearers by initiating an EPS bearer context deactivation procedure.
Explanation: According to the 3GPP specification, when the detach type is "IMSI detach" and the UE has a PDN connection for emergency bearer services active, the MME shall send a DETACH ACCEPT message and deactivate all non-emergency EPS bearers by initiating an EPS bearer context deactivation procedure. This ensures that the UE's emergency services remain active while other non-emergency services are deactivated.