Question: What triggers the increment of the 'Number of check IMEI unknown equipment responses' performance counter in a Core Network (CN) Packet Switched (PS) domain?
Option 1: Receipt of a successful 'MAP_CHECK_IMEI' response from the EIR with a known equipment identifier.
Option 2: A DNS query timeout during a domain address translation for an SGSN-initiated request.
Option 3: Receipt of a 'MAP_CHECK_IMEI' response from the EIR with a 'user error' parameter indicating an unknown equipment.
Option 4: Completion of a successful packet switching handover between two SGSNs.
Explanation: This performance counter specifically measures responses from the Equipment Identity Register (EIR) that indicate an unknown equipment, based on the 'MAP_CHECK_IMEI' response with a 'user error' parameter. This directly relates to equipment identity checks in the core network, rather than DNS queries or handover procedures. Therefore, the correct trigger is the receipt of such a response indicating an unknown equipment.