Question: In a Network Initiated Application (SIP) De-registration scenario for administrative reasons (e.g., subscription termination), which home network element may initiate the de-registration procedure using the Cx-Deregister interface, leveraging its existing knowledge of the serving S-CSCF?
Option 1: Home Location Register (HLR)
Option 2: Home Subscriber Server (HSS)
Option 3: Serving Call Session Control Function (S-CSCF)
Option 4: Packet Data Network Gateway (PDN-GW)
Explanation: The correct answer is the Home Subscriber Server (HSS) because, as per the provided context, the HSS already knows which S-CSCF is serving the user and utilizes the Cx-Deregister interface for initiating the de-registration procedure. This highlights the HSS's role in managing subscriber information and its capability to trigger administrative actions across the network, impacting SIP registrations.