Question: What is a key function of the Registrar / Application Service Selection entity in supporting mission-critical services, aside from registration and location management?
Option 1: Managing Quality of Service (QoS) for non-SIP transactions
Option 2: Providing authentication for identities within SIP signalling
Option 3: Directly handling media transfer for all mission-critical communications
Option 4: Exclusively storing application service selection information in the public network's SIP database
Explanation: Authentication of identities within SIP signalling is crucial for securing mission-critical services. The Registrar / Application Service Selection entity supports this function, ensuring that only authenticated users can access and utilize these critical services, thereby maintaining the integrity and security of the communications network.