Question: What triggers the BM-SC to initiate the MBMS session update procedure?
Option 1: When a new MBMS bearer context is created.
Option 2: When service attributes (e.g., Service Area, MBMS cell list, Access indicator or ARP) for an ongoing MBMS session need to be modified.
Option 3: When the BM-SC receives an RAA message from the MBMS GW.
Option 4: When a new MME/SGSN is added to the list of downstream nodes.
Explanation: The BM-SC initiates the MBMS session update procedure when service attributes for an ongoing MBMS session need to be modified, such as changes in the Service Area, MBMS cell list, Access indicator, or ARP. This ensures that the MBMS GWs and downstream MMEs/SGSNs are updated with the new service attributes, maintaining consistency across the network.