Question: What is the primary purpose of the MBMS session start message sent by the BM-SC to the MBMS GWs?
Option 1: To update the list of MBMS control plane nodes.
Option 2: To indicate the end of a data stream for an MBMS bearer service.
Option 3: To trigger the modification of QoS attributes in the affected MBMS GWs.
Option 4: To announce the arrival of data for a MBMS bearer service and establish bearer resources.
Explanation: The MBMS session start message is used by the BM-SC to inform the MBMS GWs about the upcoming data transmission for an MBMS bearer service, including its attributes such as QoS, MBMS service area, or MBMS-Cell-List. This allows the MBMS GWs to establish the necessary bearer resources and prepare for the data delivery.