Question: What is the primary purpose of the MBMS session start message sent by the BM-SC to the GGSN?
Option 1: To trigger the update of MBMS session attributes in the affected GGSNs.
Option 2: To indicate the end of the data stream for an MBMS bearer service.
Option 3: To trigger the bearer resource establishment and announce the arrival of data for a MBMS bearer service.
Option 4: To request the GGSN to use IP unicast mechanism for user plane data distribution.
Explanation: The MBMS session start message is used by the BM-SC to trigger the bearer resource establishment and announce the arrival of data for a MBMS bearer service, along with the attributes of the data to be delivered, such as QoS or MBMS service area. This allows the GGSN to prepare for the incoming data stream and ensure that it is delivered correctly to the intended recipients.