Section: 17.5.11 Session Update Procedure
Content: The BM-SC initiates the MBMS session update procedure when the service area for an ongoing MBMS session shall be modified. This procedure is defined only for MBMS broadcast services. The MBMS session update procedure is initiated towards one or more of the GGSNs in the list of downstream nodes in the BM-SC, according to the changes in the service area. NOTE: In addition, when the MBMS Service Area for an ongoing broadcast session is changed in the BM-SC, then GGSN(s) may be added to, or removed from, the list of downstream nodes in the BM-SC. The BM-SC will initiate MBMS session start procedures or MBMS session stop procedures towards these GGSNs accordingly. The attributes that can be modified by the session update procedure are the MBMS Service Area, and the list of downstream nodes for GGSN. When a session update message is received, GGSN will update its MBMS Bearer Context accordingly and inform its downstream SGSNs of the changed MBMS service area. If a list of downstream SGSNs is included in the session update message, GGSN shall initiate a session start procedure towards the new downstream SGSNs, and a session stop procedure towards the SGSNs that have been removed from the list. Figure 27a: MBMS Session Update procedure 1. The BM-SC sends a RAR message to the GGSNs that need to update its session attributes. 2. GGSN stores the updated session attributes in the MBMS Bearer Context, initiates session start, session stop or session update procedures towards the SGSNs in its list of downstream nodes and sends an RAA message to the BM-SC. An AAR message is not mandated for the Gmb application in response to a RAR-RAA command exchange.