Section: 20.3.2 Session update procedure
Content: The BM-SC initiates the MBMS session update procedure when service attributes (e.g. Service Area, MBMS cell list, Access indicator or ARP) for an ongoing MBMS session shall be modified. The MBMS session update procedure update procedure is initiated towards one or more of the MBMS GWs 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 MBMS GW(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 MBMS GWs accordingly. The attributes that can be modified by the RAR message are the MBMS Service Area, the MBMS-Cell-List, the ARP, the Access indicator and the list of MBMS control plane nodes (MMEs, SGSNs). When a session update message is received, the MBMS GW updates its MBMS Bearer Context accordingly and informs its downstream MMEs/SGSNs of the changed service attributes. If a list of MBMS control plane nodes (MMEs, SGSNs) is included in the session update message, MBMS GW shall initiate a session start procedure towards the new MMEs/SGSNs, and a session stop procedure towards the MMEs/SGSNs that have been removed from the list. Figure 20.3.2.1: MBMS Session Update procedure 1. The BM-SC sends an RAR message to all MBMS GWs listed in the "list of downstream nodes" parameter of the affected MBMS Bearer Context to indicate that the MBMS session is updated. 2. The MBMS GW stores the new session attributes in the MBMS Bearer Context, initiates session start, session stop or session update procedure towards the MMEs/SGSNs in its list of MBMS control plane nodes and sends an RAA message to the BM-SC. An AAR message is not mandated for the SGmb application in response to an RAR- RAA command exchange.