Section: 5.10.4 MME triggered Serving GW relocation
Content: The MME triggered Serving GW relocation procedure for E-UTRAN is depicted in figure 5.10.4-1. The procedure allows the MME to trigger Serving GW relocation due to events other than those described in the mobility scenarios (see clause 5.3.3.1 and clause 5.5.1). Such scenario exists during the establishment of a SIPTO at local network PDN connection with stand-alone GW or during the establishment of a SIPTO above RAN PDN connection. Figure 5.10.4-1: MME triggered Serving GW relocation 1. The Serving GW relocation procedure may be triggered by the MME due to events that may benefit from a Serving GW relocation other than those described in the mobility events scenarios. 2. If the MME determines that the Serving GW is to be relocated then it selects a new Serving GW according to clause 4.3.8.2. The MME sends a Create Session Request (bearer context(s) with PDN GW addresses and TEIDs (for GTP-based S5/S8) or GRE keys (for PMIP-based S5/S8) at the PDN GW(s) for uplink traffic, eNodeB address(es) and TEIDs for downlink user plane for the existing EPS bearers, the Protocol Type over S5/S8, Serving Network) message per PDN connection to the new Serving GW. The new Serving GW allocates the S-GW addresses and TEIDs for the uplink traffic on S1_U reference point (one TEID per bearer). The Protocol Type over S5/S8 is provided to Serving GW which protocol should be used over S5/S8 interface. If the PDN GW requested UE's location info, the MME also includes the User Location Information IE in this message. If the PDN GW requested UE's User CSG information (determined from the UE context), the MME includes the User CSG Information IE in this message if the User CSG Information has changed. 3. The new Serving GW assigns addresses and TEIDs (one per bearer) for downlink traffic from the PDN GW. The Serving GW allocates DL TEIDs on S5/S8. It sends a Modify Bearer Request (Serving GW addresses for user plane and TEID(s), Serving Network) message per PDN connection to the PDN GW(s). The S-GW also includes User Location Information IE and/or UE Time Zone IE and/or User CSG Information IE if it is present in step 2. The PDN GW updates its context field and returns a Modify Bearer Response (Charging Id, MSISDN, etc.) message to the Serving GW. The MSISDN is included if the PDN GW has it stored in its UE context. The PDN GW starts sending downlink packets to the new GW using the newly received address and TEIDs. These downlink packets will use the new downlink path via the new Serving GW to the eNodeB. This step is performed for all connected PDN-GWs for that specific UE. 4. The new Serving GW sends a Create Session Response (Serving GW addresses and uplink TEID(s) for user plane) message back to the MME. The MME starts a timer, to be used in step 6. 5. The MME sends a Bearer Modify Request (Serving GW addresses and uplink TEID(s) for user plane, Secondary RAT usage data request) message to eNodeB. The eNodeB starts using the new Serving GW address(es) and TEID(s) for forwarding subsequent uplink packets and sends a Bearer Modify Response message to the MME. If the PLMN has configured secondary RAT usage reporting, the MME may request the eNodeB for Secondary RAT usage data in the Bearer Modify request message. If the eNodeB has Secondary RAT usage data, it provides it in the Bearer Modify Response message. 5a. If Secondary RAT usage data is included in the previous message and if PDN GW Secondary RAT usage reporting is active, the MME uses the Secondary RAT usage data reporting procedure as described in clause 5.7A.3 figure 5.7A.3-2 to provide this information to the Serving GW and PDN GW. The MME includes a flag that the Serving GW shall not process this information and forward it to the PDN GW. 6. When the timer has expired after step 4, the MME releases the bearer(s) in the old Serving GW by sending a Delete Session Request message (Cause, Operation Indication, Secondary RAT usage data). The operation Indication flag is not set, that indicates to the old Serving GW that the old Serving GW shall not initiate a delete procedure towards the PDN GW. The old Serving GW acknowledges with Delete Session Response messages. The MME includes the Secondary RAT usage data if the eNodeB had provided it to the MME in step 5. If the Serving GW relocation procedure towards a new Serving GW fails, based on operator policy, the MME should go back to the old Serving GW and disconnects the affected PDN connections (e.g. SIPTO at local network) that are no longer allowed to remain connected.