UE Source eNodeB Target BSS Source MME Target Target SGSN Serving GW Serving GW Uplink and Downlink User Plane PDUs 1. Handover Command 2. HO from E-UTRAN Command PDN GW HSS 4. GERAN A/Gb Access Procedures 5. XID Response Downlink User Plane PDUs Sending of uplink data possible Only if ”Direct Forwarding” applies Only if ”Indirect Forwarding” applies 6. PS Handover Complete 7. XID Response 8. Forward Relocation Complete Notification 8a. Forward Relocation Complete Acknowledge 9. Modify Bearer Request For Serving GW relocation Steps 9, 10 and 11, and the following User Plane path, will be handled by Target Serving GW 12. XID Negotiation for LLC ADM 12a. SABM UA exchange re-establishment and XID negotiation for LLC ABM) Uplink and Downlink User Plane PDUs 13. Routeing Area Update procedure 14. Delete Session Request 14b. Release Resource (B) 14a. Delete Session Response (A) 10. Modify Bearer Request 10a. Modify Bearer Response 11. Modify Bearer Response
1.1.3 3G到LTE的切换
3G到LTE的具体切换信令流程： 1. 切换准备信令流程（3GPP 23.401）
UE Source RNC Target eNodeB Source SGSN Target MME Serving GW Target Serving GW PDN GW HSS Uplink and Downlink User Plane PDUs (via Source SGSN in case Direct Tunnel is not used) 1. Handover Initiation 2. Relocation Required 3. Forward Relocation Request 4. Create Session Request 4a. Create Session Response 5. Handover Request 5a. Handover Request Acknowledge 6 . Create Indirect Data Forwarding Tunnel Request 6 a . Create Indirect Data Forwarding Tunnel Response 7 . Forward Relocation Response . Create Indirect Data Forwarding Tunnel Request8 8a. Create Indirect Data Forwarding Tunnel Response
2. 切换执行信令流程（3GPP 23.401）
UE Source RNC Target eNodeB Source SGSN Target MME Target Serving GW Serving GW PDN GW HSS Uplink and Downlink User Plane PDUs (via Source SGSN if Direct Tunnel is not used) 1. Relocation Command 2. HO from UTRAN Command 4. E-UTRAN Access Procedures 5. HO to E-UTRAN Complete Sending of uplink data possible Downlink Payload User Plane PDUs (via Source SGSN if Direct Tunnel is not used) If Direct Forwarding applies Via Source SGSN in case Direct Tunnel is not used If Indirect Forwarding applies 6. Handover Notify 7. Forward Relocation Complete Notification 7a. Forward Relocation Complete Acknowledge 8. Modify Bearer Request For Serving GW relocation Steps 8, 9 and 10, and the following User Plane path, will be handled by Target Serving GW (A) 10. Modify Bearer Response 9. Modify Bearer Request 9a. Modify Bearer Response Uplink and Downlink User Plane PDUs 11. Tracking Area Update procedure 12. Delete Session Request 12b. Iu Release Procedure (B) 12a. Delete Session Response 13. Delete Bearer
1.1.4 2G到LTE的切换
2G到LTE的具体切换信令流程： 1. 切换准备信令流程（3GPP 23.401）
UE Source BSS Target eNodeB Source SGSN Target MME Target Serving GW Serving GW PDN GW HSS Uplink and Downlink User Plane PDUs 1. Handover Initiation 2. PS Handover Required 3. Forward Relocation Request 4. Create Session Request 4a. Create Session Response 5. Handover Request 5a. handover Request Acknowledge 6. Create Indirect Data Forwarding Tunnel Request 6a. Create Indirect Data Forwarding Tunnel Response 7. Forward Relocation Response 8. Create Indirect Data Forwarding Tunnel Request 8a. Create Indirect Data Forwarding Tunnel Response
2. 切换执行信令流程（3GPP 23.401）
