LTE: X2 interface
•
38 likes•41,341 views
The document discusses the X2 interface and X2 handover procedure in LTE networks. The X2 interface connects two neighboring eNodeBs and establishes an X2 connection through the X2 setup procedure. The X2 handover procedure allows handing over a UE's connection from a source eNodeB to a target eNodeB, involving preparation where the target allocates resources and the UE connects to it, and execution including a path switch to route data to the target eNodeB. Key information like UE context and bearers is exchanged between eNodeBs through the X2 interface to enable smooth handover.
Recommended
More Related Content
What's hot
What's hot (20)
Similar to LTE: X2 interface
Similar to LTE: X2 interface (20)
More from Schwannden Kuo
More from Schwannden Kuo (15)
Recently uploaded
Recently uploaded (20)
LTE: X2 interface
- 1. 4G - X2 Interface Schwannden Kuo NCTU CS schwannden@gmail.com
- 2. X2 Protocol Stack • X2 control plan • X2 data plan
- 3. X2 Protocol Stack • X2 has exactly the same stack as S1, simplifying data forwarding operation.
- 4. X2 Initiation X2 initiation sets up the connection between two eNodeB’s via X2 Interface
- 5. X2 Initiation • X2 manages two functions: load or interference related function and handover related information. • The two functions are independent of each other. UE eNodeB MME S-GW P-GW HSS User Equipment Evolved NodeB Serving Gateway PDN Gateway Mobility Management Entity Home Subscriber Server Uu S1-MME S1-U E-UTRAN Core Network (EPC) UE Uu eNodeB X-2
- 6. X2 Initiation • A full mesh between all eNodeB’s is not mandated in an E-UTRAN network. UE eNodeB MME S-GW P-GW HSS User Equipment Evolved NodeB Serving Gateway PDN Gateway Mobility Management Entity Home Subscriber Server Uu S1-MME S1-U E-UTRAN Core Network (EPC) UE Uu eNodeB X-2
- 7. X2 Initiation • Initialization begins with the identification of a suitable neighbor, either by configuration of ANRF (Automatic Neighbor Relation Function). • Once it decides to establish an X2 interface, it establishes a TLN (transport Layer Network) connection by target’s IP (found via configuration or query over S1 interface). UE eNodeB MME S-GW P-GW HSS User Equipment Evolved NodeB Serving Gateway PDN Gateway Mobility Management Entity Home Subscriber Server Uu S1-MME S1-U E-UTRAN Core Network (EPC) UE User Equipment Uu eNodeB X-2 X2 setup request
- 8. X2 Initiation Information passed in X2 setup request Mandatory 1. Physical Cell IDs for the cell it manages 2. Frequency Band 3. Tracking Area Identity and/or the associated PLMN Optional 1. Neighbor Information 2. Number of Antenna Ports 3. PRACH Configuration 4. MBSFN Subframe Info 5. CSG ID (mandatory if the eNB is CSG Cell or Hybrid Cell) 6. MultibandInfoList UE eNodeB MME S-GW P-GW HSS User Equipment Evolved NodeB Serving Gateway PDN Gateway Mobility Management Entity Home Subscriber Server Uu S1-MME S1-U E-UTRAN Core Network (EPC) UE Uu eNodeB X-2 • Informations passed during X2 setup request facilitate automatic self-configuration of the Physical Cell Identities (PCIs) and RACH self-optimization. X2 setup request
- 9. X2 Initiation • Once the X2 setup procedure has been completed by X2 Setup Response, the X2 interface is operational. UE eNodeB MME S-GW P-GW HSS Evolved NodeB Serving Gateway PDN Gateway Mobility Management Entity Home Subscriber Server Uu S1-MME S1-U E-UTRAN Core Network (EPC) UE Uu eNodeB X-2 X2 Setup Response
- 10. X2 Handover X2 Handover (HO) handover a UE from one eNodeB (source eNodeB) to the other (target eNodeB).
- 11. X2 Handover UE eNodeB MME S-GW P-GW PDN (PSS, IME) HSS User Equipment Evolved NodeB Serving Gateway PDN Gateway Packet Data Network (packet Switch Stream, IP Multimedia System) Mobility Management Entity Home Subscriber Server Downlink Data Uu S1-MME S1-U E-UTRAN Core Network (EPC) UE Uu eNodeB X-2 Uplink Data Recall the normal data flow:
- 12. X2 Handover • The network sets the measurement thresholds for sending measurement reports. UE eNodeB MME S-GW P-GW HSS Evolved NodeB Serving Gateway PDN Gateway Mobility Management Entity Home Subscriber Server Uu S1-MME E-UTRAN Core Network (EPC) eNodeB X-2 RRC Measurement ControlRRC Measurement Report S1-U Decision Request Preparation Path Switch Complete
- 13. X2 Handover • Neighboring cell signal quality is now better than the serving cell. • The RRC uses the latest measurement to decide if a handover is needed to another cell. The target cell is selected. The eNodeB for the target cell is identified. UE eNodeB MME S-GW P-GW HSS Evolved NodeB Serving Gateway PDN Gateway Mobility Management Entity Home Subscriber Server Uu S1-MME S1-U E-UTRAN Core Network (EPC) eNodeB X-2 R R C M easurem entC ontrol R R C M easurem entR eport Decision Request Preparation Path Switch Complete
- 14. X2 Handover • The Source eNodeB initiates the handover with the Handover Request message, including information about active E-RABs. Some UE historical information (UE/Cell RRM related information, list of visited cells) is passed in Handover Request message. UE eNodeB MME S-GW P-GW HSS Evolved NodeB Serving Gateway PDN Gateway Mobility Management Entity Home Subscriber Server S1-MME S1-U E-UTRAN Core Network (EPC) eNodeB X2AP Handover Request Uu Decision Request Preparation Path Switch Complete
- 15. X2 Handover • GTP connect for the uplink side is established between the Target eNodeB and the serving SGW. UE eNodeB MME S-GW P-GW HSS Evolved NodeB Serving Gateway PDN Gateway Mobility Management Entity Home Subscriber Server Uu S1-MME S1-U E-UTRAN Core Network (EPC) eNodeB Uplink S1 Bearer Establishment Decision Request Preparation Path Switch Complete
- 16. X2 Handover • Meanwhile Target eNodeB allocates resources, assigns RACH preamble and new C-RNTI to UE. UE eNodeB MME S-GW P-GW HSS Evolved NodeB Serving Gateway PDN Gateway Mobility Management Entity Home Subscriber Server S1-MME S1-U E-UTRAN Core Network (EPC) eNodeB Uplink S1 Bearer Establishment Uu Decision Request Preparation Path Switch Complete
- 17. X2 Handover • The Target eNodeB responds back to the source eNodeB with a Handover Request Acknowledge message. This message carries the Handover Command message (RRC Connection Reconfiguration Request) in a transparent container (including the RACH preamble and C-RNTI generated previously). UE eNodeB MME S-GW P-GW HSS Evolved NodeB Serving Gateway PDN Gateway Mobility Management Entity Home Subscriber Server S1-MME S1-U E-UTRAN Core Network (EPC) eNodeB X2AP Handover Request Ack. Uu Decision Request Preparation Path Switch Complete
- 18. X2 Handover • An X2 GTP connection is established between the Source and the Target eNodeBs. This channel will carry the user data during the handover. • UE is ready to buffer downlink data from this point on. UE eNodeB MME S-GW P-GW HSS Evolved NodeB Serving Gateway PDN Gateway Mobility Management Entity Home Subscriber Server S1-MME S1-U E-UTRAN Core Network (EPC) eNodeB X2 TEID X2 TEID Uu Decision Request Preparation Path Switch Complete
- 19. X2 Handover • Source eNodeB initiates HO commands to UE in RRC Reconfiguration Request. The messages contain the new RACH preamble and C-RNTI of UE. • PDCP Sequence Number (SN) are sent from source eNodeB to target eNodeB. UE eNodeB MME S-GW P-GW HSS Evolved NodeB Serving Gateway PDN Gateway Mobility Management Entity Home Subscriber Server S1-U E-UTRAN Core Network (EPC) eNodeBSNTransferStatus RRC Reconfiguration Request Uu Decision Request Preparation Path Switch Complete
- 20. X2 Handover UE eNodeB MME S-GW P-GW HSS Evolved NodeB Serving Gateway PDN Gateway Mobility Management Entity Home Subscriber Server S1-U E-UTRAN Core Network (EPC) eNodeB Downlink Data Uplink Data Data flow during handover preparation: Downlink data is buffered Decision Request Preparation Path Switch Complete
- 21. X2 Handover • During SN Transfer Status, source eNodeB freeze its transmitter/receiver status. • Mean while UE is switching to target eNodeB. UE eNodeB MME S-GW P-GW HSS Evolved NodeB Serving Gateway PDN Gateway Mobility Management Entity Home Subscriber Server S1-U E-UTRAN Core Network (EPC) eNodeBSNTransferStatus Detach from source eNodeB RRC IDLE UE status Uu Decision Request Preparation Path Switch Complete
- 22. X2 Handover • Mean while UE is switching to target eNodeB. UE eNodeB MME S-GW P-GW HSS Evolved NodeB Serving Gateway PDN Gateway Mobility Management Entity Home Subscriber Server S1-U E-UTRAN Core Network (EPC) eNodeBSNTransferStatus RACH Preamble RRC IDLE UE status Uu Decision Request Preparation Path Switch Complete
- 23. X2 Handover • Mean while UE is switching to target eNodeB. UE eNodeB MME S-GW P-GW HSS Evolved NodeB Serving Gateway PDN Gateway Mobility Management Entity Home Subscriber Server S1-U E-UTRAN Core Network (EPC) eNodeBSNTransferStatus Random Access Response RRC IDLE UE status Uu Decision Request Preparation Path Switch Complete
- 24. X2 Handover • Mean while UE is switching to target eNodeB. UE eNodeB MME S-GW P-GW HSS Evolved NodeB Serving Gateway PDN Gateway Mobility Management Entity Home Subscriber Server S1-U E-UTRAN Core Network (EPC) eNodeBSNTransferStatusUu RRC Connection Reconfiguration Complete RRC CONNECTED UE status Decision Request Preparation Path Switch Complete
- 25. X2 Handover UE eNodeB MME S-GW P-GW HSS Evolved NodeB Serving Gateway PDN Gateway Mobility Management Entity Home Subscriber Server S1-U E-UTRAN Core Network (EPC) eNodeBUu Downlink Data Uplink Data Data flow during handover execution (after ho preparation): Buffered downlink data is delivered Decision Request Preparation Path Switch Complete
- 26. X2 Handover • Path switch eNodeB MME S-GW P-GW HSS Evolved NodeB Serving Gateway PDN Gateway Mobility Management Entity Home Subscriber Server S1-U E-UTRAN Core Network (EPC) eNodeB RRC CONNECTED UE status S1AP Path Switch Req. UE Decision Request Preparation Path Switch Complete
- 27. X2 Handover • Path switch eNodeB MME S-GW P-GW HSS Evolved NodeB Serving Gateway PDN Gateway Mobility Management Entity Home Subscriber Server S1-U E-UTRAN Core Network (EPC) eNodeB RRC CONNECTED UE status S1AP Path Switch Req. Modify Bearer Req . (EPS Bearer ID, S1 TEID, ECGI, TAI) UE Decision Request Preparation Path Switch Complete
- 28. X2 Handover • Path switch eNodeB MME S-GW P-GW HSS Evolved NodeB Serving Gateway PDN Gateway Mobility Management Entity Home Subscriber Server S1-U E-UTRAN Core Network (EPC) eNodeB RRC CONNECTED UE status S1AP Path Switch Req. Modify Bearer Req . (EPS Bearer ID, S1 TEID, ECGI, TAI) Modify Bearer Req . (EPS Bearer ID, ECGI, TAI) UuUEUE Decision Request Preparation Path Switch Complete
- 29. X2 Handover • Path switch eNodeB MME S-GW P-GW HSS Evolved NodeB Serving Gateway PDN Gateway Mobility Management Entity Home Subscriber Server S1-U E-UTRAN Core Network (EPC) eNodeB RRC CONNECTED UE status S1AP Path Switch Req. Modify Bearer Req . (EPS Bearer ID, S1 TEID, ECGI, TAI) Modify Bearer Res. UuUE Decision Request Preparation Path Switch Complete
- 30. X2 Handover eNodeB MME S-GW P-GW HSS Evolved NodeB Serving Gateway PDN Gateway Mobility Management Entity Home Subscriber Server S1-U E-UTRAN Core Network (EPC) eNodeB RRC CONNECTED UE status S1AP Path Switch Req. Modify Bearer Res. Modify Bearer Res. • Path switch UuUE Decision Request Preparation Path Switch Complete
- 31. X2 Handover eNodeB MME S-GW P-GW HSS Evolved NodeB Serving Gateway PDN Gateway Mobility Management Entity Home Subscriber Server S1-U E-UTRAN Core Network (EPC) eNodeB RRC CONNECTED UE status Uu S1AP Path Switch Res. • When path switch is complete, downlink data is sent directly from S-GW to target eNodeB. And an end marker is inserted from S-GW to source eNodeB indicate the last frame from old path. Downlink data EndMarker End Marker UE Decision Request Preparation Path Switch Complete
- 32. X2 Handover UE eNodeB MME S-GW P-GW HSS Evolved NodeB Serving Gateway PDN Gateway Mobility Management Entity Home Subscriber Server S1-U E-UTRAN Core Network (EPC) eNodeBUu Downlink Data Uplink Data Data flow after hand over (after path switch complete): Buffered downlink data is delivered Decision Request Preparation Path Switch Complete
- 33. X2 Handover • The Source eNodeB initiates the handover with the Handover Request message, including information about active E-RABs. UE eNodeB MME S-GW P-GW HSS Evolved NodeB Serving Gateway PDN Gateway Mobility Management Entity Home Subscriber Server S1-MME S1-U E-UTRAN Core Network (EPC) eNodeB X2AP Path Switch Req. Ack. Uu Uu X1AP UE Context Release Decision Request Preparation Path Switch Complete
- 34. X2 HO modes 1. Seamless HO 2. Lossless HO 3. Selective Retransmission 4. Multiple Preparation • Mobility Robustness Handling (more in later presentation) • Mobility towards Home eNodeBs via X2 (more in later presentation)
- 35. X2 Feature on HO • Seamless or lost less mode are set during X2AP HO Req., and acknowledged by the corresponding response. The mode is set on a per EPS bearer basis.
- 36. X2 Feature on HO • Selective Retransmission: Do not retransmit 1. Downlink packets already received by UE before handover. 2. Uplink packets already received by source eNodeB.
- 37. X2 Feature on HO • Multiple Preparation • Trigger handover preparation to multiple eNobeB's concurrently. • Note the source receive only one RELEASE RESOURCE from selected target eNodeB. And is recommended to send cancel procedure to unselected eNodeB’s. http://www.google.com/url?sa=i&rct=j&q=&esrc=s&source=images&cd=&ved=0CAcQjRw&url=http%3A%2F%2Fwww.understandingrelationships.com%2Fdating- multiple-women%2F10254&ei=8jkFVfv5DIzc8AXAuIHwAg&bvm=bv.88198703,d.dGc&psig=AFQjCNGPJ7XPg-HlhqlDQho2qLs8iXARsQ&ust=1426492166316130
- 38. X2 Management X2 Load and Interference Management MMEMME
- 39. X2 Management • Interference Management: Frequency domain and time domain interference. • Informations are exchanged in the order of tens of milliseconds to optimize RRM process.
- 40. X2 Management • Load Balancing: X2 facilitates the exchange of load information. • In Release 10, partial reporting is allowed per cell and per measurement. Therefore, if a serving eNodeB does not support some measurements, it will still report the other measurements that it does support.
- 41. References 1. LTE: the UMTS long term evolution. New York: John Wiley & Sons, 2009. 2. LTE X2 handover sequence diagram: https://www.eventhelix.com/lte/ handover/x2/lte-x2-handover-sequence-diagram.htm#.VQOlCELfK-I 3. TE X2 Handover Messaging: http://www.eventhelix.com/lte/handover/ LTE-X2-Handover-Messaging.pdf 4. summary of LTE Identifiers: http://www.nmcgroups.com/en/expertise/lte/ identifiers.asp 5. Random Access Procedure in LTE: http://4g-lte-world.blogspot.tw/ 2013/04/random-access-procedure-rach-in-lte.html 6. 4G introduction: http://www.slideshare.net/schwannden/4g-intriduction