lte: x2 interface
TRANSCRIPT
X2 Protocol Stack
• X2 control plan • X2 data plan
X2 Protocol Stack
• X2 has exactly the same stack as S1, simplifying data forwarding operation.
X2 Initiation
X2 initiation sets up the connection between two eNodeB’s via X2 Interface
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
MobilityManagement
Entity
HomeSubscriber
Server
Uu
S1-MME
S1-U
E-UTRAN Core Network (EPC)
UE UueNodeB
X-2
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
MobilityManagement
Entity
HomeSubscriber
Server
Uu
S1-MME
S1-U
E-UTRAN Core Network (EPC)
UE UueNodeB
X-2
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
MobilityManagement
Entity
HomeSubscriber
Server
Uu
S1-MME
S1-U
E-UTRAN Core Network (EPC)
UEUser Equipment
UueNodeB
X-2
X2 setup request
X2 Initiation
Information passed in X2 setup requestMandatory
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
MobilityManagement
Entity
HomeSubscriber
Server
Uu
S1-MME
S1-U
E-UTRAN Core Network (EPC)
UE UueNodeB
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
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
MobilityManagement
Entity
HomeSubscriber
Server
Uu
S1-MME
S1-U
E-UTRAN Core Network (EPC)
UE UueNodeB
X-2
X2 Setup Response
X2 HandoverX2 Handover (HO) handover a UE from one
eNodeB (source eNodeB) to the other (target eNodeB).
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)
MobilityManagement
Entity
HomeSubscriber
Server
Downlink Data
Uu
S1-MME
S1-U
E-UTRAN Core Network (EPC)
UE UueNodeB
X-2
Uplink Data
Recall the normal data flow:
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
MobilityManagement
Entity
HomeSubscriber
Server
Uu
S1-MME
E-UTRAN Core Network (EPC)
eNodeB
X-2RRC Measurement Control RRC Measurement Report S1-U
Decision Request Preparation Path Switch Complete
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
MobilityManagement
Entity
HomeSubscriber
Server
Uu
S1-MME
S1-U
E-UTRAN Core Network (EPC)
eNodeB
X-2
RRC Measurement Control
RRC Measurement Report
Decision Request Preparation Path Switch Complete
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
MobilityManagement
Entity
HomeSubscriber
Server
S1-MME
S1-U
E-UTRAN Core Network (EPC)
eNodeB
X2AP Handover Request
Uu
Decision Request Preparation Path Switch Complete
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
MobilityManagement
Entity
HomeSubscriber
Server
Uu
S1-MME
S1-U
E-UTRAN Core Network (EPC)
eNodeBUplink S1 Bearer Establishment
Decision Request Preparation Path Switch Complete
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
MobilityManagement
Entity
HomeSubscriber
Server
S1-MME
S1-U
E-UTRAN Core Network (EPC)
eNodeBUplink S1 Bearer Establishment
Uu
Decision Request Preparation Path Switch Complete
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
MobilityManagement
Entity
HomeSubscriber
Server
S1-MME
S1-U
E-UTRAN Core Network (EPC)
eNodeB
X2AP Handover Request Ack.
Uu
Decision Request Preparation Path Switch Complete
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
MobilityManagement
Entity
HomeSubscriber
Server
S1-MME
S1-U
E-UTRAN Core Network (EPC)
eNodeB
X2 TEID
X2 TEID
Uu
Decision Request Preparation Path Switch Complete
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
MobilityManagement
Entity
HomeSubscriber
Server
S1-U
E-UTRAN Core Network (EPC)
eNodeB
SN Transfer Status
RRC Reconfiguration
Request
Uu
Decision Request Preparation Path Switch Complete
X2 Handover
UE
eNodeB
MME
S-GW P-GW
HSS
Evolved NodeB
Serving Gateway PDN Gateway
MobilityManagement
Entity
HomeSubscriber
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
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
MobilityManagement
Entity
HomeSubscriber
Server
S1-U
E-UTRAN Core Network (EPC)
eNodeB
SN Transfer Status
Detach from source eNodeB
RRC IDLEUE status
Uu
Decision Request Preparation Path Switch Complete
X2 Handover• Mean while UE is switching to target eNodeB.
UE
eNodeB
MME
S-GW P-GW
HSS
Evolved NodeB
Serving Gateway PDN Gateway
MobilityManagement
Entity
HomeSubscriber
Server
S1-U
E-UTRAN Core Network (EPC)
eNodeB
SN Transfer Status
RACH Preamble
RRC IDLEUE status
Uu
Decision Request Preparation Path Switch Complete
X2 Handover• Mean while UE is switching to target eNodeB.
UE
eNodeB
MME
S-GW P-GW
HSS
Evolved NodeB
Serving Gateway PDN Gateway
MobilityManagement
Entity
HomeSubscriber
Server
S1-U
E-UTRAN Core Network (EPC)
eNodeB
SN Transfer Status
Random Access Response
RRC IDLEUE status
Uu
Decision Request Preparation Path Switch Complete
X2 Handover• Mean while UE is switching to target eNodeB.
UE
eNodeB
MME
S-GW P-GW
HSS
Evolved NodeB
Serving Gateway PDN Gateway
MobilityManagement
Entity
HomeSubscriber
Server
S1-U
E-UTRAN Core Network (EPC)
eNodeB
SN Transfer Status
Uu
RRC Connection Reconfiguration Complete
RRC CONNECTEDUE status
Decision Request Preparation Path Switch Complete
X2 Handover
UE
eNodeB
MME
S-GW P-GW
HSS
Evolved NodeB
Serving Gateway PDN Gateway
MobilityManagement
Entity
HomeSubscriber
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
X2 Handover• Path switch
eNodeB
MME
S-GW P-GW
HSS
Evolved NodeB
Serving Gateway PDN Gateway
MobilityManagement
Entity
HomeSubscriber
Server
S1-U
E-UTRAN Core Network (EPC)
eNodeB
RRC CONNECTEDUE status
S1AP Path Switch Req.
UE
Decision Request Preparation Path Switch Complete
X2 Handover• Path switch
eNodeB
MME
S-GW P-GW
HSS
Evolved NodeB
Serving Gateway PDN Gateway
MobilityManagement
Entity
HomeSubscriber
Server
S1-U
E-UTRAN Core Network (EPC)
eNodeB
RRC CONNECTEDUE status
S1AP Path Switch Req.
Modify Bearer Req .(EPS Bearer ID, S1 TEID, ECGI, TAI)
UE
Decision Request Preparation Path Switch Complete
X2 Handover• Path switch
eNodeB
MME
S-GW P-GW
HSS
Evolved NodeB
Serving Gateway PDN Gateway
MobilityManagement
Entity
HomeSubscriber
Server
S1-U
E-UTRAN Core Network (EPC)
eNodeB
RRC CONNECTEDUE 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
X2 Handover• Path switch
eNodeB
MME
S-GW P-GW
HSS
Evolved NodeB
Serving Gateway PDN Gateway
MobilityManagement
Entity
HomeSubscriber
Server
S1-U
E-UTRAN Core Network (EPC)
eNodeB
RRC CONNECTEDUE 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
X2 Handover
eNodeB
MME
S-GW P-GW
HSS
Evolved NodeB
Serving Gateway PDN Gateway
MobilityManagement
Entity
HomeSubscriber
Server
S1-U
E-UTRAN Core Network (EPC)
eNodeB
RRC CONNECTEDUE status
S1AP Path Switch Req.
Modify Bearer Res.
Modify Bearer Res.
• Path switch
UuUE
Decision Request Preparation Path Switch Complete
X2 Handover
eNodeB
MME
S-GW P-GW
HSS
Evolved NodeB
Serving Gateway PDN Gateway
MobilityManagement
Entity
HomeSubscriber
Server
S1-U
E-UTRAN Core Network (EPC)
eNodeB
RRC CONNECTEDUE 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
End Marker
End Marker
UE
Decision Request Preparation Path Switch Complete
X2 Handover
UE
eNodeB
MME
S-GW P-GW
HSS
Evolved NodeB
Serving Gateway PDN Gateway
MobilityManagement
Entity
HomeSubscriber
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
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
MobilityManagement
Entity
HomeSubscriber
Server
S1-MME
S1-U
E-UTRAN Core Network (EPC)
eNodeBX2AP Path Switch Req. Ack.
Uu
Uu
X1AP UE Context Release
Decision Request Preparation Path Switch Complete
X2 HO modes1. 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)
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.
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.
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.
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X2 Management
X2 Load and Interference Management
MMEMME
X2 Management• Interference Management: Frequency domain and
time domain interference.
• Informations are exchanged in the order of tens of milliseconds to optimize RRM process.
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.
References1. 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