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LTE X2 Handover
X2 Handover Case
The X2 handover is used when direct connectivity exits between the source and target eNBs with X2 interface
Handover Preparation
The X2 handover procedure is performed without Evolved Packet Core (EPC) involvement, i.e. preparation messages are directly
exchanged between Source eNB and Target eNB
Handover Execution
There is direct tunnel formed between source and target eNBs for downlink data forwarding in handover execution time
1. Before Handover
3
MME
2. Handover Preparation
1
3
SAE-GW
MME
2
3. Handover Execution
1
MME
2
MME
2
Source
Target
Source
Target
Source
eNB
eNB
eNB
eNB
eNB
3
UE
SAE-GW
2
1
· Source eNB makes the HO
decision
· Direct Tunnel Setup (X2
Transport Bearer between
Source eNB and Target eNB)
DRB (Data Radio Beaer)
Signaling in Radio
UE
SAE-GW
2
2
2
Source
Target
Source
Target
eNB
eNB
eNB
eNB
eNB
1
1
3
UE
moving
· Detach from Source eNB
· Direct Packet Forwarding (DL
traffic from Source eNB to
Target eNB)
S1 Bearer (GTP Tunnel)
S1 Signaling
MME
Target
4
moving
1
3
SAE-GW
2
1
3
UE
moving
3
5. After Handover
1
3
2
2
1
1
3
SAE-GW
4. Handover Completion
moving
3
UE
· Path Switch procedure
between Target eNB and
MME
· Release S1 Bearer of Source
eNB
· Release X2 Transport Bearer
for direct Packet Forwarding
X2 Bearer (GTP Tunnel)
X2 Signaling
S11 (GTP-C) Signaling
UL Traffic
DL Traffic
About NMC Consulting Group
NMC Consulting Group was founded on year 2002 and is advanced, professional network consulting company which is specialized for IP Network area like FTTH, Metro Ethernet and
IP/MPLS, Service area like IPTV and IMS lastly, Wireless network area like Mobile WIMAX and LTE.
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LTE X2 Handover
1
LTE X2 Handover
UE
EMM-Registered
ECM-Connected
RRC-Connected
Source
Target
eNB
eNB
MME
S-GW
P-GW
HSS
PCRF
EMM-Registered
ECM-Connected
RRC-Connected
DRB (UL/DL)
S1 Bearer (UL/DL)
Measurement Report
S5 Bearer (UL/DL)
Internet
Triggering the UE to send a Measurement Report based on
triggering event (e.g., when a neighboring cell becomes better
than the serving cell in terms of radio signal strength)
Serving Cell (Radio Signal Strength),
Target Cell (PCI, Radio Signal Strength)
Handover Decision by Source eNB
Derive KeNB*
Handover Preparation
Handover Request
ECGI of the Target Cell (of Target eNB), UE-AMBR, UE Security Capability, KeNB*, E-RAB to be setup (E-RAB ID, QCI, ARP, S1 S-GW TEID)
eNB generates AS keys from KeNB (KeNB = KeNB*)
Receive S1 TEID (UL)
S1 TEID (UL) for Source eNB
S1 Bearer Established (UL)
Handover Request Ack
Receive Target eNB
TEID (DL)
Admission control with E-RAB QoS (QCI, ARP)
Target eNB reserves RRC resource and allocates a new C-RNTI (e.g., Target C-RNTI)
X2 Transport Bearer’s {E-RAB ID, Target eNB TEID},
Handover Command (Target C-RNTI, Target DRB ID (UL/DL), Target eNB AS Security Algorithm)
Create Target eNB TEID (DL)
X2 Transport Bearer (DL)
Established
DL: From Source eNB to Target eNB
Create DRB ID (UL/DL)
Handover Command (RRC Connection Reconfiguration)
Target C-RNTI, Target DRB ID (UL/DL), Target eNB AS Security Algorithm)
UE can not send & receive packets
Handover Execution
Receive DRB ID (UL/DL)
Detach from
the source eNB
RRC-Idle
UE generates KeNB (KeNB = KeNB*) & AS keys
SN (Sequence Number) Status Transfer
DL Count (DL PDCP SN), UL Count (UL PDCP SN)
S1 Bearer (UL/DL)
S5 Bearer (UL/DL)
Internet
S5 Bearer (UL/DL)
Internet
S5 Bearer (UL/DL)
Internet
X2 Transport Bearer (DL)
Synchronization
Buffering DL Packets
UL Allocation
Handover Confirm (RRC Connection Reconfiguration Complete)
Target C-RNTI
Start to send the Buffered DL Packets to the UE
DRB Established (UL/DL)
RRC-Connected
AS Security setup
S1 Bearer (UL/DL)
X2 Transport Bearer (DL)
DRB (UL/DL)
DRB (UL/DL)
S1 Bearer (UL)
Path Switch Request
Create S1 TEID (DL)
ECGI, TAI, Received UE Security Capability, E-RAB to be switched in DL (E-RAB ID, S1 Target eNB TEID)
Modify Bearer Request
Handover Completion
EPS Bearer in DL (EPS Bearer ID, S1 Target eNB TEID), ECGI, TAI
Receive S1 TEID (DL)
S1 Bearer Established (DL)
Modify Bearer Request
Indication of IP-CAN Session Modification (CCR)
EPS Bearer ID, ECGI, TAI
IMSI, UE IP, ECGI, TAI
Acknowledge of IP-CAN Session Modification (CCA)
Modify Bearer Response
EPS Bearer ID
Modify Bearer Response
EPS Bearer in DL (EPS Bearer ID)
Switch to DL path
S1 Bearer (UL/DL)
X2 Transport Bearer
EM (DL)
EM
Target eNB sends packets to UE from old path
(X2) until receiving “End-Marker”, after then it
sends packets from new path (S1)
DRB (UL/DL)
S1 Bearer (UL/DL)
Send “End Marker” Packet
For re-ordering in the Target eNB, the S-GW sends “End-Marker”
packet(s) on the old path immediately after switch the path
S5 Bearer (UL/DL)
Internet
Path Switch Request Ack
UE Conext Release
E-RAB to be switched in UL (E-RAB ID)
3
X2 Transport Bearer (DL)
Released
EMM-Registered
ECM-Connected
RRC-Connected
S1 Bearer Released (UL/DL)
RRC-Connected
Source
Target eNB
Source eNB release UE context
EMM-Registered
ECM-Connected
UE
3
4
5
eNB
EM
5
EM End Marker
4 EM
S-GW
LTE X2 Handover
2