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LTE X2 Handover Messaging
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All Rights Reserved
LTE X2 Handover Sequence
Diagram
UE
Source
eNodeB
Target
eNodeB
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MME
SGW
X2AP Handover Request
X2AP Handover Request Acknowledge
Handover Command
Begin forwarding downlink
data from source to
destination eNodeB
X2AP SN Transfer Status
Handover Confirm
S1AP Path Switch Request
Modify Bearer
Request
Modify Bearer
Response
S1AP Path Switch Acknowledge
Marks the end for forwarding
X2AP UE Context Release
Tracking Area Update Request
Tracking Area Update Response
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Source eNodeB 
Target eNodeB
X2AP Handover Request
•
•
•
Target Cell Id
eUTRAN Cell
Identifier
PLMN Id
GU Group Id
GUMMEI Id
MME Group Id
MME Code
MME UE S1AP ID
UE Context
Information
Security parameters are also
included in the message
QoS parameters
GTP Tunnel Information
The message also includes RRC
context information
UE Security
Capabilities
AS Security
Information
Key eNB
eRAB Id
Information about the radio
bearers is included in the message.
The per RAB information includes
•
•
•
Handover desired
for radio reasons
PLMN Identity
UE reported RRC downlink signal
quality measurements
Uplink signal quality measured at
the eNodeB
eNodeB picks the target cell id for
the handover.
X2 handover is initiated if the If
the target cell is served by the
same MME as the current cell
The message includes UE context
information that identifies the UE
on the S1AP interface.
•
•
Cause
eNodeB decides to initiate an X2
handover based on:
•
•
Old eNB UE X2AP
Id
X2AP Handover Request
(Initiating Message)
•
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eRAB 0
eRABs to be Setup
List
…
RRC Context
eRAB N-1
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eRAB Level QoS
Parameters
UL GTP Tunnel
Endpoint
QCI
Transport Layer
Address
GTP TEID
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Target eNodeB 
Source eNodeB
X2AP Handover Request
Acknowledge
•
•
The target eNodeB receives
performs admission control on
receipt of the Handover Request.
The target eNodeB responds with
X2AP Handover Request
Acknowledge.
Information about the accepted
RABs is included in the message.
•
•
•
The Uplink and Downlink GTP
Tunnel information is included
for each RAB.
The tunnel assignments are
made at the target to transport
traffic during the handover.
A Handover Command message
sent via a transparent container.
•
eRAB Id
X2AP Handover Request Acknowledge
(Successful Outcome)
•
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Old eNB UE X2AP Id
UL GTP Tunnel
Endpoint
New eNB UE X2AP Id
eRAB Admitted Item 0
eRABs Admitted List
…
eRAB Admitted Item N1
Transparent Container
Transport Layer
Address
GTP TEID
DL GTP Tunnel
Endpoint
Transport Layer
Address
GTP TEID
Target to Source eNB
Payload
The source eNodeB send this
message to the UE.
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Source eNodeB 
Target eNodeB
X2AP SN Transfer Status
•
The source eNodeB now sends the
SN Transfer Status
The following fields are present for
each RAB
•
•
•
•
•
The uplink PDCP sequence
number
Uplink Hyper Frame Number
The downlink PDCP sequence
number
Downlink Hyper Frame Number
These fields are needed for
continuing ciphering and integrity
protection after the handover.
X2AP SN Status Transfer
(Initiating Message)
•
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eRAB Id
Old eNB UE X2AP
Id
PDCP SN
UL Count Value
New eNB UE X2AP
Id
eRAB Subject to
Status Transfer 0
HFN
eRABs Subject to
Status Transfer
…
PDCP SN
DL Count Value
eRAB Subject to
Status Transfer N-1
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HFN
5
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Target eNodeB  MME
S1AP Path Switch Request
•
•
•
The target eNodeB requests
switching of the S1-U GTP tunnel
towards the target eNodeB.
The MME identifies the UE with
the “eNB to UE S1AP ID”
The message includes the new cell
id and the tracking area id
Security capabilities of the target
eNodeB are also included.
S1AP Path Switch Request
•
eRAB Id
eNB UE S1AP ID
eRABs to be
Switched DL Item 0
Transport Layer
Address
eRABs to be
Switched DL List
…
GTP TEID
eRABs to be
Switched DL Item N-1
PLMN Id
EUTRAN CGI
Cell Id
PLMN ID
TAI
TAC
UE Security
Capabilities
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Encryption
Algorithms
Integrity Protection
Algorithms
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•
•
The MME requests the SGW to
switch the path to the target
eNodeB.
The S1-U TEID received from the
target eNodeB is passed to the
SGW.
Modify Bearer Request
(initiating message)
MME  SGW
Modify Bearer Request
Bearer Context to
be Updated
TEID/GRE Key
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EPS Bearer Id
S1-U eNodeB FTEID
7
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SGW  MME
Modify Bearer Response
SGW updates the bearer and
responds back
Modify Bearer Response
(initiating message)
•
Bearer Context
Updated
EPS Bearer Id
S1 SGW F-TEID
TEID/GRE Key
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•
•
•
•
The target eNodeB requests
switching of the S1-U GTP tunnel
towards the target eNodeB.
The MME identifies the UE with
the “eNB to UE S1AP ID”
The message includes the new cell
id and the tracking area id
Security capabilities of the target
eNodeB are also included.
S1AP Path Switch Request
S1AP: MME Target eNodeB
S1AP Path Switch
Acknowledge
MME UE S1AP ID
eNB UE S1AP ID
eRABs to be
Switched UL List
eRABs to be
Switched UL Item
eRAB Id
GTP TEID
Security Context
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•
Sent when the target eNodeB has
successfully completed the path
switching and radio signaling for
the handover.
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X2AP UE Context Release
(Initiating Message)
Target eNodeB 
Source eNodeB
X2AP UE Context Release
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Old eNB UE X2AP
ID
New eNB UE X2AP
ID
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UE-NAS  MME-NAS
Tracking Area Update
Request
PLMN Id
Sent if the just completed
handover resulted in a Tracking
Area Update
eUTRAN-CGI
Tracking Area Update Request
•
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Cell Id
Uplink NAS
Transport
PLMN Id
TAI
TAC
EPS Update Time
TA Updating
Old GUTI
Tracking Area
Update Request
M-TMSI
Last Visited
Tracking Area
EPS Bearer Context
Status
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Bearer Context
Active
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MME-NAS  UE-NAS
Tracking Area Update
Accept
Sent if the just completed
handover resulted in a Tracking
Area Update
Tracking Area Update Request
•
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EPS Update Result
Tracking Area
Update Result
© 2013 EventHelix.com Inc.
TA Updated
T3412
T3402
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