LTE X2 Handover Messaging - EventHelix.com
LTE X2 Handover Messaging - EventHelix.com
LTE X2 Handover Messaging - EventHelix.com
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<strong>EventHelix</strong>.<strong>com</strong><br />
• design with sequence diagrams<br />
• tele<strong>com</strong> systems engineering<br />
• object oriented design<br />
<strong>LTE</strong> <strong>X2</strong> <strong>Handover</strong> <strong>Messaging</strong><br />
© 2013 <strong>EventHelix</strong>.<strong>com</strong> Inc.<br />
All Rights Reserved
<strong>LTE</strong> <strong>X2</strong> <strong>Handover</strong> Sequence<br />
Diagram<br />
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UE<br />
Target<br />
eNodeB<br />
Source<br />
eNodeB<br />
MME<br />
SGW<br />
<strong>Handover</strong> Confirm<br />
<strong>X2</strong>AP <strong>Handover</strong> Request<br />
<strong>X2</strong>AP <strong>Handover</strong> Request Acknowledge<br />
<strong>Handover</strong> Command<br />
<strong>X2</strong>AP SN Transfer Status<br />
S1AP Path Switch Request<br />
Begin forwarding downlink<br />
data from source to<br />
destination eNodeB<br />
Modify Bearer<br />
Request<br />
<strong>X2</strong>AP UE Context Release<br />
S1AP Path Switch Acknowledge<br />
Modify Bearer<br />
Response<br />
Marks the end for forwarding<br />
Tracking Area Update Request<br />
Tracking Area Update Response<br />
© 2013 <strong>EventHelix</strong>.<strong>com</strong> Inc. 2
<strong>X2</strong>AP <strong>Handover</strong> Request<br />
(Initiating Message)<br />
Source eNodeB <br />
Target eNodeB<br />
<strong>X2</strong>AP <strong>Handover</strong> Request<br />
• eNodeB decides to initiate an <strong>X2</strong><br />
handover based on:<br />
• UE reported RRC downlink signal<br />
quality measurements<br />
• Uplink signal quality measured at<br />
the eNodeB<br />
• eNodeB picks the target cell id for<br />
the handover.<br />
• <strong>X2</strong> handover is initiated if the If<br />
the target cell is served by the<br />
same MME as the current cell<br />
• The message includes UE context<br />
information that identifies the UE<br />
on the S1AP interface.<br />
• Security parameters are also<br />
included in the message<br />
• Information about the radio<br />
bearers is included in the message.<br />
The per RAB information includes<br />
• QoS parameters<br />
• GTP Tunnel Information<br />
• The message also includes RRC<br />
context information<br />
Old eNB UE <strong>X2</strong>AP<br />
Id<br />
Cause<br />
Target Cell Id<br />
GUMMEI Id<br />
UE Context<br />
Information<br />
eRABs to be Setup<br />
List<br />
RRC Context<br />
<strong>Handover</strong> desired<br />
for radio reasons<br />
PLMN Identity<br />
eUTRAN Cell<br />
Identifier<br />
GU Group Id<br />
MME Code<br />
MME UE S1AP ID<br />
UE Security<br />
Capabilities<br />
AS Security<br />
Information<br />
eRAB 0<br />
…<br />
eRAB N-1<br />
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PLMN Id<br />
MME Group Id<br />
Key eNB<br />
eRAB Id<br />
eRAB Level QoS<br />
Parameters<br />
UL GTP Tunnel<br />
Endpoint<br />
QCI<br />
Transport Layer<br />
Address<br />
GTP TEID<br />
© 2013 <strong>EventHelix</strong>.<strong>com</strong> Inc. 3
<strong>X2</strong>AP <strong>Handover</strong> Request Acknowledge<br />
(Successful Out<strong>com</strong>e)<br />
Target eNodeB <br />
Source eNodeB<br />
<strong>X2</strong>AP <strong>Handover</strong> Request<br />
Acknowledge<br />
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• The target eNodeB receives<br />
performs admission control on<br />
receipt of the <strong>Handover</strong> Request.<br />
• The target eNodeB responds with<br />
<strong>X2</strong>AP <strong>Handover</strong> Request<br />
Acknowledge.<br />
• Information about the accepted<br />
RABs is included in the message.<br />
• The Uplink and Downlink GTP<br />
Tunnel information is included<br />
for each RAB.<br />
• The tunnel assignments are<br />
made at the target to transport<br />
traffic during the handover.<br />
• A <strong>Handover</strong> Command message<br />
sent via a transparent container.<br />
Old eNB UE <strong>X2</strong>AP Id<br />
New eNB UE <strong>X2</strong>AP Id<br />
eRABs Admitted List<br />
Transparent Container<br />
eRAB Admitted Item 0<br />
…<br />
eRAB Admitted Item N-<br />
1<br />
Target to Source eNB<br />
Payload<br />
eRAB Id<br />
UL GTP Tunnel<br />
Endpoint<br />
DL GTP Tunnel<br />
Endpoint<br />
Transport Layer<br />
Address<br />
GTP TEID<br />
Transport Layer<br />
Address<br />
GTP TEID<br />
• The source eNodeB send this<br />
message to the UE.<br />
© 2013 <strong>EventHelix</strong>.<strong>com</strong> Inc. 4
<strong>X2</strong>AP SN Status Transfer<br />
(Initiating Message)<br />
Source eNodeB <br />
Target eNodeB<br />
<strong>X2</strong>AP SN Transfer Status<br />
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• The source eNodeB now sends the<br />
SN Transfer Status<br />
• The following fields are present for<br />
each RAB<br />
• The uplink PDCP sequence<br />
number<br />
• Uplink Hyper Frame Number<br />
• The downlink PDCP sequence<br />
number<br />
• Downlink Hyper Frame Number<br />
• These fields are needed for<br />
continuing ciphering and integrity<br />
protection after the handover.<br />
Old eNB UE <strong>X2</strong>AP<br />
Id<br />
New eNB UE <strong>X2</strong>AP<br />
Id<br />
eRABs Subject to<br />
Status Transfer<br />
eRAB Subject to<br />
Status Transfer 0<br />
…<br />
eRAB Subject to<br />
Status Transfer N-1<br />
eRAB Id<br />
UL Count Value<br />
DL Count Value<br />
PDCP SN<br />
HFN<br />
PDCP SN<br />
HFN<br />
© 2013 <strong>EventHelix</strong>.<strong>com</strong> Inc. 5
S1AP Path Switch Request<br />
Target eNodeB MME<br />
S1AP Path Switch Request<br />
• The target eNodeB requests<br />
switching of the S1-U GTP tunnel<br />
towards the target eNodeB.<br />
• The MME identifies the UE with<br />
the “eNB to UE S1AP ID”<br />
• The message includes the new cell<br />
id and the tracking area id<br />
• Security capabilities of the target<br />
eNodeB are also included.<br />
eNB UE S1AP ID<br />
eRABs to be<br />
Switched DL List<br />
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eRABs to be<br />
Switched DL Item 0<br />
…<br />
eRABs to be<br />
Switched DL Item N-1<br />
PLMN Id<br />
eRAB Id<br />
Transport Layer<br />
Address<br />
GTP TEID<br />
EUTRAN CGI<br />
Cell Id<br />
PLMN ID<br />
TAI<br />
TAC<br />
UE Security<br />
Capabilities<br />
Encryption<br />
Algorithms<br />
Integrity Protection<br />
Algorithms<br />
© 2013 <strong>EventHelix</strong>.<strong>com</strong> Inc. 6
Modify Bearer Request<br />
(initiating message)<br />
MME SGW<br />
Modify Bearer Request<br />
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• The MME requests the SGW to<br />
switch the path to the target<br />
eNodeB.<br />
• The S1-U TEID received from the<br />
target eNodeB is passed to the<br />
SGW.<br />
Bearer Context to<br />
be Updated<br />
TEID/GRE Key<br />
EPS Bearer Id<br />
S1-U eNodeB F-<br />
TEID<br />
© 2013 <strong>EventHelix</strong>.<strong>com</strong> Inc. 7
Modify Bearer Response<br />
(initiating message)<br />
SGW MME<br />
Modify Bearer Response<br />
• SGW updates the bearer and<br />
responds back<br />
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Bearer Context<br />
Updated<br />
TEID/GRE Key<br />
EPS Bearer Id<br />
S1 SGW F-TEID<br />
© 2013 <strong>EventHelix</strong>.<strong>com</strong> Inc. 8
S1AP Path Switch Request<br />
S1AP: MME Target eNodeB<br />
S1AP Path Switch<br />
Acknowledge<br />
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• The target eNodeB requests<br />
switching of the S1-U GTP tunnel<br />
towards the target eNodeB.<br />
• The MME identifies the UE with<br />
the “eNB to UE S1AP ID”<br />
• The message includes the new cell<br />
id and the tracking area id<br />
• Security capabilities of the target<br />
eNodeB are also included.<br />
MME UE S1AP ID<br />
eNB UE S1AP ID<br />
eRABs to be<br />
Switched UL List<br />
Security Context<br />
eRABs to be<br />
Switched UL Item<br />
eRAB Id<br />
GTP TEID<br />
© 2013 <strong>EventHelix</strong>.<strong>com</strong> Inc. 9
<strong>X2</strong>AP UE Context Release<br />
(Initiating Message)<br />
Target eNodeB <br />
Source eNodeB<br />
<strong>X2</strong>AP UE Context Release<br />
• Sent when the target eNodeB has<br />
successfully <strong>com</strong>pleted the path<br />
switching and radio signaling for<br />
the handover.<br />
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Old eNB UE <strong>X2</strong>AP<br />
ID<br />
New eNB UE <strong>X2</strong>AP<br />
ID<br />
© 2013 <strong>EventHelix</strong>.<strong>com</strong> Inc. 10
Tracking Area Update Request<br />
UE-NAS MME-NAS<br />
Tracking Area Update<br />
Request<br />
• Sent if the just <strong>com</strong>pleted<br />
handover resulted in a Tracking<br />
Area Update<br />
Uplink NAS<br />
Transport<br />
eUTRAN-CGI<br />
TAI<br />
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PLMN Id<br />
Cell Id<br />
PLMN Id<br />
TAC<br />
EPS Update Time<br />
TA Updating<br />
Old GUTI<br />
Tracking Area<br />
Update Request<br />
M-TMSI<br />
Last Visited<br />
Tracking Area<br />
EPS Bearer Context<br />
Status<br />
Bearer Context<br />
Active<br />
© 2013 <strong>EventHelix</strong>.<strong>com</strong> Inc. 11
Tracking Area Update Request<br />
MME-NAS UE-NAS<br />
Tracking Area Update<br />
Accept<br />
• Sent if the just <strong>com</strong>pleted<br />
handover resulted in a Tracking<br />
Area Update<br />
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EPS Update Result<br />
TA Updated<br />
Tracking Area<br />
Update Result<br />
T3412<br />
T3402<br />
© 2013 <strong>EventHelix</strong>.<strong>com</strong> Inc. 12
Thank You<br />
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• design with sequence diagrams<br />
• tele<strong>com</strong> systems engineering<br />
• object oriented design<br />
Thank you for visiting <strong>EventHelix</strong>.<strong>com</strong>. The following links provide more information<br />
about tele<strong>com</strong> design tools and techniques:<br />
Links<br />
<strong>LTE</strong> <strong>X2</strong> <strong>Handover</strong> Sequence Diagrams<br />
EventStudio System Designer<br />
VisualEther Protocol Analyzer<br />
Tele<strong>com</strong> Call Flows<br />
TCP/IP Sequence Diagrams<br />
Tele<strong>com</strong> • Networking • Software<br />
Description<br />
Detailed message flow analysis of <strong>LTE</strong> <strong>X2</strong><br />
handovers<br />
Sequence diagram based systems engineering<br />
tool.<br />
Wireshark based visual protocol analysis and<br />
system design reverse engineering tool.<br />
GSM, SIP, H.323, ISUP, <strong>LTE</strong> and IMS call flows.<br />
TCP/IP explained with sequence diagrams.<br />
Real-time and embedded systems, call flows<br />
and object oriented design articles.<br />
© 2013 <strong>EventHelix</strong>.<strong>com</strong> Inc.<br />
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