LTE X2 Handover Messaging

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• design with sequence diagrams • telecom systems engineering • object oriented design

LTE X2 Handover Messaging

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LTE X2 Handover Sequence

Diagram

Target

eNodeB

MME

UE

X2AP Handover Request Acknowledge

SGW

X2AP Handover Request

Source

eNodeB

X2AP SN Transfer Status

S1AP Path Switch Request _{Modify Bearer}

Request

Modify Bearer Response

S1AP Path Switch Acknowledge

X2AP UE Context Release

Tracking Area Update Request

Tracking Area Update Response Handover Command

Handover Confirm

Begin forwarding downlink data from source to destination eNodeB

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Source eNodeB 

Target eNodeB

X2AP Handover Request

X2AP H an d o ve r Re q u es t (In itia tin g Mes sage)

Old eNB UE X2AP Id

Cause Handover desired _{for radio reasons}

Target Cell Id PLMN Identity eUTRAN Cell Identifier GUMMEI Id GU Group Id PLMN Id MME Group Id MME Code UE Context Information MME UE S1AP ID UE Security Capabilities AS Security

Information Key eNB

eRABs to be Setup List

eRAB 0

eRAB Id eRAB Level QoS

Parameters QCI UL GTP Tunnel Endpoint Transport Layer Address GTP TEID … eRAB N-1 RRC Context

• eNodeB decides to initiate an X2 handover based on:

• 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.

• Security parameters are also included in the message

• Information about the radio

bearers is included in the message. The per RAB information includes

• QoS parameters

• GTP Tunnel Information

• The message also includes RRC context information

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Target eNodeB 

Source eNodeB

X2AP Handover Request

Acknowledge

X 2A P H an do ve r R equ est A ck no wl edg e (S uc ce ss ful O ut co m e)

Old eNB UE X2AP Id

New eNB UE X2AP Id

eRABs Admitted List

eRAB Admitted Item 0

eRAB Id UL GTP Tunnel Endpoint Transport Layer Address GTP TEID DL GTP Tunnel Endpoint Transport Layer Address GTP TEID …

eRAB Admitted Item N-1

Transparent Container Target to Source eNB Payload

• 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.

• The source eNodeB send this message to the UE.

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Source eNodeB 

Target eNodeB

X2AP SN Transfer Status

X2AP SN Sta tu s Tra n sf er (In itia tin g Mes sage)

Old eNB UE X2AP Id

New eNB UE X2AP Id eRABs Subject to Status Transfer eRAB Subject to Status Transfer 0 eRAB Id UL Count Value PDCP SN HFN DL Count Value PDCP SN HFN … eRAB Subject to Status Transfer N-1

• 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.

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Target eNodeB  MME

S1AP Path Switch Request

S1

AP

Pa

th

Swit

ch

R

eques

t

eNB UE S1AP ID eRABs to be Switched DL List eRABs to be Switched DL Item 0 eRAB Id Transport Layer Address GTP TEID … eRABs to be Switched DL Item N-1 EUTRAN CGI PLMN Id Cell Id TAI PLMN ID TAC UE Security Capabilities Encryption Algorithms Integrity Protection Algorithms

• 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.

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MME  SGW

Modify Bearer Request

Mod

if

y

Bear

er

R

eques

t

(initia

ting

me

ss

ag

e)

Bearer Context to

be Updated

EPS Bearer Id

S1-U eNodeB

F-TEID

TEID/GRE Key

• 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.

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SGW  MME

Modify Bearer Response

Mod

if

y

Bear

er

R

esponse

(initia

ting

me

ss

ag

e)

Bearer Context

Updated

EPS Bearer Id

S1 SGW F-TEID

TEID/GRE Key

• SGW updates the bearer and responds back

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S1AP: MME Target eNodeB

S1AP Path Switch

Acknowledge

S1

AP

Pa

th

Swit

ch

R

eques

t

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

• 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.

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Target eNodeB 

Source eNodeB

X2AP UE Context Release

X2AP U

E Con

te

xt R

ele

ase

(I

nitia

ting

Messag

e)

Old eNB UE X2AP

ID

New eNB UE X2AP

ID

• Sent when the target eNodeB has successfully completed the path switching and radio signaling for the handover.

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UE-NAS  MME-NAS

Tracking Area Update

Request

Tr ac king A rea U pd at e R eq ue st Uplink NAS Transport eUTRAN-CGI PLMN Id Cell Id TAI PLMN Id TAC Tracking Area Update Request

EPS Update Time TA Updating

Old GUTI

M-TMSI

Last Visited Tracking Area EPS Bearer Context

Status

Bearer Context Active • Sent if the just completed

handover resulted in a Tracking Area Update

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MME-NAS  UE-NAS

Tracking Area Update

Accept

Tr ac king A rea U pd at e R eq ue st Tracking Area Update Result

EPS Update Result TA Updated

T3412

T3402 • Sent if the just completed

handover resulted in a Tracking Area Update

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