SAE-GWMME

eNB

Source Target

eNB

UE

DRB (Data Radio Beaer) S11 (GTP-C) SignalingS1 Bearer (GTP Tunnel)

S1 Signaling

moving

SAE-GWMME

eNB

Source Target

eNB

UEmoving

Signaling in Radio

X2 Bearer (GTP Tunnel)

X2 Signaling

SAE-GWMME

eNB

Source Target

eNB

UEmoving

MME

eNB

Source Target

eNB

UEmoving

1. Before Handover 2. Handover Preparation 3. Handover Execution 4. Handover Completion

· Detach from Source eNB· Direct Packet Forwarding (DL

traffic from Source eNB to Target eNB)

· Source eNB makes the HO decision

· Direct Tunnel Setup (X2 Transport Bearer between Source eNB and Target eNB)

· Path Switch procedure between Target eNB and MME

· Release S1 Bearer of Source eNB

· Release X2 Transport Bearer for direct Packet Forwarding

SAE-GWMME

eNB

Source Target

eNB

UE

5. After Handover

UL Traffic

DL Traffic

2

1

2

3

31

2

1

2

3

31

1

22

3

4

31

1

2

3

31

1

2

3

31

SAE-GW

2 2

LTE X2 Handoverwww.nmcgroups.com

X2 Handover CaseThe X2 handover is used when direct connectivity exits between the source and target eNBs with X2 interface

Handover PreparationThe 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 ExecutionThere is direct tunnel formed between source and target eNBs for downlink data forwarding in handover execution time

LTE X2 Handover 1

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. Copyright © 2002-2011 NMC Consulting Group. All rights reserved.

LTE X2 Handover

UE

EMM-Registered

RRC-ConnectedECM-Connected

EMM-RegisteredECM-Connected

RRC-Connected

S-GW P-GWMME PCRFHSSeNB

Source Target

eNB

S5 Bearer (UL/DL)S1 Bearer (UL/DL)DRB (UL/DL) Internet

Handover Decision by Source eNB

Han

do

ver

Pre

par

atio

n

Derive KeNB*

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)

Handover Request

Receive S1 TEID (UL)

Admission control with E-RAB QoS (QCI, ARP)Target eNB reserves RRC resource and allocates a new C-RNTI (e.g., Target C-RNTI)

Handover Request Ack

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)Receive Target eNB TEID (DL)

DL: From Source eNB to Target eNB

Create DRB ID (UL/DL)

X2 Transport Bearer (DL) Established

S1 Bearer Established (UL)

S1 TEID (UL) for Source eNB

eNB generates AS keys from KeNB (KeNB = KeNB*)

Path Switch Request

ECGI, TAI, Received UE Security Capability, E-RAB to be switched in DL (E-RAB ID, S1 Target eNB TEID)Create S1 TEID (DL) Modify Bearer Request

EPS Bearer in DL (EPS Bearer ID, S1 Target eNB TEID), ECGI, TAI

Modify Bearer Request

EPS Bearer ID, ECGI, TAI Indication of IP-CAN Session Modification (CCR)IMSI, UE IP, ECGI, TAI

Acknowledge of IP-CAN Session Modification (CCA)

EPS Bearer IDModify Bearer Response

Modify Bearer Response

EPS Bearer in DL (EPS Bearer ID)Switch to DL path

Send “End Marker” PacketS1 Bearer (UL/DL)

X2 Transport Bearer (DL)

EM

S5 Bearer (UL/DL) InternetS1 Bearer (UL/DL)DRB (UL/DL)

For re-ordering in the Target eNB, the S-GW sends “End-Marker” packet(s) on the old path immediately after switch the pathTarget eNB sends packets to UE from old path

(X2) until receiving “End-Marker”, after then it sends packets from new path (S1)

Path Switch Request Ack

E-RAB to be switched in UL (E-RAB ID)UE Conext Release

EM

Source eNB release UE context

EMM-Registered

RRC-ConnectedECM-Connected EMM-Registered

ECM-ConnectedRRC-Connected

S1 Bearer Established (DL)

Receive S1 TEID (DL)

X2 Transport Bearer (DL)Released

S1 Bearer Released (UL/DL)

Measurement Report 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)

Han

do

ver

Exe

cuti

on

UE

can

not

send

& r

ecei

ve p

acke

ts

S5 Bearer (UL/DL)S1 Bearer (UL/DL) Internet

SN (Sequence Number) Status Transfer

DL Count (DL PDCP SN), UL Count (UL PDCP SN)

X2 Transport Bearer (DL)

Buffering DL PacketsSynchronization

UL Allocation

Handover Confirm (RRC Connection Reconfiguration Complete)

Target C-RNTI

DRB Established (UL/DL)

Start to send the Buffered DL Packets to the UE

S5 Bearer (UL/DL)S1 Bearer (UL/DL) Internet

UE generates KeNB (KeNB = KeNB*) & AS keys

AS Security setup

RRC-Connected

RRC-IdleDetach from the source eNB

Handover Command (RRC Connection Reconfiguration)

Target C-RNTI, Target DRB ID (UL/DL), Target eNB AS Security Algorithm)

Receive DRB ID (UL/DL)

X2 Transport Bearer (DL)

DRB (UL/DL)

S5 Bearer (UL/DL) InternetS1 Bearer (UL)DRB (UL/DL)

Han

do

ver

Co

mp

leti

on

LTE X2 Handover 2

eNB

Source

S-GWEnd Marker

UE

EM3 4

5

3 4 5 EM

EM

Target eNB