EMM Procedure 6. Handover without TAU - Part 2. X2 Handover

March 21, 2014 | By Netmanias (tech@netmanias.com)

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SUMMARY

This document will describe the procedure for X2 handover performed in an intra-LTE environment, as defined as EMM Case 6 in our technical document, “Eleven EMM Cases in an EMM Scenario”. First, features related to handover on X2 protocol will be discussed, followed by detailed procedures of X2 handover. We will learn how a handover between eNBs is prepared and performed without EPC intervention, and how DL packets are forwarded through a direct tunnel between two eNBs during the handover interruption time for seamless service provision. We will also look into how EPC gets involved in switching the EPS bearer path after a handover. Finally, we will examine how the information elements in EPS entities are different before and after the X2 handover procedure.

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Table of Contents

1. Introduction
2. Concept of X2 Handover

2.1 X2 Protocol Stacks

2.2 X2AP Functions

2.3 X2 Messages Relating to Mobility Management Function

2.4 X2 Handover Procedure at a Glance

2.5 UE State and Connection Information Before and After X3 handover

3. Procedure of X2 Handover

4. EPS Entity Information: Before/After X2 Handover

4.1 Before X2 Handover

4.2 After X2 Handover

5. Closing

1. Introduction

In the previous document, “EMM Case 6. Handover without TAU – Part 1. Overview of LTE Handover” [1], we have discussed the procedures related to LTE handover, and learned that LTE handover is a “UE-assisted/network-controlled” process that is reported by UE, and determined by eNB.

This document will focus on procedures for X2 handover that is performed through the x2 interface in an intra-LTE environment. So, we will assume both source and target eNBs are connected to the same MME/S-GW, and are located in a TA that IS in the Tracking Area Identifier (TAI) list for the associated UE.

Chapter 2 describes the concept of X2 handover, and

Chapter 3 provides detailed procedures of X2 handover. Finally,

Chapter 4 will summarize how information elements in EPS entities are different before and after the X2 handover.

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knokej 2014-04-09 06:03:39

2 questions regarding AS security. The ordering of steps 15 and 16 make it appear as though buffered DL data can start flowing to the UE BEFORE AS security is in force. Could steps 15 and 16 be reversed?

In order for the UE to derive new keys, it must know the PCI and frequency of the target cell. Which message from the source eNB contains the indication of which cell is the target? (Probably it is the Handover command, but the target cell/eNB indication isn't shown as data in that message)

Netmanias 2014-04-12 10:11:16

Hi knokej,

Thank you for your comment.
- As you noted, step 15 and 16 should be switched. We've fixed it and posted the updated file.
- "MobilityControlInfo" filed in the Handover Command message (i.e., RRCConnectionReconfiguration message) contains the PCI and carrier frequency of the target cell.

Siraj 2016-08-04 17:54:17

Hi Team,

I wanted to know is it possible to have X2 link b\w two eNodeBs which are being controlled by two different MMEs.

Creation of X2 link is part of planning or it happens dynamically.

ABHISHEK RANJAN 2017-12-17 15:48:06

Hi Siraj,

Yes, it is possible to have x2 links between two eNB's which are connected to different MME's.

It can be done by defining the X2 neighbor relation or by ANR.

Hope this helps.

Many Thanks,

Abhishek

himanshu g 2017-08-22 15:43:00

Hi ,

I have a query is it mandatory for source eNB to send "handover restriction list" IE in "handover request" message to target eNB in the case of inter PLMN X2 handover ?

BR,

Himanshu Gupta

ABHISHEK RANJAN 2017-12-17 17:26:13

Hi Himanshu,

it is an optional parameter, from specs 36.4113:

=====================================================================================

The target eNB shall use the information in Handover Restriction List IE if present in the HANDOVER REQUEST message to

- determine a target for subsequent mobility action for which the eNB provides information about the target of the mobility action towards the UE;

- select a proper SCG during dual connectivity operation.

If the Handover Restriction List IE is not contained in the HANDOVER REQUEST message, the target eNB shall consider that no roaming and no access restriction apply to the UE.

=====================================================================================

In this case if HRL is not provided and HO fails because of this, then it may make a HO via S1.

Many Thanks,

Abhishek

Manuel Ortiz 2018-04-25 11:29:36

Is there more information in regards to what happens during a Handover Cancel message? Does this apply to a terminal when it moves back to the serving eNB before the handover is complete and serving eNB signal strentgh becomes stronger than the target eNB?

Thank you!

Ajeet Srivastava 2020-06-12 16:29:18

x2 handover failure invalid mme group why this error??

chandan dubey 2020-07-23 18:05:47

Great explanation for the X2 handover thanks Netmania for your efforts...

Ganesh 2020-10-12 20:23:31

ON Handover Preparation stage of X2 handover,

Under this sction: "5) [Target eNB] Preparing X2 Handover"

I could see this notes as "If available, it establishes an UL S1 bearer connecting to S-GW, by using the UL S1 bearer information (S1 S-GW TEID) stored at the source eNB."

Anyone could you explain this process in detail that , How the target eNB knows and establishes the UL S1 bearer tunnel with the S-GW (which is receiving UL packets from Source EnodeB)

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