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EMM Procedure 6. Handover without TAU - Part 3. S1 Handover
April 08, 2014 | By Netmanias (tech@netmanias.com)
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This document will describe the procedure for S1 handovers 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 handovers on S1 protocol will be discussed, followed by detailed procedures of S1 handover. We will learn how EPC (MME) intervenes in preparation of a handover between eNBs, and how DL packets are forwarded through an indirect tunnel that passes S-GW 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 information elements in EPS entities are different before and after the S1 handover procedure.
Page 1 of 5

Table of Contents  

1. Introduction
2. Concept of S1 Handover

2.1 S1 Protocol Stacks

2.2 S1AP Procedures and Messages Relating to Handover

2.3 S1 Handover Procedure at a Glance

2.4 UE State and Connection Information Before and After S1 handover

3. Procedure of S1 Handover

4. EPS Entity Information: Before/After S1 Handover

5. Closing



1. Introduction


In the previous document, “EMM Case 6. Handover without TAU – Part 2. Overview of X2 Handover” [1], we have discussed the procedures related to an X2 handover that is performed solely by two eNBs.


This document will focus on procedures for an S1 handover performed between two eNBs with intervention of EPC. As in X2 handovers discussed previously, we will assume both source and target eNBs are connected to the same MME/S-GW (intra-LTE environment), 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 S1 handover, and

Chapter 3 provides detailed procedures of S1 handover. Finally,

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




Page 1 of 5
knokej 2014-04-26 04:05:42
As usual, nicely written and nicely diagrammed. Keep up the good work Netmanias!
Pongtep C 2014-07-16 18:15:26

It's very good in details. Thank you

Kumar 2015-03-29 12:41:52

Excellenbt details to make everythings clear and easy to understand.


Looking forward to similiar document for CSFB to 3G/2G and VoLTE Call Setup.

weiwei 2015-07-22 20:53:53

So good!But a question,why not use C/D instead B/A?   Because C/D+RRC=E-RAB

anhdt 2016-06-24 12:15:00

Thanks for your detail document. Can you publish document relating inter-RAT handover

sinai 2018-11-11 19:32:48

I am blown away,

this is the best informative website ever. could you tell me which tools do you use to draw your figure?

gautam kashyap 2021-02-03 21:14:10

Hi Team,

In the handover preparation diagram, it shows ECM and EMM state at Target ENB.

Shouldn's they appear as MME states?




Shahreen Noor Ahmed 2021-02-28 08:40:36

Excellent learning material, thanks. Do you have content for the two other S1 handover procedure especially on Inter-LTE HO to understand why actually that is triggered? 


• Inter-LTE Handover: UE’s serving MME and/or S-GW is changed after handover

■ Inter-MME Handover: UE’s serving MME is changed, but S-GW remains unchanged after handover

■ Inter-S-GW Handover: UE’s serving S-GW is changed, but MME remains unchanged after handover

■ Inter-MME/S-GW Handover: Both UE’s serving MME and S-GW are changed after handover


• Inter-RAT Handover: Handover between networks that use different radio access technology


■ E-UTRAN to UTRAN, etc.

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