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EMM Procedure 1. Initial Attach - Part 1. Cases of Initial Attach
December 26, 2013 | By Netmanias (tech@netmanias.com)
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Comments (27)
The Initial Attach document discusses procedures for initial attach by a user attaching to an LTE network for the first time, as defined as EMM Case No. 1 in our previous document, “Eleven EMM Cases in an EMM Scenario”. The document consists of two companion documents: this document (Part 1. Cases of Initial Attach) and the subsequent document (Part 2. Call Flow of Initial Attach). In this document, we will distinguish different cases of initial attach (Attach Case 1 ~ Attach Case 5), and different function blocks involved in each case. Then we will explain different procedures required in each case of initial attach. In the subsequent document, detailed initial attach procedures in Attach Case 1 will be described.
Page 1 of 4

Table of Contents  

1. Introduction
2. Cases of Initial Attach

        2.1 Unknown UE

        2.2 Known UE
3. Simplified Call Flow in Each Case

4. Closing



1. Introduction


This document discusses initial attach, as defined as EMM Case 1 in our previous document, “Eleven EMM Cases in an EMM Scenario” [1]. During this phase, a user turns on his device (UE) for the first time after subscribing to an LTE network/service, and attempts to attach to the network, sending an IMSI to the network (The initial procedure will be the same if the user is using an IMSI as his UE ID, even when he has previously attached to the network).


The initial attach procedure may vary depending on circumstances. It can be as in EMM Case 1, to be explained below, or as in EMM Case 11 (initial attach in another city), to be covered in the subsequent document later. Or it can be other types depending on whether or not a user (UE) kept the information used for his last attach to the network (hereinafter referred to as the “Last Attach Information1, or whether the network (MME) has information about the user, including UE ID, (hereinafter referred to as the “Last UE Context”2). So, this document (Part 1 of Initial Attach) will describe different initial attach types, and find out their characteristics and differences from one another. The subsequent document (Part 2) will focus on EMM Case 1, providing the related procedures in details.


This document is organized as follows:


Chapter 2 identifies different initial attach cases, and

Chapter 3 briefly discusses different initial attach procedures of each case, by listing the functions to be performed by MME.






Page 1 of 4
Dirk Van Aken 2013-12-31 07:39:05
Hello Netmanias,

I have been an xDSL/GPON/Ethernet/IP System Architect for the past 15 years and recently I started to work on LTE.
During one of my searches on detailed LTE information I came accross your site.
Looking at your papers and presentations, I must admit I never saw more detailled drawings than yours !
Congratulations for those who created them !

I wonder which software package are you using ?
I'm a regular Microsoft Visio user but IMO this package falls short for this kind of drawings ..
Netmanias 2013-12-31 15:55:57
Hello Mr. Dirk Van Aken,

Thanks for having interest in our site.

Most of our diagrams were created using Microsoft Visio. And we didn't use any special SW package.
While providing professional consulting services in various technical fields over 10 years, we have created quite a lot of Visio templates ourselves.

Paulos Rizos 2014-01-02 18:11:13
Top quality as always, really appreciating these english versions. Looking forward to the next technical document.
Boonchai 2014-03-18 16:37:55
Excellent Thank you
Boonchai 2014-03-18 16:38:25
Excellent Reference to understand
ASHOK 2014-04-04 22:04:23
Very nice and easy to understand...
ASHOK 2014-04-04 22:04:52
Very nice and easy to understand.. and more informative as well
Naresh Babu 2014-06-26 20:38:02

This is what I need to understand call flows. Finally I am in right place.

tuan 2014-08-07 18:48:38

thanks you

vipin garg 2014-10-01 14:57:33

Hi Netmanias,


Thanks for these excellent explanations. Recently I cleared an interview in which your tutorials played a major role.  :)


Keep goind and enlighten the world with technology knowledge

gvsathyam 2014-11-28 19:18:39

Hi Netmanias,


Could you please provide your tutorials on LTE RACH & HARQ procedures ?



Raghu 2014-12-09 20:25:33

Nice to understand and very intresting

gecuili 2015-01-23 19:20:06

 I think, the scene ii) the MME doesn’t have the valid Last UE Context of the UE can cover the  scene iv) the UE was detached from other MME last time.

Kendy Nowax 2015-04-03 19:00:44

Thanks for this material.

Very helpful and easy to understand. Hope that you will upload some material about AS side.


Ayaz mohammed 2015-07-02 21:26:38

Thanks Alot....

Nice and simple..Please share more documents on LTE..

Ritzmond M. Josef 2015-07-12 23:23:10

Very informative... Thanks a lot netmanias.com. Keep it up!

Tony Lee 2015-09-22 07:13:58

Your material is great! A good introdution before one dive into 3gpp spec

aleksandar 2016-02-27 14:32:18


Do you maybe have call flow for the following case:

1.Subscriber is attached to 2-3G network.

2.Subscriber is not provisioned on HSS

3.Subscriber is using LTE capable phone.

Subscriber has active 2-3G PDP session. What is the scenario when UE attempts to handover to 4G in this case? Can Modify Bearer request from MME to SGW happen before successfull ULR/ULA (that obviously can not  be successfull, as subscriber is not provisioned in HSS)?

Thank You!

prahladbutola 2016-03-05 21:31:34

In a very simple way the subject is explained nicely...

AnnaG 2016-04-27 03:01:49

Very good documentation. Easy to understand and to the point.

Had a question- Figure 2 Attache case 3. If the NAS security setup is already done with OLD MME, when the UE attaches to the new MME, How can the new MME process these messages and contact the old MME ? Wont the integrity check fail as the integrity setup of the UE was with old MME and not the new MME ?

santosh.patra 2016-06-24 12:16:23

Very useful and easy to understand the flow end to end...Kudos to the team.


Thank U.

chamod 2019-09-17 15:02:20

Man who is uploading this contain deservers a nobel ;-)

Mahmood 2019-10-29 01:22:08

Thank you for breaking down the call flow into easy to understand steps supported with beautuful visuals

May Allah guide you to the ultimate truth.

Fahd Ali Khan 2020-01-15 23:22:30

Need IMS end to end call flow..

MRawat 2020-02-19 18:32:06

Dear Netmania Team,

If new MME sucessfully identified old MME , then how Old MME not able to provide UE identity to New MME. As per this scenario UE previously attached to Old MME and must have UE identifucation. Pls let me know what causes "Identification Response (error cause)"

Attach Case 3: Unknown UE, MME Changed

The UE requests for initial attach using its Old GUTI. So, the MME (New MME) asks the Old MME for the Last UE Context relating to the UE, but fails to receive any. So, the MME requests the UE for a UE ID, and acquires an IMSI.

[UE → New MME]                Attach Request (Old GUTI)
[New MME → Old MME]        Identification Request (Old GUTI)
[Old MME]                         No IMSI
[New MME ← Old MME]        Identification Response (error cause)
[UE ← New MME]                Identity Request (UE ID = IMSI)
[UE → New MME]                Identity Response (IMSI)


chandan dubey 2020-05-26 17:41:20

Great explanation Netmanias!!!!!!!!

chandan dubey 2020-05-26 17:41:49

Great explanation Netmanias

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