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EMM Procedure 1. Initial Attach - Part 2. Call Flow of Initial Attach
January 16, 2014 | By Netmanias (tech@netmanias.com)
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This document, Part 2 of Initial Attach, discusses detailed initial attach procedures in EMM Case 1. EMM Case 1 concerns initial attach procedures for a user attaching to a network for the first time, as categorized as Attach Case 1 in Part 1. Below, we will provide a description of those procedures and their related function blocks explained in Part 1. We will also summarize what kinds of information are set in the EPS entities after the procedures.
Page 1 of 4

Table of Contents  

1. Introduction
2. Initial Attach Procedure 

        2.1 IMSI Acquisition
        2.2 Authentication
        2.3 NAS Security Setup
        2.4 Location Update
        2.5 EPS Session Establishment
3. EPS Entity Information

        3.1 Before Initial Attach
        3.2 After Initial Attach
4. Closing



1. Introduction


Now, we will look into EMM Case 1: Initial Attach by Unknown UE in "Eleven EMM Cases in an EMM Scenario"[1]. This case is when a user turns on his UE and attempts to attach to a network for the first time after subscribing to the LTE network/service. It was previously categorized as Attach Case 1 in Part 1 [2].


This document, Part 2, is organized as follows:


Chapter 2 explains the detailed procedures in EMM Case 1 (Initial Attach with IMSI), focusing on the functions required in the case, as described in Part 1.

Chapter 3 summaries what information is changed in each EPS entity after initial attach in Attach Case 1.





Page 1 of 4
fanrui 2014-07-08 19:20:50

Very good document. I have a question.

In the attach accept message, the IP address allocated to UE is mandatory. However, there is no DNS related information. I checked spec, in the PCO (protocol configuration options) DNS server address can be included. However PCO is optional. So I just wonder, without DNS server in PCO, how could UE access the internet?

nn 2018-01-21 22:21:29

Hi, Did you find the answer anywhere ? 


Hi Netmania ,


Can you please clarify points put by fanrui. Thanks in advance.



broncadocrl 2018-10-20 00:20:30



Normally, DNS server address is always included in PCO. If not, UE is only able to access internet using IPs (no DNS server to translate the domain, etc.) or manually defining the DNS server in the UE.

Nagarjuna 2014-09-18 22:07:16

Hi Netmanias,


                   First i have to thank you ,that you shred a very valuable document with us regarding lte stuffs , but here i have one small suggestion , in point of initial attach part-1 &2 ,document maily focusing on core network rather on RRC messages (i.e., between UE and Enodeb).can you please add one more document(which highlits UE and Enodeb messages and transcations) related also,so that end-to-end connection would be clear for us.






Vijayakumar 2015-04-12 21:45:11

Hi Netmanias,


Really appreciate the depth of the information provided. Can you please also add "Selection of MME" in the eNodeB (using static/DNS queried)?

HUAWENJIAN 2015-07-19 12:59:52

Very Excellent!Thanks for your sharing! I have got lot of information about my concern topic.

Siva Dugana 2015-10-20 11:58:00

Thanks for the detailed explanation. Appriciate to have a thought to share the knowledge like this...

DAniel 2016-04-05 09:37:32

Man, congratulations and thank you

Chandru 2016-05-25 18:40:09

Really Really Super. Thanks Guys

Netmanias 2016-05-29 04:15:31

Glad you liked it. And thank you for your interest!

Stay tuned!

Markus 2016-09-21 20:03:50

Great explanation! Thank you very much.

venkat 2016-10-17 13:31:10

Good explanation!!


bijukumar 2017-05-23 15:17:34

good explanation and  well explained in main areas of call flows

Ken Hu 2017-07-21 10:53:06

There is typo in "Figure 7. Procedure for EPS Session Establishment (1)" step 14 : "Receive S5 TEID (DL)" should be "Receive S5 TEID (UL)".

Wireless 2020 2023-06-25 23:23:45

yes corrrect

Marcin 2017-09-29 18:43:43

I was thinking same as you. Thank you for notice so I know i wasn't wrong.

Above all - good explanation

netti 2017-10-05 15:47:07

Can somebody help me with the softwares required in this simulation?

yasin akar 2017-12-19 23:24:37

hi everyone,


Is it possible to establish more than 1 default bearers? I mean within an initial attach procedure the EPS can setup more than one APN default bearers for example one for internet and other one is IMS bearers? Or only one default bearer can be setup within an initial attach procedures and other additional default bearers can be setup upon demanded?


thanks for your supports.

keyur shah 2018-08-09 15:19:05


Thanks for such nice explaination.

I have one doubt.why HSS Generate more than one AV(Authentication Vector) for single UE?





gorgedowdy 2020-04-01 08:08:18

Yes i've the same doubt. Someone clarify it

Alassane MAMADOU 2018-09-25 18:11:25

Hi , 

Good explanation . 

I have one question , i got a lot of Attach rejection and when i checked ,all  are related to the case when Attach complete were not sent to the MME .

How can we analyse this issue ? 

I am using Astellia 


Tushar Waman 2019-03-21 00:06:13


In Step 14, Should'nt it be ReceiveS5 TEID (UL) instead of Receive S5 TEID (DL) on the S-GW ? Or did i miss something :/

diy 2019-03-27 06:26:19



Great Explanation, Thank You for taking the time to document in such great detail. 

I have one question regarding MSISDN, With reference to Fig 7. Create Session Request, will MSISDN be exchanged between SGW and PGW?

One more thing will IMSI be always included in the create session request message?

Thank you for all your help



Wireless 2020 2023-06-25 23:25:49

yes IMSI will be present in CSR

Wireless 2020 2023-06-25 23:25:43

yes IMSI will be present in CSR

treyon13579 2020-10-01 00:47:51

Yes MSISDN is found in ULA and CSR

Chaitu 2019-04-23 01:14:04

Hii ,

      If I want to make a volte call ,will this lte authentication happens first before going to ims part?

Saravanan 2020-03-13 18:37:00

Hi Chaitu,

Yes. UE(device) LTE authentication happens first, once UE gets IP Address then IMS Registration( authentication) happens.



Spoorthi Devanand 2019-05-21 20:00:58

Thabk you for the detailed explanation!

Can you please explain what NAS-MAC is ?

Dominic Antony 2019-12-11 20:07:26

Hi Folks,

Could someone please explain, how internet connectivity is established in UE? I'm looking for that sepecific message (RRC/ NAS) which upon receiving establishes a successfull internet access. Like if look at the smartphone, there should be up and down arrow at the signal range icon. Please see the attached image. So when will this happen or which message establishes this internet connection? Please help.



ANIS UR REHMAN 2020-02-05 17:26:38

Hi Netmanias,

Could you please explain the CSFB call flow ?




yasir69093735 2020-02-10 15:51:04

I Have A Quary, when UE first time attaching with the network, how it get LAI in 3G and tAI in 4G.?????

aaa12312 2020-08-23 13:22:02

Is it possible to establish more than 1 default bearers

superanimes.ru 2023-09-23 13:06:44


Thanks for such nice explaination.

I have one doubt.why HSS Generate more than one 



Anurag K Sinha 2024-01-22 20:49:11

Hi Netmanias,

After step 29 there should be step 30 but it is mistakenly written as 39.

Also, in the same step, the E-RAB Setup Response is shown between e-Nb and S-GW, but it should be between e-nb and MME.

Kindly acknowledge.

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