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LTE EMM and ECM States
September 20, 2013 | By Netmanias (tech@netmanias.com)
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Comments (20)
Page 2 of 6



Table of Contents  

1. Introduction
2. EMM, ECM and RRC States
3. EMM State Transition
4. EMM Features

5. EMM User Information

6. Closing



2. EMM, ECM and RRC States


2.1 Type of EMM Procedures


Table 1 lists the types of EMM procedures supported by NAS protocols and the specific procedures belonging to each of these types. Detailed EMM procedures will be discussed through the documents to be presented later on. We will briefly see what specific procedures are in each EMM type.


Table 1. Types of EMM procedures

There are three types of EMM procedures as follows:
  1. EMM common procedure: Refers to one that can always be initiated whilst a NAS signaling connection exists between a UE and MME. This procedure can be divided further into five procedures : globally unique temporary identifier (GUTI) allocation, authentication, identification, security mode control (SMC), and EMM information;
  2. EMM specific procedure: Refers to one that is related to user mobility (registration and location update). It can be divided further into three procedures: attach, detach and TA update (TAU). In an LTE network built with an existing 3GPP network, additional procedures (i.e. combined attach, combined detach and combined tracking area update) are also included in the procedure. However, since this document discuss an LTE network only, these procedures are excluded in Table 1 (See “LTE Interworking” technical document (TBD)) for more information about the procedures); and
  3. EMM connection management procedure: Refers to one related to establishing a NAS signaling connection. It can be divided further into three procedures: service request, paging and transport of NAS messages.


2.2 EMM/ ECM/ RRC State


EMM is a sub-layer of the NAS layer. As an EMM procedure proceeds, a UE can have one of the seven EMM states1 and an MME can have one of the four EMM states2 . Among these states, some states such as “EMM-Registered” and “EMM-Deregistered” are the common states that both entities can have. Hereinafter, EMM states will refer to these two states (See [1] for more information about other EMM states.).


In order for a UE and an MME exchange NAS messages with each other, a signaling connection for exchanging NAS messages must be established between them. This connection is called EPS Connection Management (ECM) connection. It is a logical connection consisting of RRC connection between a UE and an eNB, and S1 signaling connection between the eNB and an MME as seen in Figure 1. This means, when an ECM connection is established/terminated, the RRC and S1 signaling connections are all established/terminated as well. To a UE, an established ECM connection means having an RRC connection established with an eNB, and to an MME it means having an S1 signaling connection established with the eNB.


Figure 1 illustrates EMM, ECM and RRC states associated with the UE and MME3. EMM can be in either “EMM-Registered” or “EMM-Deregistered” state depending on whether the UE is attached to, or detached from, the network. ECM can have either “ECM-Connected” or “ECM-Idle” depending on whether a NAS signaling connection (i.e. ECM connection) is established or not. Likewise, RRC can be in either “RRC-Connected” or “RRC-Idle” depending on whether an RRC connection is established or not. Table 2 shows in what conditions a UE stays in a particular state.


Figure 1. EMM, ECM and RRC states


Table 2. Description of EMM, ECM and RRC states

Page 2 of 6
Netmanias 2015-05-25 03:10:12

There is an error in Figure 2 and it will be fixed later.

Find comments belows (from Jakub Bluszcz in LinkedIn):

RLF doesn’t necessarily result in transition from RRC-CONNECTED to RRC-IDLE [3GPP 36.331 5.3.11].

If cell selection and reestablishment procedures are successful the UE remains in RRC-CONNECTED. In case there is no suitable cell or reestablishment fails the UE goes to RRC-IDLE.

Some selected cases are presented here: http://www.sploty.com/en/rlf-and-connection-reestablishment

Moreover, the RLF and subsequent transition to RRC-IDLE doesn’t result in transition from EMM-REGISTERED to EMM-DEREGISTERED. The correct state should be EMM-REGISTERED substate NO-CELL-AVAILABLE [see 24.301], which means that after new cell selection followed by new RRC Connection Establishment the EMM Service Request procedure can be successful (i.e. no Attach nor default EPS bearer establishment is necessary). 

Deepak 2016-09-14 22:51:17


Lajos Pajtas 2015-06-11 17:20:48

Probably the best description available on net.

Keep up the good work!

Mateenuddin 2015-09-09 01:43:12

The best document ever to read for begginers.

Thanks for effort, please keep it up.

vidyasagarrao7 2017-01-10 22:11:21



Can you please share this document? I am unable to download that..

Mail id: vidyasagarrao7@gmail.com

Netmanias 2017-01-11 13:10:21

You can download PDF file after login.

santoshi 2016-01-29 00:20:18

how cna we attach a ue to network through coding?

Basheer 2016-04-12 15:54:28

Explanation on UE transitions are awsome...Thank you very much

MANOJ 2016-05-04 13:58:13



Thanx for very good docs for LTE..


Can you explain in which order we should read LTE docs

Nikita Raju Nikrad 2017-06-13 16:19:47



You can find the index with above link for the LTE Technical Documents.

CHEN 2016-10-14 14:29:45

Thank you for your every good documents for LTE, easy to understand :)

ANTONIO ALONSO 2016-10-21 05:23:39

I think there is an error when explaining transition from D to C. Current text provides two conditions [numbered as "i)" and "ii)"] and use "AND" between both (so it means the two conditions must be fulfilled) ... should it not be an "OR" instead? Any of them means transtioning from D (EEM-Registered, ECM-Idle/RRC-Idle)  to C (EEM-Registered, ECM-Connected/RRC-Connected).

Thierry Van de Velde 2017-03-01 04:23:46

This article has become a timeless classic and a must-read for any 4G and 5G network engineer.

jullien 2017-03-02 20:46:14

Dear Netmanias;

From where i can download all the technical documents ? Thanks.

Netmanias 2017-03-04 10:04:57

Thank you for your interest and inquiry. 

Unfortunately, you can't download all documents at a single location at once.
Instead, each page provides a red box saying "Download PDF File", and you can simply click it and download the document on the page. 

Hope it helps.

CHEN 2017-05-25 17:03:27

Dear Netmanias,

There is a question, what will the LTE client do if the network will drop the LTE PDP context because the inactivity timer in the P-GW has expired and therefore release the client IP address?

Apparently, there is a 24-hr inactivity timer in the P-GW. Will the LTE client re-establish a new PDP Context activation request to the network to keep LTE connected?





justin king 2020-02-04 16:03:40

Wonderful document that can be understood even by beginners thanks a lot


jassim.mohamed@asiacell.com 2020-04-05 18:09:30


firstly thanks alot for sharing such vlauble, comprehensive doucment.

I would like to asking if there are a way to find out all documntes related to LTE basic detials sequentially arranged.

thanks in advance


Yijie 2020-10-15 15:32:08

Thanks for your effort, it is super useful.

HMK 2020-10-23 19:37:18

Anyone can u pls help Im confusing on below two things I would like to know what is 
1.Number of Dynamic 4G bearers(This parameter indicates the number of activated 4G bearers) and
2. Number of dynamic 4G MMs(This parameter indicates the number of attached 4G users. The users are those in the ECM-CONNECTED and ECM-IDLE states)

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