EMM Procedure 5. Periodic TAU

February 19, 2014 | By Netmanias (tech@netmanias.com)

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SUMMARY

This document, as the fifth of the EMM Procedure series, will describe the periodic TAU procedure, as defined as EMM Case 5 in our technical document, “Eleven EMM Cases in an EMM Scenario”. This procedure is used when a UE, while staying inactive in ECM/RRC-Idle state, informs the MME of its current location (TAI, Tracking Area Identity) upon expiration of the periodic TAU timer. Between a UE and MME, no bearer (E-RAB) is set up, and only a signaling connection (ECM connection) is established. We will learn how the ECM connection is released and the UE returns to Idle state once a new TA is reported. We will also look into how information elements in EPS entities are different before and after the periodic TAU procedure.

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Table of Contents

1. Introduction
2. Concept of Periodic TAU
3. Procedure of Periodic TAU
4. EPS Entity Information: Before/After Periodic TAU

4.1 Before Periodic TAU

4.2 After Periodic TAU

5. Closing

1. Introduction

This document describes the periodic TAU (Tracking Area Update) procedure defined as EMM Case 5 in our technical document, “Eleven EMM Cases in an EMM Scenario” [1]. This procedure is performed when a UE which entered ECM/RRC-Idle state in the network due to no traffic periodically updates the network (MME) with its current location.

A TAU procedure is initiated when a UE enters a new TA that is not in the list of TAIs allocated by the MME at the time of the UE’s attach, or when the TAU timer expires. The EMM Case 5 presented in this document is related to the periodic TAU procedure performed by a UE upon expiration of the TAU timer. A UE in Idle state reports its current location to the MME by sending a TAU Request message when the TAU timer expires. After transiting to ECM/RRC-Connected state, and performing the periodic TAU procedure, the UE returns to Idle state.

This document explains the periodic TAU procedure performed by a UE in Idle state in an LTE network.

Chapter 2 explains the concept of periodic TAU, and

Chapter 3 describes its specific procedures. Finally,

Chapter 4 summarizes how information elements in EPS entities are different before and after the procedure.

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Paulos Rizos 2014-02-19 23:32:42

Great explaination as always. Eagerly awaiting the next installment.

Paulos Rizos 2014-02-26 00:47:57

As an idea it would be very good if you could produce some VoLTE technical documents.

Avadh 2014-03-28 17:23:29

Very crisp and clear explanation. Pretty helpful to understand the scenario thoroughly. Have been following Netmanias LTE documents for quite sometime now.

Two suggestions (please treat it as general comment for all LTE documents) :

1) Even though message names are clear yet sometimes its difficult to relate to exact information (message) element within a given message. It would be nice - if you can mention exact information element as well.

2) It would be better - if you also attach respective wireshark dumps along with the scenario covered in that document. That way, one can better relate the call flow to actual message flow.

-Thanks & Rgds
Avadh

LedZep 2015-05-03 16:11:20

Hi Netmanias,

Thanks for such a wonderful blog on LTE procedures. Very useful for those who are trying to get an end to end picture. Once again, amazing work and kudos to your team!

Also, I would like to point out a small error in Section "III. Procedure of Periodic TAU", steps "4), 5), 6) [UE → MME] ECM Connection Establishment Request and TA Report" as below:

>>> "TAU Request message is sent integrity-protected with the NAS integrity key (KNASint) and encrypted with the encryption key (KNASenc)."

As I know, at UE Initial NAS messages(Specifically, TAU and Attach Request are never sent Ciphered) are sent only Integrity Protected and not ciphered. Please refer to TS 24.301, Section 4.4.5 for further reference.

Thanks!

surajbora.bias 2015-05-25 00:16:21

Very Nice explained

Madhav 2015-06-17 17:20:28

no E-RAB (between UE and MME) ---- I think E-RAB always set up in between UE-eNodeB -SGW....

mani 2016-03-16 16:08:30

Hi,

Please clarrify me this procedure,

UE is in Attached to Netowrk with Some HPLMN(111-210) and in Equilent HPLMN list (112-210) is present , same has been configured in USIM as well

So , my question is when UE moved from (111-210) to (112-210) what could be the UE behaviour ?

Does UE intiate TAU procedure or Attach request ?

I have tried with Samsung Mobile it is intiating TAU when in the case UE is already attached with HPLMN(111-210) then UE moved from HPLMN - to Equilent HPLMN(112-210) since its already configured in USIM as well as Mobile configuration

But in the case if i removed Equilent HPLMN(112-210) in mobile NAs configuration UE will intiate Attach reuqest in this case

where as when i tried with other mobile like LG and other mobile altair ..it's intiating attach request procedure when UE changed from HPLMN(111-210) to Equilent HPLMN(112-210) instead of TAU even though SIM configured E-HPLMN list (111-210 && 112-210)

what is the ideal Behaviour ?

please share your valuable comments ..

bala nagarjuna 2019-09-10 16:09:27

Hi Mani,

IF Equivalent HPLMN or Equivalent PLMN is present and once UE moves from HPLMN to Equivent HPLMN/ PLMN then UE performs TAU instead of Attach if there is no detach happened. If detach happens then UE trigger again Attach Request.

I think TAU sending is ideal Behaviour in your case.

Ajay 2016-06-24 12:20:09

(ii) During the procedure, the UE stays in EMM-Registered, ECM-Connected and RRC-Connected state. The periodic TAU procedure is different from procedures for initial attach or service request in that no E-RAB (between UE and MME) is set up, and only the signaling connection (ECN signaling connection) for delivering periodic TAU-related NAS messages is set up during this procedure.

In above paragraph, small typo mistake.(ECN signaling connection) should be (ECM signaling connection)

Netmanias 2016-09-23 17:41:31

Thanks Ajay - the typo is corrected.

Federico Gennari 2016-09-22 22:31:06

Outstanding description and neat sequence diagram.

But, if I'm not wrong, it looks like that is missing any reference to timer T3440, that's letting the NAS Signalling Connection Release after the TAU.

wenjiangong 2017-01-24 14:52:21

how download this？

Netmanias 2017-01-24 15:21:15

Click the "Download PDF File" button on the left side after log in.

vishwas vasant rasal 2019-06-07 16:26:27

Hi Netmania ,

Thanks for this document , really meaningfull document .

I have one query , When UE is in connected state , and TA get changed which is not in TAI list , then TAU will get triggered , I request you to clarify this condition , especially please mention the rrc message name in which this TAU request will get piggybacked .

Thanks in advance ,

eagarly waiting for you response .

Response from other readers also most welcome....!!!

Devanshu Tyagi 2020-05-03 07:36:06

Hi Netmanias, Thanks for the nice articles covering great details on each topic.
I have a query, Let's say that Timer 3412 is not expired yet and UE moved from TA1 to T2 (both are in TAI List so TAU will not happen). Now if any downlink service request comes for the UE. Will MME still page it in TA1 first or how this will work ?

Devanshu Tyagi 2020-05-10 20:51:31

There's a small Typo in the below message :

The MME sends the UE a TAU Accept message, as integrity-protected and encrypted. This message is delivered through a Downlink NAS Transport message, an S1AP message, from the eNB to the MME, and then through a DL Information Transfer message, an RRC message, from the UE to the eNB.

which should be Like :

The MME sends the UE a TAU Accept message, as integrity-protected and encrypted. This message is delivered through a Downlink NAS Transport message, an S1AP message, from the MME to the eNB, and then through a DL Information Transfer message, an RRC message, from the eNB to the UE.

Nurgun G 2024-04-23 20:06:18

great explanation, thanks

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