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EMM Procedure 5. Periodic TAU
February 19, 2014 | By Netmanias (tech@netmanias.com)
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Page 2 of 5

 

 

 

2. Concept of Periodic TAU

 

A UE, while in Connected state, has an established end-to-end EPS bearer connecting the UE to the network (P-GW). The network (MME) keeps track of which cell the UE is located in. So, if there is any traffic destined to the UE, the network can deliver it immediately.

 

However, if a UE enters Idle state, the signaling connection and bearers (E-RAB bearer) between the UE and the network (MME) are all released. Then, the network (MME) loses track of the UE’s location. The network should always be aware of the current location of all UEs, whether in Active or Idle state, in order to deliver traffic to UEs that are in Idle state. So, those in Idle states should report their current location, i.e. in which TA (Tracking Area) they are located, to the network (MME) periodically even when there is no data to deliver. A TA is a group of cells, and managed by MME. The location of UEs in Idle state is recognized at a TA level.

 

To this end, MME provides a UE with a TAI list and TAU timer (T3412) as included in a Attach Accept message when the UE initially attaches the network. Using them, the UE performs a TAU procedure upon expiration of the TAU timer.

 

When MME receives TA information from a UE, it updates the UE’s current location information (TA, cell) to keep the latest information. In case there is traffic destined to the UE while it is in Idle state, the MME informs the UE about the new traffic by sending a Paging Message to the cells in the TA that has been reported by the UE as its current location1.

 

Figure 1 shows an example of a TAU procedure performed by a UE in Idle state. The UE (UE1) has attached Cell 2 in eNB1 upon its initial attach, and has been allocated a TAI list (e.g. TAI={TAI1, TAI2}) and TAU timer (e.g. T3412=60 mins) through the Attach Accept message sent from the MME. Later, after transiting to Idle state, it travels from  →  →  → . For the purposes of this example, we assume i) the UE travels between only the TAs that are in the TAI list initially allocated through the Attach Accept message, ii) the MME the UE reports its TA information to is the one the UE’s context is stored in, and iii) both the UE and the MME have kept the NAS security context (KNASint, KNASenc, etc.) valid.

 

 

Figure 1. Concept of Periodic TAU 

 

After attaching and transiting to Idle state in Cell 2, UE1 wakes up and establishes an ECM signaling connection with the MME when the TAU timer expires at T1. Next, it sends the MME a TAU Request (TAI=TAI1, Last Visited TAI=TAI1) message that includes the TAI of its current cell, and the last visited TAI (the one reported through the last TAU request) (). Then, UE1 returns to Idle state when it receives a TAU Accept message. After receiving the TAU Request message from the Idle UE, the MME checks whether the UE’s last TAI (TAI of Last TAU) has been changed or not, and updates it with the latest information if it has been changed. If there is traffic heading to UE1, and thus the S-GW notifies the MME of the DL traffic, the MME finds out which TA the UE was in last time based on the TAI of Last TAU information, and sends a Paging Message to the cells in the TA (see our LTE technical document, “LTE EMM Procedure 4. Service Request” [2] for detailed information). Later, if UE1 moves to Cell 4, and the TAU timer expires (), UE1 sends the MME a TAU Request (TAI=TAI1, Last Visited TAI=TAI1) message as it is still in TA1. If UE1 moves to Cell 11, and the timer expires (), it sends the MME a TAU Request (TAI=TAI2, Last Visited TAI=TAI1) message, including TAI2 instead of TAI1, as now it belongs to TA2. Then the MME accordingly updates UE1’s TAI of Last TAU with TAI2.

 

Figure 2 shows the connections established, and the states of UE and MME in the user and control planes before and after the periodic TAU procedure.

 

 

Figure 2. Connections and States before/after TAU

 

A UE’s states before, during and after a periodic TAU procedure are as follows:

 

(i) Before the procedure, the UE stays in EMM-Registered, ECM-Idle and RRC-Idle state, and any resources allocated by E-UTRAN, i.e. the E-RAB (except for the uplink S1 bearer) and ECM signaling connection between the UE and the MME, are kept released.

(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 (ECM signaling connection) for delivering periodic TAU-related NAS messages is set up during this procedure.

(iii) After the procedure, the ECM signaling connection set between the two entities is released, releasing the E-UTRAN resources. Then, the UE returns to EMM-Registered, ECM-Idle and RRC-Idle state again.

 

Figure 3 is another illustration that shows the state transition of a UE before/after a periodic TAU procedure. Upon the expiration of T3412, the UE which has been in Idle state sends the MME a TAU Request message to report its current TA and last TA (then it transits to Connected state). Then the MME returns a TAU Accept message to the UE after updating the UE’s location information (TA). As a result, the signaling connection between the two entities is released, and the UE transits back to Idle state.

 

 

Figure 3. State Transition of a User Performing Periodic TAU

 

Page 2 of 5
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 ..

 

 

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....!!!

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