Periodic Tracking Area Update (TAU) - T3412 timer

Today I was asked about the Periodic Tracking Area Update. Question was what is happening when the UE is in one place cell or TA in general.

So here is what the Spec Rel 9. is saying about it.

Periodic TAU

Periodic tracking area updating is used to periodically notify the availability of the UE to the network. The procedure is controlled in the UE by the periodic tracking area update timer (timer T3412). The value of timer T3412 is sent by the network to the UE in the ATTACH ACCEPT message and can be sent in the TRACKING AREA UPDATE ACCEPT message. The UE shall apply this value in all tracking areas of the list of tracking areas assigned to the UE, until a new value is received.

No Periodic TAU case

If the timer T3412 received by the UE in an ATTACH ACCEPT or TRACKING AREA UPDATE ACCEPT contains an indication that the timer is deactivated or the timer value is zero, then the timer T3412 is deactivated and the UE shall not perform periodic tracking area updating procedure.

This part is a small change in Spec comparing to Rel 8. I feel this is because Machine to Machine (M2M) communication will take more part in overall. Because what else could be a reason to resign from (Periodic) TAUs?

Timer's behavior

The timer T3412 is reset and started with its initial value, when the UE goes from EMM-CONNECTED to EMM-IDLE mode. The timer T3412 is stopped when the UE enters EMM-CONNECTED mode or EMM-DEREGISTERED state.

EPS Mobility Management (EMM) and Connection Management (ECM) states

The EPS Mobility Management (EMM) states describe the Mobility Management states that result from the mobility management procedures e.g. Attach and Tracking Area Update procedures.
There are two EMM states described in Specs:

• EMM-DEREGISTERED
• EMM-REGISTERED

The EPS Connection Management (ECM) states describe the signalling connectivity between the UE and the EPC.
There are two ECM states described in Specs:

• ECM-IDLE
• ECM-CONNECTED

In general, the ECM and EMM states are independent of each other. Transition from EMM-REGISTERED to EMM-DEREGISTERED can occur regardless of the ECM state, e.g. by explicit detach signalling in ECM-CONNECTED or by implicit detach locally in the MME during ECM-IDLE. However there are some relations, e.g. to transition from EMM-DEREGISTERED to EMM-REGISTERED the UE has to be in the ECM-CONNECTED state.

Tracking Area Update (TAU) procedure

In this article major topic will be Tracking Area Update (TAU) procedure. What cause it, how the procedure made and what it causes for UE.

Because there are couple of reasons why TAU can take place we won't start from picture as always.

Almost all what you will see below you can read in details in 3GPP TS 23.401.
Also, maybe you are looking for Periodic Tracking Area Update, don't you huh?

Triggers for TAU procedure

When TAU procedure is taking place SGW can be changed or not, but what really bring Tracking Area Update procedure to live is what we should ask about.
According to the spec I've mentioned above a standalone tracking area update occurs when a GPRS-attached or E UTRAN-attached UE experiences any of the following conditions:

• UE detects it has entered a new TA (Tracking Area) that is not in the list of TAIs (Tracking Area Indicators) that the UE registered with the network;
• the periodic TA update timer has expired;
• UE was in UTRAN PMM_Connected state (e.g. URA_PCH) when it reselects to E-UTRAN;
• UE was in GPRS READY state when it reselects to E-UTRAN;
• the TIN indicates "P-TMSI" when the UE reselects to E-UTRAN (e.g. due to bearer configuration modifications performed on GERAN/UTRAN);
• the RRC connection was released with release cause "load re-balancing TAU required";
• the RRC layer in the UE informs the UE's NAS layer that an RRC connection failure (in either E-UTRAN or UTRAN) has occurred;
• when the UE changes the radio capability of at least one of the following radio access technologies: GERAN, UTRAN or CDMA 2000;  *
• for a UE supporting CS fallback, or configured to support IMS voice, or both, a change of the UE's usage setting or voice domain preference for E-UTRAN.
• for a SR-VCC capable UE, a change of MS Classmark 2 and/or MS Classmark 3 and/or Supported Codecs.
• UE manually selects a CSG cell whose CSG ID is absent from both the UE's Allowed CSG list and the UE's Operator CSG list.

For all cases except case "periodic TA update timer" UE shall set the EPS update type IE to "TA updating", but for "TA update timer" case the UE shall set the EPS update type IE to "periodic updating".
For trigger marked with "*" the UE shall include a UE radio capability information update needed IE in the TRACKING AREA UPDATE REQUEST message.
The TAU procedure is initiated by an UE in either ECM-IDLE state or ECM-CONNECTED state. The decision to perform SGW change during the tracking area update procedure is made by the MME independently from the triggers above.

The eNodeB shall include TAI+ECGI of the current cell in every S1AP Uplink NAS Transport message it sends. There are situations where eNodeB can contain cells from more than one TA and cell changes that happen on same sNodeB are not notified to the MME. However to finish TAU procedure with accept message MME needs to know UE's current TAI.