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.