1. Tracking Area Update

Tracking Area Update(TAU) in LTE is similar to a Location Area Update/Routing Area Update in GSM and WCDMA. There are 3 kinds of Tracking Area Update.

  1. Normal TAU - It is initiated when the tracking area on which the UE is attached is different from the previously registered cell or when UE re-selects/does handover from 2G/3G to LTE.

  2. Periodic TAU(Timer T3412) - The periodic TAU procedure is controlled in the UE by the periodic tracking area update timer (timer T3412). It is initiated when the timer T3412 in the UE is expired.  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. 

  3. Combined TAU - The UE operating in CS/PS mode 1 or CS/PS mode 2, in state EMM-REGISTERED, shall initiate the combined tracking area updating procedure. Within a combined TAU procedure the messages TRACKING AREA UPDATE ACCEPT and TRACKING AREA UPDATE COMPLETE carry information for the tracking area updating and the location area updating.

A successful Tracking Area Update Procedure is shown below. TAU Request shall contain EPS Update Type, Old GUTI or valid IMSI and the last visited Tracking Area Identifier (TAI). TAU Accept shall contain new GUTI and a list of Tracking Area Codes(TAC) belonging to one PLMN.

LTE Tracking Area Update

1.1 Trigger for Periodic or Normal TAU

  • UE detects it has entered a new TA that is not in the list of TAI’S that the UE registered with the network.

  • The periodic TA update timer has expired.

  • UE was in UTRAN GMM_Connected state when it reselects to E-UTRAN.

  • UE was in GPRS READY state when it reselects to E-UTRAN.

  • 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.

  • 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.

1.2 Trigger for Combined TAU

  • When the UE that is attached for both EPS and non-EPS services detects entering a tracking area that is not in the list of tracking areas that the UE previously registered in the MME.

  • When the UE that is attached for EPS services wants to perform attach for non-EPS services. In this case the EPS update type IE shall be set to "combined TA/LA updating with IMSI attach".

  • When the UE receives an indication from the lower layers that the RRC connection was released with cause "load balancing TAU required".

  • When the UE changes the UE network capability information or the MS network capability information or both.

  • When the UE reselects to an E-UTRAN cell while it was in GPRS READY state or GMM-CONNECTED mode.

  • When due to manual CSG selection the UE has selected a CSG cell whose CSG identity is not included in the UE's Allowed CSG list.

2. Tracking Area Update Reject

There is a possibility for the network to reject TAU request sent by the UE. This is indicated by Tracking Area Update Reject. The TAU reject contains EMM cause information element which is to indicate the reason why an EMM request from the UE was rejected by the network.

As per 3GPP specification 24.301 the possible reject causes can be any of the below.

EMM cause information element

Any other value received by the mobile station shall be treated as 0110 1111, "protocol error, unspecified". Any other value received by the network shall be treated as 0110 1111, "protocol error, unspecified".