Tracking area update(TAU) in LTE

Before jumping to Tracking area update procedure lets first understand what is Tracking Area(TA) and why we need TA(Tracking Area).

What is Tracking Area?- Tracking Area in LTE is geographical combination of several eNodeBs.
                       Each TA has two main identities:- Tracking Area code(TAC) and Tracking Area Identity(TAI).
    TAI= PLMN ID+TAC.

Why we need Tracking Area?- In Idle when there is no signalling and data bearers are associated with it the location of UE to MME is known by Tracking area(TA) only.

Now as we know what is TA and need of TA we are ready to discuss Tracking are update.

Tracking Area Update:- Tracking area update initiated by UE(direction is from UE to NW) only after the UE completes the attach procedure and moves into the EMM(EPS Mobility Management) state.

Note:- UE can trigger TAU in RRC idle state or in connected state but procedure ends only when the UE is in RRC connected state.

Reasons for TAU?- There are many reasons for Tracking Area Update(TAU). Some are listed below:-

> When a UE moves to a new Tracking Area.
> When the Periodic update timer T3412 expired. The value of T4312 is in the attach accept message.
> Registering with the Non EPS services(CS Domain) when the UE is already attached for EPS services. This includes completing an IMSI attach as part of TAU procedure.
> Registering for an EPS services after an inter RAT changed( From GERAN/UMTS to LTE).
> Re-Registering to LTE after CS fallback.
> In case of MME load balancing( UE initiates TAU procedure when eNodeB releases the RRC connection with cause "Load Balancing TAU required".

Note:- EPS update type should be as below:
"TA updating", "Combined LA/TA updating", "combined LA/TA updating with IMSI attach", "Periodic updating".