Let’s look at how the Tracking Area Updates work from the point of view of the network.
Let’s take an example of a UE which has been sent the Tracking Area List TA0 and TA1, which is currently in ECM_IDLE state served by eNBs in Tracking Area 1.
The UE is moving towards another eNB in Tracking Area 2. As the UE listens on the Broadcast Channel the power of the new eNB overtakes that of the previous eNB, but the UE notes the Tracking Area of the new eNB, which is not on the UE’s Tracking Area List.
So the UE must make a Tracking Area Update to inform the network.
The first thing to do is to establish a radio connection.
Once the radio connection is setup a S1-AP connection is setup, upon which an NAS message – EMM Tracking Area Update Request is sent which contains the GUIT and old Tracking Area ID, which is sent to the MME.
The MME then sends back a new Tracking Area List for the UE and new TMSI to update the GUTI of the subscriber.
The UE updates it’s GUTI, updates it’s Tracking Area List, sends an EMM TRACKING AREA UPDATE COMPLETE and the UE returns to ECM_IDLE state.