When the UE is in connected mode i.e in RRC-Connected state, its location will be known in Cell Level for example in cell2 in eNB3.
But when the UE is in Idle mode i.e RRC-IDLE state, its location will be known in TA level.
Operators define a group of neighboring eNB’s as a TA.
what is the need of TA?
When the UE is in idle state, if there is an incoming data, then UE has to wake up. The waking up will be done by sending Paging message from eNB.
But for the eNB to know which area the UE is registered to, it needs TA.
Once the TA is known, the network will send paging message to every eNB in that TA. Then each eNB will broadcast paging message over the radio link.
UE when in idle state, wakes up at certain period to check for Paging message for incoming data. If it gets, then it will be in active state to receive data.
TA is always initiated by UE.
Identities in Tracking Area:
TAC: Tracking Area Code: It is the code for a particular tracking area inside a network.
TAI: Tracking Area Identity:
It is the combination of TAC + PLMN-ID.
So how does the Network (MME) will know the location of the UE?
When the UE is attached to the network, it will get a TAI of the network.
If the UE is traveling between “TAC” of the same TAI, it doesn’t need to send TAU to the network. Because we know that TAI can have multiple TAC.
But if the UE moves to the new TA, UE needs to send a TAU.
Additionally, when the UE is in idle mode, UE has to send TAU periodically to MME even-though UE has not moved. Otherwise the network cannot tell if the UE is in idle mode or not able to communicate.
UE will trigger TAU after UE completes Attach Procedure.
UE can trigger TAU in RRC Idle state or RRC Connected state, but the procedure will only end when the UE is in RRC Connected State.
What are the triggers for TAU Procedure:
1. UE moves to a new Tracking Area.
2. The Periodic update timer T3412 expired. The value of T4312 is in the attach accept message.
3. Registering with the Non EPS services(CS Domain) when the UE is already attached for EPS services.
4. Re-Registering to LTE after CS fallback.
5. MME load balancing
6. When UE changes the UE specific DRX parameter;
7. Normal TAU: When UE enters a tracking area that is not in the list of tracking areas in MME that the UE has currently registered.
8. Combined TAU: When the UE is operating in CS/PS and enters a tracking area that is not in the list of tracking area in MME that the UE has currently registered.
9. Periodic TAU: Upon expiry of T3412, Periodic TAU procedure is initiated to notify the availability of UE to network.
TRACKING AREA UPDATE REQUEST
UE will initiate the tracking area updating procedure by sending a TRACKING AREA UPDATE REQUEST message to the MME.
Some of the IE’s included are:
1. EPS update type: It will be used to identify if the update Request is TA updating’ or ‘combined TA/LA updating’ or ‘combined TA/LA updating with IMSI attach’ or ‘periodic updating. It will be based on the first 3 bits
2. Old GUTI
3. Additional update type: If the bit is 0 then no additional update type. If 1, then it is SMS.
4. Last Visited Registered TAI
5. Device properties
TRACKING AREA UPDATE ACCEPT
If the Tracking Area Update Request has been accepted by MME, then MME will send Tracking Area Update Accept to UE.
Below are some of the IE’s that will be sent:
1. T3412 value
2. GUTI : If this value is present, then UE will use this GUTI for EPS services and store the new GUTI.
3. TAI list
4. Equivalent PLMNs
5. EPS bearer context status
TRACKING AREA UPDATE COMPLETE
Tracking Area Update Complete will be sent by UE to MME as a response to Tracking Area Update Accept.
TRACKING AREA UPDATE REJECT
MME will send Tracking Area Update Reject if Tracking Area Update cannot be accepted by MME.
Refer: 3GPP TS 24.301