In an LTE EPC network, each tracing area (TA) corresponds to one tracking area identity (TAI), and one or more TAIs form a TA list. A TA list is the paging scope of the EPC network and the basic unit for location management. That is, the network side only needs to aware the TA list of a UE.
UE locations are updated following the tracking area update (TAU) procedure. A TAU procedure is used to register UE location information so that the network can learn UE locations in a timely manner. A TAU is initiated by a UE in the ECM-IDLE or ECM-CONNECTED states. It can be triggered:
- In the event that the UE TAI is not in the TA list registered on the network
- During periodic TAU configuration
- In the event that the UE RAT type (such as GERAN, UTRAN, or E-UTRAN) changes
- For load balancing on the network side
- For a handover
a/ Intra RAT
a. 4G → 4G
b/ Inter RAT
a. 2G → 4G
b. 3G → 4G
- A handover procedure is conducted on the RAN and initiated by the eNodeB. If a UE is in the ECM-CONNECTED state and there are data connections for the UE, once the UE moves between two eNodeBs, a handover is initiated to ensure smooth data connections. This procedure focuses on bearer resource allocation for UEs.
- A TAU procedure is conducted on the core network and initiated by the UE. After a UE completes a handover procedure, if the UE finds that the current TAI is not in the TA list on the network where the UE is registered, the UE may further initiate a TAU procedure.