When an operator deploys its LTE network in a particular region, it divides the whole region into small areas known as tracking areas. Each tracking area consists of a groups of eNodeBs/cells. In LTE, a cell can be part of only one tracking area though it can broadcast up to 6 PLMN (first primary and others considered as secondaries).
During S1-setup procedure, eNodeB sends its supported TAI list to MME and MME stores these details.
There are three possibilities at cell level:
- Each cell broadcasting same TAC and same PLMN.
- Few cells broadcasting different TACs (consecutive or non-consecutive TACs) but broadcasting same PLMN
- Combination of TACs and Different PLMNs.
Based on these above details, MME maintains different types (type 00, 01 and 10) of TAI list. The purpose of maintaining different types of TAI list is to configure TAI list at UE through different ways. Each type uses different format to prepare TAI list. For the UE, it is just a list of TAI and there is no change in TAU procedure because of different types.