When we talk about "Idle" mode mobility, it mean track an UE when it doesn't have RRC connection with an eNodeB. When UE is in ECM-IDLE or RRC-IDLE, it's location can't be tracked at the cell level granularity.
In LTE, the geographical location is divided into cell(s) and tracking area(s). One tracking may consist of a number of cell(s). MME is the node which maintains UE's location at the granularity of tracking area when an UE is in ECM-DILE or we can say when there is no radio connection between UE and eNodeB. So there are two possibilities may occurs:
- Down link data is coming: In that case, SGW informs to MME and MME initiates paging for that UE.
- UE's tracking area is changed due to its mobility: UE is moved to new tracking area to it's mobility.