In order to keep radio resources free, if a UE doesn’t send or receive data for a predefined threshold, it’ll detach from the network and call back to Idle mode.
If the UE has data to send to the network, the UE will re-attach to the network, whereas if the network has data to send to the UE, it’ll Page the UE in the tracking area it’s currently in, the UE is always listening for it’s identifier (s-TMSI) on the paging channel, and if it hears it’s identifier called, the UE will re-attach.
I’ve also attached a PCAP file of the packet flow between the eNB and the MME.
The first packet is sent by the eNB to the serving MME to indicate the user wishes to detach from the network.
The next packet is sent from the MME back to the eNB confirming UE is releasing from the network.
Finally after the UE has released it’s radio resources the eNB sends a UEContextReleaseComplete so the MME knows the UE is now in Idle state and will need to be paged.