One of the common themes we cover over and over in the 4G discussion is the desire to preserve energy on the UE RF side of things, to extend battery life as much as possible.
The 3GPPs requirements for LTE also included the smallest round trip times, defining less than 5 ms in unload condition, so traffic to the UE must be routed as quickly as possible.
Mobiles are by their very nature, mobile.
This requires UEs to constantly monitor the RF conditions and the signal measurements from different base stations so the UE can determine if it’s time to handoff to another cell due to going further from one eNB and closer to another, or another eNB offering better RF conditions (Strong signal etc).
This requires regular exchanges of messages and checks, but this would take a lot of energy and eat up battery usage.
Instead we avoid maintaining the radio connection all the time with the aid of an inactivity timer on the eNB.
For as long as user data is flowing over the air interface the connection is maintained, for example web browsing, the inactivity timer is constantly reset as traffic flows.
However when the eNB detects no packets sent or received by the UE the timer starts counting down from it’s set value.
When the inactivity timer reaches 0 the RRC Connection is released and the UE no longer has an RNTI.
The UE is still listening to an eNB, it’s just not sending data to it it and visa-versa.
As the radio bearer has been removed the UE the S1-AP and S1-UP bearers between the eNB and the MME and the eNB and the S-GW respectively, can be torn down.
This means the MME is no long sure of exactly which eNB the UE is listening on.
This is referred to as ECM_IDLE state as there is no radio connection, and the network is unaware of the precise location of the UE.
An ECM_ACTIVE state is the state when the UE is connected to an eNB with an RNTI and it’s inactivity timer has not reached 0.
The dotted line bearers shown in the image above frequently change between active and inactive based on the ECM_ACTIVE / ECM_INACTIVE state of the bearers.