LTE: What's relation between RRC-Connected and ECM-Connected ?

Question

If I am right, an eNodeB can support multiple cells and each cell has different pci.
Now suppose enodeB has two cells i.e cell1 and cell2. If I am going right, both the cell(s) should have different PCIs.
I have assumed UE is attached to cell1 and due to mobility, it is moved to cell2. During this movement, what message shall be exchanged between UE and enodeB. And Is there any message send to MME regarding the new cell ?

Answer 1

RRC layer in LTE maintains two states i.e. RRC-IDLE and RRC-CONNECTED. Similarly MME node at core networks maintains two states "ECM-IDLE" and "ECM-CONNECTED".

When an UE establishes Radio connection with an eNodeB it's and eNodeB RRC layers move from RRC-IDLE to RRC-CONNECTED. This state transition doesn't mean that data flow can be done. RRC connection just provides a way or path to signalling messages between UE and eNodeB.
But when UE establishes at least one data bearer with network then only data flow can start.

In short, when UE is in ECM-CONNECTED state then data and signalling can flow but not true for RRC-CONNECTED state. RRC-CONNECTED is a sub set of ECM-CONNECTED.

Comments

Just want to add if RRC connection is established RRC state transition will be from RRC_IDLE to RRC_CONNECTED. And once UE attaches to network ECM state transition will be from ECM_IDLE to ECM_CONNECTED.
And if there is no data over S1 path state can be change from ECM_CONNECTED to ECM_IDLE. This will also force RRC_CONNECTED state to move to RRC_IDLE
Please refer to EMM State Transition diagram in the link below.
http://www.netmanias.com/en/?m=view&id=techdocs&no=5909