EMM, ECM, RRC States, TAU and Handovers

Its very interesting to observe the connection between 5 terms mentioned in the subject line.

Refer here and here for details on EMM, ECM and RRC states.

EMM has two states:- EMM Registered and EMM De-registered. When a UE is connected to the network it moves from EMM De-registered to EMM Registered. Also the ECM state moves to ECM connected from ECM idle. Before these two happening the RRC state in UE is moved from RRC idle to RRC connected. So when UE is actively connected to the network the states in UE/network are RRC Connected, ECM Connected and EMM Registered.

When UE moves to Idle mode first the RRC connection is released. That is RRC state moves from RRC connected to idle. Then ECM state moves to ECM idle from connected. This mean eNB has released the RRC connection and network has released the UE bearer context information but EMM state is Registered. This is termed as S1 release in 3GPP TS 23.401. It would be interesting to know that ECM idle state is valid only on S1 and S11 interface. The UE information is actively maintained in PGW but is released in eNB and MME. Also the behavior of network in ECM idle mode is different in different cases. (future posts). The basic idea of idle mode is UE is not know to the network and in connected mode UE is known to the network. The term "known" refers to UE location, bearer information etc.

TAU - Tracking Area Update. A tracking area is a group of cells that are being tracked by a SGW. When UE is connected to the network, a list of tracking areas are sent to it. When a UE moves to new tracking area it will trigger a tracking area update procedure as defined in 3GPP TS 23.401. A TAU can be triggered when UE is in idle mode or connected mode.

Now at any given point of time UE can be in idle or connected mode. If UE is in connected mode and moving from one place to other then handovers take place. Note that handovers are transparent to UE except that UE now camps on to a new eNB. Also handovers are network initiated. This means a handover can happen when the UE is known to the network, i.e when UE is in connected state. Also after the handover if UE finds that it is present in new tracking area then it can initiate a tracking area update.

But a UE need not be in a connected state all the time. What happens when a UE is in idle mode and is moving from one place to other? When UE is in idle mode, MME will have the UE's last know location. So if some data arrives to UE, while it is in idle mode, MME will initiate paging process to find UE. Now if UE has moved from the last know location how will the MME know the UE's new location. This is where TAU becomes extremely important. In this case it is the responsibility of UE to inform about its location to network. This is done by sending TAU. As I said when UE is attached to the network MME sends a list of tracking areas. So if MME wants to find UE i.e is in idle mode, it will simply send paging message to the eNB and eNB will page for the UE. But if UE is in new tracking area, that is not in the list received during initial attach, it will initiate a TAU. Once network receives TAU it will store the UE's location info.

To initiate a TAU UE should have a RRC connection. First thing it does is it camps on to a nearest eNB by doing cell re-selection process. After RRC connection is established UE may send TAU to network. Note that ECM state in network is still idle for that UE. But TAU may move the ECM state in network from idle to connected. (future post)

Thus if we look at overall picture a handover can happen when UE is in EMM Registered, ECM connected and RRC connected state. A TAU may be initiated by UE when it is in EMM Registered, ECM idle and RRC idle state. Both the procedures help UE in its movement from one place to other.