Take a look at the picture below. It shows you the architecture of 4G LTE. a.k.a Evolved Packet System (EPS).
In order to enable "MOBILITY" you need to provide Signalling connection available to the UE even when it is mobile anywhere in the network.
MOBILITY is possible by having a signaling connection to the UE, so that it can be tracked by the network either at eNodeB level or Tracking area level.
In order to have signaling connection available to the UE, there needs to be CONNECTION MANAGEMENT.
In signaling context CONNECTION MANAGEMENT means that signaling connection exists between UE and the Network even if the UE is Mobile .
Based on these two aspects. There are two states defined for MOBILITY MANAGEMENT and CONNECTION MANAGEMENT in EPS.
- EPS MOBILITY MANAGEMENT ( EMM)
- EPS CONNECTION MANAGEMENT (ECM)
EPS Mobility Management (EMM)
EMM states are maintained by higher layers between MME and UE to make sure:
- If UE is reachable by the network or not?
- Can UE receive services from the network or not?
Based on the two bullet points discussed above. There are two main EMM states
- EMM REGISTERED
- EMM DEREGISTERED
The UE is in EMM DEREGISTERED state when it is switched on. At that time MME does not have knowledge of UE at cell or tracking area level as UE is not registered yet.
In this state UE cannot be paged and it cannot have any data traffic etc. because there are no user planes established in this state.
When UE is powered on, it tries to perform UE Attach procedure and move into EMM REGISTERED whenever possible.
UE can move into EMM-REGISTERED state by completing Tracking Area Update or after a successful handover or Attach procedure.
- Location of UE in EMM Registered State is known by the MME to at least an accuracy of the Tracking Area list allocated to the UE.
- UE always has at least one active Packet Data Network (PDN) connection when in EMM-REGISTERED state.
Moving into De-registered State
Deactivating all PDN connections causes the UE to move back to EMM-DEREGISTERED state.
UE can always make a transition from EMM-REGISTERED state to EMM-DEREGISTERED state by completing the Detach procedure or by having a Tracking area update rejected.
EPS connection Management (ECM)
ECM states are maintained by higher layers (NAS layer) between UE and MME.
These ECM states determine the signalling connectivity between UE and Core Network. Based on the signaling and connectivity availability, the following ECM states are defined.
There are two ECM states:
- A UE in ECM-IDLE state does not have signaling connection to the MME.
- UE can only performs cell selection and cell reselection procedures in ECM IDLE state.
- Location of UE in ECM-IDLE state is known by MME to an accuracy of Tracking Area i.e., the set of Tracking Areas with which UE is registered.
Procedures in ECM Idle
UE can perform the following procedures in ECM Idle.
- Tracking Area Updates triggered by Mobility and periodic TAU timer (T3412)
- TAU triggered for MME load balancing i.e., relocating the UE's signalling connection to different MME
- TAU triggered for UE to signal a change of its core network Capability or its UE specific Discontinuous Reception Cycle (DRX) cycle.
- Responding to paging messages by performing a Service request procedure
- Performing Service Request Procedure when Uplink data is to be sent
ECM connected state
UE enters ECM connected state after signalling connection has been established between UE and MME.
From UE perspective this signaling connection means RRC connection between UE and eNodeB.
From MME perspective this signaling connection means establishing S1 signaling connection between eNodeB and MME.
The following procedures result in transition from ECM Idle to ECM Connected
- Attach Request
- Tracking Area Update Request
- Service Request
- Detach Request
- The location of UE in ECM Connected State is known by the MME to an accuracy of serving eNodeB level
- UE mobility is handled using handovers not through cell selection or reselection.
conclusion: can we represent both eMM and eCM together?
As said earlier both MOBILITY and SIGNALING states are very important. Therefore in order to summarize, we can represent both Mobility Management and Signaling Connection Establishment States together in one diagram as shown below.
example of emm and ecm states
As an example, both the states of ECM and EMM are represented as an example in this diagram below.
Thank you for reading. Please leave your questions or comments below
Glad it was helpful for you Krunal