# 蔡瀚興-讀書會-21/07/31
## <font color="ff0000">Introduction</font>
**A state:** EMM-Deregistered + ECM-Idle + RRC Idle
**B state:** EMM-Registered + ECM-Connected + RRC-Connected
**C state:** EEM-Registered + ECM-Idle + RRC-Idle

## <font color="ff0000">EMM Scenario Environment</font>
- Two cities ,in geographically separated areas ,served by one LTE network operator
- The network may or may not have user information of the user for authentication


## <font color="ff0000">EEM Cases</font>
### <font color="0000ff">Case 01: Initial Attach (A -> B)</font>
1. UE in A state
2. Perform PLMN and cell search ,Attach Request ,sychronizely.
3. Become B state
- **Note:** If succeeds ,UE stay in B state. If fails ,UE transits to A state
### <font color="0000ff">Case 02: Detach (B -> A)</font>
- UE may need to detach ,or to be detached
### <font color="0000ff">Case 03: S1 Release due to User Inactivity (B -> C)</font>
- If UE stops using the service for a certain period of time
- Inactivity can be detected by a UE or an MME
### <font color="0000ff">Case 04: Service Request due to New Traffic (C -> B)</font>
- If new user traffic is generated for the UE in Idle
- Transits to B state through a **service request procedure**
### <font color="0000ff">Case 05: Periodic TAU (C -> B -> C)</font>
1. UE in Idle **performs periodic TAU** when TAU timer expires(TA may or not be update)
2. Transiting to B state ,and perform TAU procedure
3. Back to C state
- **Note:** If TAU fails ,UE will do it until success
### <font color="0000ff">Case 06: Handover without TAU (B -> B)</font>
- **UE(Activate)** move to TA which is **in TA list** ,so no TAU procedure is performed
### <font color="0000ff">Case 07: Cell Reselection with TAU (C -> C)</font>
- **UE(Idle)** move to TA which is **in TA list** ,so no TAU procedure is performed
### <font color="0000ff">Case 08: Handover with TAU (B -> B)</font>
- **UE(Activate)** move to TA which is **not in TA list** ,so TAU procedure is performed **after case 6 done**
### <font color="0000ff">Case 09: Cell Reselection with TAU (C -> B -> C)</font>
- **UE(Idle)** move to TA which is **in not TA list** ,so TAU procedure is performed **after case 7 done**
- This kind of TAU procedure is **similar to case 5** ,but triggered by a different event
### <font color="0000ff">Case 10: Move to Another City (B or C -> A)</font>
- UE is using services or in idle state
### <font color="0000ff">Case 11: Initial Attach in Another City (A -> B)</font>
1. **If City2 have info of UE:** Using its GUTI provide by City1 MME
2. **If City2 have no info of UE:** Requests the UE for IMSI to be used as UE ID