These are my lecture notes from IMT’s NET02x (4G Network Essentials) course, I thought I’d post them here as they may be useful to someone. You can find my complete notes here.
As we discussed before when no data has been sent by a UE for a period of time the eNB will switch from an ECM-Connected state to an ECM-Idle state where there is no radio connection.
![](https://nickvsnetworking.com/wp-content/uploads/2019/04/3-Passing-to-Standby-Mode-After-a-Long-Period-of-Inactivity-_-NET02x-C.png)
So let’s look at the release procedure.
When the transmission timeout (typically 10 – 30 seconds) has expired, meaning a user hasn’t sent data for that length of time, the eNB sends the MME a S1-AP UE Context Release Request with the cause of User Inactivity to denote why the change is being made.
The MME then sends a GTP-C message requesting release of the tunnel between the S-GW and the eNB (GTP-C Release request).
The S-GW sends back a GTP-C Release Access Bearers response, indicating it has cleared down the GTP tunnel between itself and the eNB,
The MME then sends a S1-AP UE Context Release Command to the eNB, and the eNB sends an RRCConnectionRelease which releases the RNTI assigned to that UE removing it’s radio resources.
Finally a S1-AP UE Context Release Complete is sent from eNB to the MME to let the MME know the process has completed.
![](https://nickvsnetworking.com/wp-content/uploads/2019/04/3-Passing-to-Standby-Mode-After-a-Long-Period-of-Inactivity-_-NET02x-D.png)
At this stage the RNTI is no longer active so the UE cannot use the RNTI and therefore cannot be assigned radio resources.
The UE is now in ECM_Idle mode, however as it still has an IP Address allocated and can be bought back it’s in EMM_Regsitered mode.
States
EMM-Deregistered State
UE is disconnected from the network with no radio resources and does not have an IP Address
![](https://nickvsnetworking.com/wp-content/uploads/2019/04/EMM-Deregistered.png)
EMM-Registered & ECM-Connected
- UE is connected to the network with an IP address
- Radio resources (RNTI allocated)
- Location of the UE known
- All tunnels & connections established
![](https://nickvsnetworking.com/wp-content/uploads/2019/04/EMM-Registered-ECM-Connected.png)
EMM-Regsitered & ECM-Idle
- UE has an IP address & appears to be connected
- No radio resources (RNTI) currently in use
- No tunnels or connection from the eNB to the S-GW & MME.
- Tunnel between S-GW and P-GW and the tunnel between the MME and S-GW
- A relative location (tracking area) of the UE is available
![](https://nickvsnetworking.com/wp-content/uploads/2019/04/ECM-Idle-EMM-Registere.png)