If you are speaking about S1 Release procedure, the answer below should do the job.
S1 Release is initiated due to repeated RRC signaling integrity check failure or UE activity.
EnB starts a timer (Inactivity Timer). If the UE remains inactive for a certain amount of time, EnodeB initiates the S1 Release Procedure where the UE context maintained at the EnodeB is deleted but NOT at the MME.
Enode sends sends the MME a UE Context Release Request message.The MME requests the S-GW for release of resources associated with the eNB, by sending Release Access Bearers Request message. All the tunnels (bearers) established are released & the UE is moved into ECM IDLE mode but remains in EMM Registered mode.
In short S1 siganlling & RRC connections in Control plane are released along with the the downlink S1 bearer and Data Radio Bearer (DRB) in user plane, that are associated with a UE.
The S-GW releases all the downlink S1 bearer resources associated with the UE (S1 TEID), but keeps the uplink S1 bearer resources (uplink S1 TEID) intact for delivering packets in case of uplink data.
The S-GW responds back by sending a Release Access Bearers Response message to the MME. S-GW buffers the incoming packets for the UE, and delivers them after the downlink S1 bearer is re-established.
The MME responds to the eNB by sending a UE Context Release Command message to release the UE context stored at the eNB.
The eNB sends a RRC Connection Release message to the UE to release it. The eNB then releases all the radio resources/bearers allocated to the UE, and deletes the UE contexts.
The eNB responds back to the MME with a UE Context Release Complete message.
So now the MME deletes the S1 TEID for downlink, ECGI, S1AP UE ID.
Now comes the question that how & when does UE move back to the ECM CONNECTED mode. It is again done by either of the 2 ways->
1. UE Initiated.
2. Network initiated.
If a UE tries to send a message/email (Uplink data), it sends a Service Request to the MME. MME sends an ERAB setup request to the EnodeB to establish a DRB & S1 bearer for downlink. After the security procedures, the UE is moved into ECM Connected mode & can avail the services.
If there is an incoming call / data for the UE, SGW buffers those packets & informs MME with a downlink data notification message. MME sends a paging message to all the enBs in the Tracking Area where the UE was last seen. The enB then broadcasts this on its PCH(Paging channel) so that the UE can receive it. The UE now sends a service request & again in the same way the RRC Connection & bearers/tunnels are reestablished & the UE is moved into ECM CONNECTED Mode.
Well there are a lot of messages that will be exchanged during these procedures. This is just a high level info.
Hope this helps!
