LTE: What is the use of "S-TMSI" when it receives in the RRC Connection Request message ?

Question

It may be an easy question but still I have some doubt.
How RRC Connection setup procedure gets impacted with the presence of "S-TMSI" as an UE-Identity within the RRC Connection Request message ?
Note: RRC Connection request message can carry either 5 bytes long (S-TMSI) or 5 bytes long random value.

Answer 1

I know about three scenarios when UE sends RRC Connection Request message to the eNodeB.

1. Attach Request 2. Service Request 3. Tracking area update.

2. When Ue registers first time to the network, it sends 5 bytes long random value as an UE Identity within the RRC Connection Request and UE doesn't send registeredMME (optional) information with the RRC Connection setup complete since UE is first time doing network attach.

3. When UE is in ECM-IDLE mode and it has to send some data to the network or it expects some data from the network. In that case, RRC Connection Request message carriers S-TMSI and it also receives registeredMME information within the RRC Connection Setup Complete.

4. If this is normal tracking area update when UE is moving out of coverage of registered MME then it sends S-TMSI and registeredMME within the RRC Connection Request and RRC Connection Setup Complete messages respectively.
   There is another reason when UE sends TAU due to cause "Load Balancing TAU Required". In that situation, UE sends RRC messages to eNodeB without S-TMSI and registeredMME information.

Comments

Thanks Vimal.

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S-TMSI is assigned by MME, it is used for identifying UE context