LTE: How the UE knows if ISR is active or not?

Question

Will the encryption of traffic between eNB and LTE core network be implemented in LTE networks? Will cellular operators apply IPsec encryption over communication links between LTE eNodeB and SAE Gateway or LTE core network?
Isn’t IPsec encryption a mechanism which each subscriber may decide to use (or not) between the UE and its peer?

Answer 1

For ISR
1) UE should know whether it can support ISR or not and it tells to network.
2) UE should know whether ISR is activated after requesting or not from network.

If UE supports ISR then it request through these two messages to network to activate ISR
* Routing area update request / Attach request Message ---> MS network capability ----> ISR flag
ISR flag :- UE tells about whether it supports ISR or not.

After applying for request if network accepted and activated the UE should be informed about this ISR activation.
* About the activation UE knows when the ISR activation status status comes in
* TAU Accept and RAU Accept ----> Update result flags

For more u can refer 24.004 , 24.008