LTE : A UE can initiate re-establishment procedure if and only if security has been established why ?

Question

Upon detecting Radio Link Failure UE will do RRC Connection re-establishment but to do RRC Connection re-establishment security should be activated in UE, otherwise UE will go Idle mode.So here(After detecting RLF) we are resetting all PHY,MAC,RLC,PDCP Configurations but why we are making security should be there to do RRC Connection re-establishment.

Answer 1

IF UE sent reestablishment request to other eNB ,neighbor to RLF happened eNB, will try to find UE context in the existing UE contexts with C_RNTI ,But there may be chance presence of other UE context with same C_RNTI ,just to verify whether this context is belongs to this UE or not we need "shortMAC-I" .

Thats the reason If AS security is enable then only UE will initiate reestablishment procedure otherwise UE will initiates RRC connection request procedure.

Comments

I have one doubt, for another eNB it should be RRC Connection request why reestablishment request?

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yes you may be right ,but in RRC spec 36.331 says  if RLF happened and AS security is active then UE will initiate RRC connection reestablishment request. Because of this only Forward handover concept come into picture.

Answer 2

There is a mandatory IE "shortMAC-I" within the RRC Connection Reestablishment Request message. This shortMAC-I can be generated only when access stratum security has already been activated otherwise how UE will generate this mandatory IE.

In RRC specification it is clearly mentioned that if AS security is not activated then only UE can execute RRC Re-establishment procedure.