what happens when AS security fails? after what procedure will perform after AS security fails

Question

I saw a scenario where an Identity is requested by MME after successful Authentication and Security mode complete.. As a Response to this message UE is Sending IMEI? what is the need of checking IMEI number after successful authentication? If IMEI is in block list then ongoing Attach procedure is going to be Terminate...? Identity Request intention is mainly to check the IMEI belongs to any one of Black, while or Green list then why cannt MME check for this before Security Mode?

Answer 1

S1 release procedure (i. e, rrc and s1 connection release) initiated by mme after AS security fails and perfom rrc connection reestablishment procedure.

Comments

after failing of security mode failure message form UE to enodeB  what will be the  next step of enodeB and UE. (AS security).

i.e, what happens when AS security fails ( after sending security mode command failure message what will be failure condition of as security fail where it want to go for next procedure)

Answer 2

In LTE, two levels of security is enforced for an UE. MME is responsible for enabling NAS level security, it also sends essential information to eNodeB which is used to activate access stratum (AS) security after successful activation of NAS security. That means NAS security is activate first then access security. If AS security activation fails then UE moves from RRC-CONNECTED to RRC-IDLE state and it tries fresh RRC connection establishment.

Note: If AS security succeed and after that UE looses the radio connection with eNodeB, it tires to resume the lost connection using the RRC connection re-establishment procedure.