Why we need Two types of Security modes (NAS,AS) in LTE

Question

I went through the specification and not found the exact answer for the NAS Key Set Identifier.
Three bits are used for that but the is the meaning of all bit combination, I don't know.

Answer 1

NAS Security is between MME and UE whereas AS security is UE and eNodeB both serves the different purpose.

There are two ways to do this, i.e. Integrity and Ciphering.
For C-Plane: Integrity protection applied.
For U-plane: Integrity protection and ciphering.

In general term Integrity protection means, Receiver can verify that received message is the same as that Sender/Transmitter sends. Where as Ciphering/encryption means Transmitter/sender encrypts the data with a key and that key knows to only Receiver and no middleman can understand the data even if it gets it.

Let me know if have any comment...

Answer 2

Adding in above answer.
Integrity and ciphering are applied to c-plane and u-plane as below

For C-plane: Integrity protection and ciphering.
For U-Plane: ciphering applied.

Answer 3

As NAS signalling is between the UE and MME which is through eNodeB. As eNodeB is not in the operator's location there are chances that message can be tampered between eNodeB and MME. So in order to protect NAS messages we provide ciphering and integrity protection to those messsges by NAS security(which is between UE and MME).

And AS security by its name is access stratum security. The air medium is vulnerable. So we provide this security for messages which is between UE and eNodeB.

Answer 4

I agree with Veer Pal , integrity protection is not applied to U plane data and ciphering is applied for both Control plane and user plane.Note that it is important to have NAS messages also encrypted or ciphered for example in case of RRC connction request and RRC signalling messages.