Why RRC integrity function is in PDCP?

Question

After reading security protocol of LTE, I am wondering both AS integrity and ciphering are in PDCP by using PDCP SN.

But in WCDMA, RRC integrity is RRC layer, and RRC ciphering are in RLC as user plane data..
1) Why design as this in LTE?
2) LTE RRC seems have no RRC sequence number?

Answer 1

1. In LTE security(integrity and ciphering) is bring into PDCP only. So no need to have same feature scattered in RRC/RLC.

2. In LTE the PDCP sequence number is used for the security algorithm and not RRC sequence number.

Answer 2

Want to clarify a bit. There is not RRC-integrity and RRC-ciphering. Integrity and Ciphering are two processes.

Integrity refers to maintaining and assuring the accuracy and consistency of data over its entire life-cycle.
Ciphering: put a message into secret writing (put (encode).

Now coming to your question why it is like this: In layman It is just shifted the work from one protocol to other protocol. Dividend rule :). The main purpose is to get the work done. Who is doing that's not compulsion. PDCP layer has to be there to do the ROHC so It can have more work. No need to make the lengthy code for RRC. That's what I think. Rest they might have some technical aspects also. Might other will emphasize on it.