Why PDCP is there in Control Place of LTE

Question

In UMTS PDCP does not appear for the control plane of protocol suite while in LTE PDCP is there between RLC/MAC and RRC. What is the rational behind keeping PDCP in control plane.

Comments

In LTE, PDCP does the ciphering and integrity protection of the packets. And for signaling messages above two are necessary. This might be the one of the reason why PDCP is coming in to picture for control plane message in LTE.
If i remember correctly in UMTS integrity protection was in RRC.

Answer 1

The Packet Data Convergence Protocol (PDCP) is located above the RLC layer of the user
plane and PDCP is also used for most of the RRC messages.

The Key features are:
• Ciphering and deciphering both the user plane and most of the control plane data, a functionality
that in WCDMA was located in the MAC and RLC layers.
• Integrity protection and verifi cation, to ensure that control information is coming from the
correct source.
The PDCP layer receives PDCP SDUs from the NAS and RRC and after ciphering.
Header compression and corresponding decompression of the IP packets. This is based on
the Robust Header Compression (ROHC) protocol, specified in the Internet Engineering Task
Force (IETF) and also part of the WCDMA PDCP layer. Header compression is more
important for smaller IP packets , especially in connection with the VoIP service,
as the large IP header could be a significant source of overhead for small data rates.

Answer 2

In Signaling, the only use of PDCP seems to be Integrity and Cipher protection (Security). Although it may have various other roles to play with Data.
Seems fair enough especially given that it is over the air interface and more importantly in the public domain.
EVDO has a Security layer just above the MAC layer as well, and in common to Packet Application(PA) and default signaling application (DSA), which are at a layer much above it.