PDCP protocol layer exists in UE & eNodeb, It is part of LTE air interface control and user planes.

The main services and functions of the PDCP sublayer for the user plane include:

• Header compression and decompression: ROHC only;
• Transfer of user data;
• In-sequence delivery of upper layer PDUs at PDCP re-establishment procedure for RLC AM;
• Duplicate detection of lower layer SDUs at PDCP re-establishment procedure for RLC AM;
• Retransmission of PDCP SDUs at handover for RLC AM;
• Ciphering and deciphering;
• Timer-based SDU discard in uplink.

The main services and functions of the PDCP for the control plane include:

• Ciphering and Integrity Protection;
• Transfer of control plane data.

Protocol specification

3GPP TS 36.323 - Evolved Universal Terrestrial Radio Access (E-UTRA); Packet Data Convergence Protocol (PDCP) specification