Packet Data Convergence Protocol
PDCP is an abbreviation for Packet Data Convergence Protocol. This protocol is specified by 3GPP in [1] for UMTS and [2] for LTE. The PDCP is located in the Radio Protocol Stack in the UMTS and LTE Air interface on top of the RLC layer.
PDCP provides its services to the RRC and user plane upper layers, e.g. IP at the UE or to the relay at the base station. The following services are provided by PDCP to upper layers:
- transfer of user plane data;
- transfer of control plane data;
- header compression;
- ciphering;
- integrity protection.
The header compression technique can be based on either RFC 2507 or RFC 3095. RFC 1144 can also be used for some background information, and although the techniques in the RFC are not used in modern TCP/IP implementations, it still shows what the compression/decompression technique looks like.
If PDCP is configured for No Compression it will send the IP Packets without compression; otherwise it will compress the packets according to its configuration by upper layer and attach a PDCP header and send the packet. It uses the service provided by a lower layer called Radio Link Control (RLC) that uses the Radio Link Protocol.
Different header formats are defined, dependent on the type of data to be transported. In LTE, there are e.g. header formats for Control Plane PDCP Data PDU, for User plane PDCP Data PDU with long PDCP SN (12 bits), for User plane PDCP Data PDU with short PDCP SN (7 bits) and others.