The PDCP protocol provides for the compression and decompression of the IP header, for the transfer of the user data and for the maintenance of the sequence numbers for the carrier signal. For the compression of the IP header, PDCP uses the Robust Header Compression( ROHC) method.
As for the user plane, the data packets of the PDCP protocol can be compressed and non-compressed. In the former configuration, the data packets are compressed and the header is appended in the layers above. The protocol uses the services of the underlying Radio Link Control( RLC) layers.
The PDCP header consists of two data fields: the protocol data unit( PDU) type field and the packet identifier( PID). The PDU type field indicates whether the protocol data unit is a data PDU, or a sequence number PDU. The PID data field shows the compression and data packet type.