Overview

Currently, the most common protocols for data services are IPv4 and IPv6 at the network layer and TCP and User Datagram Protocol (UDP) at the transport layer. Because new services and applications are continually being introduced, it is conceivable that in the future, new network layer protocols would be developed supporting these services. As such, it is desirable that the UTRAN be able to interwork with these emerging protocols without requiring any changes to its RLC or MAC layers. The packet data convergence protocol helps achieve this goal by adapting different network layer protocols to the RLC layer so that the user data can be transferred across the UTRAN transparently. Another function of PDCP is to improve the transmission efficiency for delay-sensitive information such as voice or video. It does this by reducing the size of the header fields of upper-layer packets using different header compression and decompression algorithms that have been standardized by IETF. In the initial version of this protocol, a separate PDCP entity is used for each radio bearer. In the later version, however, it will be possible to multiplex two or more radio bearers onto the same RLC PDU using a single PDCP entity.

PDCP receives the user data from the higher layers, performs the header compression if requested, and sends the resulting PDCP PDUs to the RLC layer. Notice that these PDUs are nonaccess stratum because they are exchanged between the UE and the core network, and do...