Packet Data Convergence Protocol (PDCP)
In the LTE radio protocol stack, the PDCP layer is located above the RLC layer and below the IP layer (in the user plane) or the RRC layer (in the control plane). 3GPP provides a PDCP specification in . From UMTS, the main function of the PDCP layer has been the header compression of IP packets – this is the reason why it is called the Packet Data Convergence Protocol.
In LTE, the PDCP layer has evolved to support a security function; that is, integrity protection and ciphering. To provide time-varying characteristics to the security function, a PDCP sequence number has been introduced in the PDCP layer. The PDCP sequence number is attached to each PDCP PDU, and it is used to generate different security output per PDCP PDU. Thanks to the PDCP sequence number, the PDCP layer is able to perform ARQ-related functions, which can improve the radio efficiency at handover.
The PDCP layer is running on top of the RLC layer. A PDCP entity which is used to perform the PDCP functions can be configured either with both transmitting and receiving sides (for a bidirectional radio bearer), or only one of them (for a unidirectional radio bearer). If a PDCP entity is configured with both transmitting and receiving sides, it is associated either with one AM RLC entity or two UM RLC entities (one for the uplink direction and one for the downlink direction). If a PDCP entity is configured with either a transmitting or a receiving side, it is associated ...