(L2). It has the responsibility for controlling the communications over the WCDMA transport channels provided by the physical layer [3GP04c]. The MAC layer maps logical channels to transport channels and performs the scheduling of radio bearers (or logical channels), as well as the selection of the data rates being used, i.e. selection of the transport format (TF) being applied. It should be noticed that both RLC and MAC protocols are also used in the user plane. WCDMA is the physical layer (L1) that is based on WCDMA technology. It is the air interface between the UE and the base station (Node B). Protocol Stack for the User Plane As introduced in section 3.2, the user plane is used for transferring user data. The user data transferred across the lub can be basically divided into the circuit-switched data such as voice and packet-switched data such as ftp or web applications carried by TCP/IP transport protocols. Figure 3.4 presents the UTRAN protocol stack of the user plane for the packet- switched domain and Figure 3.5 for the circuit-switched domain. The Radio Network Layer within the user plane consists of Packet Data Convergence Protocol (PDCP), RLC, MAC, and a set of Frame Protocols (FP) each for the corresponding transport channel. The Packet Data Convergence Protocol (PDCP) makes the UMTS radio interface capable to carry IP data packets [3GP02d]. It also performs IP header compression and decompression. The compression reduces the length of the header to a smaller number of bytes, thereby contributing to increased efficiency and a higher data rate, which is particularly important for small data packets. It is used specifically for the packet- switched domain. The RLC and MAC protocols have already been briefly introduced in the control plane protocol stack. The Frame Protocol (FP) is defined at the lub interface responsible for the relaying of transport channels between the UE and the RNC via the Node B in order to extend the radio transport channels from Node B to RNC. The set of FP define the structure