me concepts because, apart from the user traffic, some overheads are needed (which was not the case in GSM/EDGE networks). Each of these interfaces and related dimensioning will now be discussed. Protocol Stack In Figures 3.57(a) and (b) the user plane protocol stacks for several interfaces are represented (where FP is the Frame Protocol, PH the physical layer, UP the user plane, SNAP the subnetwork Access Protocol, PDCP the Packet Data Convergence Protocol and UDP the user Datagram Protocol). The part that involves transmission is in the bottom part of Iub and I interfaces. The common layers in all the interfaces are the physical layer and the ATM. The physical layer is used as the generic name in the figures since the practical solutions for a physical layer are many. The physical layer can consist of wire (e.g. coaxial cable, optical fibre) and wireless media (e.g. microwave link) and on top of them different kinds of digital hierarchies can be used (e.g. PDH, SONET, SDH), which accommodate different carriers (e.g. E1, T1, Interface Planning E.g., E.g., Vocoder Vocoder RLC-U RLC-U Node B I,-CS UP I-CS UP WCDMA WCDMA (a) CS E.g., E.g., IPv4, IPv6 3G-SGSN IPv4, IPv6 GTP-U GTP-U RLC-U RLC-U Node B LLC/SNAP LLC/SNAP WCDMA WCDMA Layer Layer (b) PS Figure 3.57 Protocol stack of user plane traffic JT1 (JT is a japanese standared the equivalent of ANSI), STM-1). In the case of the wireless physical layer such as the microwave radio link, the physical layer will also have radio framing for the digital baseband bit stream. The ATM is chosen as the layer 2 protocol for WCDMA transmission to pack the data into cell format and to provide the possibility of effectively gaining from differentiating the services according to their different kinds of service level and bandwidth requirements. AAL2 is chosen as the ATM adaptation layer in the Iub interface for all user traffic to keep the data units small in narrowband connections, but for the I interface the choices are different for I-cs and Iu-ps. I-cs uses AAL2 as well since