s with data rates. As the data rates increase, the coverage area decreases. Similarly, data rates have a big impact on transmission network planning right from the dimensioning phase. Once the number of base stations has been decided, all the RAN interfaces (except lub, Iur and Iu) should be dimensioned, as should the number of RNCs required. For the dimensioning of interfaces, some knowledge of protocol stacks for CS data and PS data is required. 8.4.1 Protocol Stacks Figure 8.8(a) is a protocol stack for the user-plane CS traffic. It basically shows the movement of the traffic from the mobile station to the MGW (media gateway). Figure 8.8(b) shows a similar movement of the traffic for PS data from a mobile station to the GGSN. Some hardware units in the equipment perform the functioning of these layers. One such example is node B (or base station). SP units perform the layer 1 functions while FP/AAL2/ATM layers are present in the application manager (as shown in Figure 8.2). 8.4.2 Overheads One important point to note from the dimensioning perspective is that, when the traffic/signal moves from one layer to another, some overhead bits are added. These bits increase the capacity requirement of the interface. Thus, when doing dimensioning each overhead has to be carefully calculated. Assume that a mobile subscriber is trying to establish a connection with the network. As the call is voice (CS), it will follow the protocols shown in Figure 8.8(a). The overheads to be attached to the call's capacity calculations would include voice activity factor (VAF), soft handover (SHO), protocol stack overhead, and signalling overhead. Voice activity factor (VAF) overhead: Once a mobile is connected to the network, the subscriber will either speak or listen (active or silent mode). In either case, some bits will be required for the functioning of that mode. Obviously, overheads during the active mode are more than in the silent mode. The type of voice connection can be defined in terms of AMR (adaptive multi-rate) codec modes.