nal, streaming, interactive and background. Of these, conversational and streaming generate the CS traffic while the PS is generated by all four classes. Air interface data channels are carried over Iub by the Frame Protocol (FP). These frame protocols are then segmented in AAL2 packets. These mini cells are then carried by ATM cells within the ATM VCCs, which are then mapped on to the physical interface, e.g. microwave/fibre optic/copper. Frame protocols are applied to transfer user data and the necessary control information between the serving RNC (SRNC) and the BTS. They are characterised by a frame structure consisting of the payload, containing the data and a header/trailer with control information. Three different kinds of FPs are specified in the UTRAN, i.e. Iub CCH, RAN DCH and Iur CCH. When data pass through each of the layers, an 'overhead' is added to it. These overheads increase the required capacity. Apart from the protocol stack overheads and frame protocol overheads, there are other overheads that should be used when doing interface dimensioning. With actual traffic, signalling is required and usually a capacity between % and 10% is reserved. Also, as voice, CS data and PS data flow on the Lub, signalling capacity requirements are the highest there. In the previous chapter, the concept of soft handover was discussed. Soft handover increases the performance of the WCDMA network, but it is a feature that increases the required capacity by almost 40%. This means that approximately 40 % of UEs are connected to two or more Node Bs at the same time. Furthermore, it can be assumed that 35 % are in a two-way SHO and 5 % in a three-way SHO. Consequently, on average every call occupies 0.6 X 1 + 0.35 X 2 + 0.05 X 3 = 1.45 physical channels. Retransmission and buffering also increases capacity requirements. The system retransmits the data to the subscriber's UE because the first time transmission had errors. This feature is used for PS services but not for CS services as it does not make any sense. Some capac