ength (ti), ses- sion inter-arrival time length (1/2j), activity factor Ai, plus signalling overheads and O&M margins. Here we assume that the ratio peak traffic over average traffic corre- sponds to the burstiness factor B. We then calculate the overall PDF(R) and CDF(R), where Ra corresponds to the ag- gregate bit rate to determine the outage probability for each value of the user bit rate. Afterwards we obtain a set of outage probabilities, which corresponds one to each user bit rate Rb. At the end we dimension the channel capacity by fixing a common outage probably value P for each service i. 7.7.1.1 lub Total Traffic As indicated in the preceding section, after we calculate the peak traffic per Node B, we take into account additional overheads and signaling loads. Thus, we obtain the total traffic at the Iub interface from the user information traffic, soft handover traffic, burstiness factor and overheads as well as signaling margins. Typical assumptions for the margins include: O&M = 10%, signaling = 20%, and ATM overhead = 40% of the lub peak user traffic, respectively. In summary, we can define the Total Iub traffic as: Total lub traffic = peak traffic + O&M + signaling + overhead Total lub traffic = average traffic X X (1 + 0.4 + + 0.1) 7.7.2 RNC Capacity To practically determine the number of RNCs required for a network, we generally con- sider the: maximum number of Node-Bs to be managed; the maximum lub, Iu and Iur connections supported; and the maximum throughput of both CS and PS services. We determine the total number of Node Bs based on the type of services offered, the number of subscribers projected and area of coverage desired among other parameters. We express the average traffic forecasted in Erlangs for CS and Mbps for PS. Plotting nominal values of PS VS. CS traffic, e.g. 64 kbps for both services we can see in Figure 7.10 that the proportion of PS and CS traffic depends on the desired load for either service. In any case, it seems that we cannot have half and half of each service type. 16