n terms of their individual capacities) is enough for the needed capacity of the network. Of course, as seen above, this depends heavily on the frequency re-use factor. There are three essential parameters required for capacity planning: estimated traffic, average antenna height, and frequency usage. Figure 2.12 Example of frequency re-use Traffic Estimates Traffic estimation or modelling is based on theoretical estimates or assumptions, and on studies of existing networks (i.e. experience). Traffic in the network is dependent on the user communication rate and user movement in the network. The user communication rate RADIO NETWORK PLANNING AND OPTIMISATION means how much traffic is generated by the subscriber and for how long. The user movement is an estimate of the user's use of the network in static mode and dynamic mode. Traffic estimation in the network is given in terms of 'erlangs'. One erlang (1 Erl) is defined as the amount of traffic generated by the user when he or she uses one traffic channel for one hour (this one hour is usually the busy hour of the network). Another term that is frequently used in network planning is 'blocking'. Blocking describes the situation when a user is trying to make a call and is not able to reach a dialled subscriber owing to lack of resources. Generally, it is assumed that a user will generate about 25 mErl of traffic during the busy hour, and that the average speaking (network usage) will be about 120 seconds. These figures may vary from network to network; some networks use average figures of 35 mErl and 90 S. Another factor is the user's behaviour in terms of mobility. In the initial years of the GSM, the ratio of static users to dynamic users was almost 0.7, but with rapid changes in technology this ratio may soon become 1.0! User mobility affects handover rates, which in turn affects network capacity planning. The actual traffic flowing in the network can be calculated by using tables that use the maximum traffic at a base station and the blocking rate. Commonly used