 optimal location for an access point with an location omnidirectional antenna will generally be close to the centre of the area to be covered, in a position that maximises line-of-sight to client stations and is clear of obstructions, particularly metal objects such as filing cabinets. An elevated location can be very effective, for example, a ceiling mounted unit. Operating Any channel that shows significant background or sporadic channel noise should be avoided. The available non-overlapping channels can then be allocated to access points based on their initial locations. A typical channel allocation pattern for the three non-overlapping channels in the 2.4 GHz band is shown in Figure 7-2. Power setting In general, the number of access points will be minimised if maximum permitted transmit power is used. Reasons to adopt a lower power setting may be to reduce out of building propagation or to avoid interference with other RF systems. Conversely high power settings may be required to combat local conditions of high RF noise or high path loss. p. 152"), it is possible to operate four access points with minimal frequency overlap on channels 1, 5, 9 and 13, potentially increasing network capacity by one-third. This would permit the additional channel allocation patterns shown in Figure 7-3. Early WLAN layouts were often developed on a trial-and-error basis, by temporarily deploying access points, taking signal strength and throughput measurement (essentially repeating a site survey) and then relocating or adding access points to fill in any identified holes in RF coverage. Planning tools such as Wireless Valley's "LAN Planner" are available that improve WLAN design by simulation and graphical display of the Chapter Seven Grid pattern Cellular pattern Figure 7-2: Channel Allocation Pattern for 802.11b Access Points with Three Non-overlapping Channels expected performance of the network. Information such as received signal strength indicator (RSSI), signal to interference ratio (SIR), signal to noise ratio (SNR), th