I need information about setting up an Aironet AP 1240. It seems that the configuration at RF setting does not seem to be at its peak. We use only the protocols 802.11 b\g and the speed is at 54 Mbps. My question is, what configuration should be used to provide the highest possible coverage? I am aware that the various metallic structures that make up the building structure detrimental to the efficient propagation of waves but even directly under the access point I just receive a signal whose signal\noise is -25dbm.
Try changing the channel on the ap to see if that gets you a better SNR. Hard to say what you can do to tweak the ap, without knowing how the wireless is setup. Moving ap's around or changing antennas type may help, but if you have a lot of interference in the 2.4GHz then you just have to work around that. Are you at least getting a -72dbi or stronger signal in areas that you required coverage?
54 Mbps is the fastest possible speed in the 802.11g standard. If you want a faster nominal rate you'll have to install an 802.11n access point.
WLAN design involves tradeoffs between speed and range. If you want the most possible throughput, you have to disable the slower data rates, which means your range will be reduced. If you want the maximum possible range, you have to allow slower connections which means less throughput for everyone.
If you're having noise problems due to multipath (metal structures), an 802.11n solution will solve a lot of problems. 802.11n uses multipath to improve its performance, unlike b/g/a which degrade in a multipath environment. If that's not an option, then make sure your APs are mounted at least 1 wavelength (2 preferably) away from any metal, and test all channels to see which gives you the best performance.