I have setup a home business with a wireless LAN using a Cisco 350 AP and PCMCIA card(s). I have tried a number of different frequencies only to find that they have not make a significant difference in the range that users have within the house. The house itself is a classic New England post-and-beam design, and should very easily be covered by a single access point. Additionally, the user has major issues when wanting to work outside of the house due to a lack of signal strength. I would appreciate any and all help from people who have setup these systems in the past and know some tricks to extending the effective range.
You might look for a better antenna for your access point. You might also find relocating it helps (Highest floor worked best for me.) Technically, line-of-site is best without obstructions but you should get minimal degradation with a wall/floor or two away. Also look for any 2.4 GHz interference. I got rid of all the 2.4 phones in my house because it knocked me offline every time someone called.
get into the AP via web browser from a non-radio attached PC. Get into setup, then Diagnostics. click on the Start Carrier Test. The resulting graphs will indicate if there is a lot of traffic or noise on the 11 possible channels. DON'T FORGET TO STOP THIS TEST. You will not be able to associate to the AP while this test is running. You may want to set the option to search for less-congested channels. The previous respondent is correct, though. If there is a lot of interference, you will not connect.
I agree with the instructions of looking at what channels are in use and picking the channel with the least interference. I would also like to add the need for a site survey to find the cell perimeters and relocating the AP to the location that would provide the largest cell perimeter. Many times, the first or most convenient physical location for an AP is not the most optimal location for radio frequency.