One option in determining where to best place your AirPort base station is to measure its signal quality from a wireless client. In theory, where the signal quality is best would make for ideal candidates for placement. This would hold true for a single or multiple base stations in an extended wireless network.
One method to calculate signal quality is to compute the Signal-to-Noise Ratio (SNR). SNR is the signal level (in dBm) minus the noise level (in dBm). Both of these values are typically represented as negative numbers. For example, a signal level of -53dBm and a noise value of -90dBm would yield an SNR of 37dB (i.e., SNR = Signal - Noise = -53 -(-90) = 37)
The calculated SNR value, as measured from a wireless client, would decrease as the range to the base station increases due to applicable free space loss. Also an increase in RF intereference from microwave ovens, cordless phones, walls, ceilings, etc, which would increase the noise level, would also decrease the overall SNR value.
- 40+dB = Excellent signal
- 25dB to 40dB = Very good signal
- 15dB to 25dB = Low signal
- 10dB to 15dB = Very low signal
- 5dB to 10dB = Little or no signal
The following are two methods to take signal & noise level readings:
Method 1 - Using iStumbler
Download and install a copy of iStumbler. Use iStumbler's Inspector feature (select Edit > Inspector from the iStumbler's menu) to take the signal and noise level readings.
Method 2 - Using OS X System Profiler
For OS X Tiger through Snow Leopard:
Click on the Apple icon on the menu bar > About This Mac > More Info... > Contents > Network > AirPort > Interfaces > en1 > Current Network Information > Find your wireless network > Signal / Noise
For OS X Lion and later:
Click on the Apple icon on the menu bar > About This Mac > More Info... > System Report... > Network > Wi-Fi > Interfaces > en1 > Current Network Information > Find your wireless network > Signal / Noise
Method 3 - Using Wi-Fi Diagnostics
For OS X Lion the Wi-Fi Diagnostics app is located in: /System/Library/CoreServices/Wi-Fi Diagnostics.app
Run Wi-Fi Diagnostics > Select "Monitor Performance" > Continue > The Signal & Noise values will be shown in the Performance table.
For OS X Mountain Lion you can access the Wi-Fi Diagnostics directly from the AirPort icon on the OS X menu bar. Once the application open, use Command+N to open the Network Utilities windows. Finally click on the Wi-Fi Scan tab.
Regardless of which method you use to gather the reading, you would plug them into the SNR formula. The desired goal value is an SNR of 25+dB. At this SNR value, wireless clients should be getting reasonable throughput performace with the base station. This is also the value you would want at the location where you would want to place a second base station to extend the first if you are planning on extending the wireless range.
If you are getting SNR values of less than 25+dB at the client, either try to locate the source of the Wi-Fi interference or try relocating the base station until they are within a 25+dB range.