When light is green will I be sure to get a usable signal?

Question

when light is green will I be sure to get a usable signal?

Answer 1

My Airport Express is plugged-in high on a wall and has a green light.
My mac is near it and I do not get a usable signal.
To verify that your Mac is getting a usable signal (or not), you will want to calculate the Signal-to-Noise Ratio (SNR) of the AirPort Express Base Station (AX) from the location of your Mac.
SNR (in dB) = Signal (in dBm) - Noise (in dBm)
You can easily find the Signal and Noise value using System Profiler on your Mac.
ref: 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
While your still in the System Profiler, note the value for Transmit Rate.
These values should be negative numbers. For example, mine currently read: -67 dBm / -95 dBm, with a transmit rate of 27 (Mbps). Plugging in these numbers yield: SNR = -67 - (-95) = 28. My Mac Mini is about four rooms away from my AEBS with about five walls between them. As you can see, the signal is still quite useable ... but the bandwidth is minimal, especially for streaming.
SNR Guideline
40dB+ SNR = Excellent signal
25dB to 40dB SNR = Very good signal
15dB to 25dB SNR = Low signal
10dB to 15dB SNR = Very low signal
5dB to 10dB SNR = No signal
If the green light indicates that is receiving and boosting the signal, why don't I get a signal?
As Bob has already mentioned, a green status light basically is telling you that the base station thinks that it is properly configured. Unfortunately, it does not indication signal or connection quality.
Does "extending" mean increasing [boosting] signal strength and extending the signal to a "close enough" computer or next "Airport Express" in a daisy chain?
Yes, "extending" will boost the signal strength at the location of the extending base station.
Is the strength of the "extended" signal dependent on the strength [adequacy] of the signal received by the Airport Express?
The strength isn't, but the bandwidth is. The following graphic does a pretty good job of explaining it.