In Debian (including Ubuntu) you can use
iwlist to scan nearby Wi-Fi access points. In the statistics like this:
Cell 01 - Address: CC:B2:55:XX:XX:XX ESSID:"X" Protocol:IEEE 802.11bg Mode:Master Frequency:2.412 GHz (Channel 1) Encryption key:on Bit Rates:1 Mb/s; 2 Mb/s; 5.5 Mb/s; 11 Mb/s; 6 Mb/s 9 Mb/s; 12 Mb/s; 18 Mb/s; 24 Mb/s; 36 Mb/s 48 Mb/s; 54 Mb/s Quality:33 Signal level:0 Noise level:0 IE: IEEE 802.11i/WPA2 Version 1 Group Cipher : CCMP Pairwise Ciphers (1) : CCMP Authentication Suites (1) : PSK Extra: Last beacon: 252ms ago
There is a field called "quality". I can understand what "signal level" means, it is RSSI (Received Signal Strength Indication). However, what does "quality" mean really? How is it measured?
Meanwhile, on my Ubuntu 12.04 LTS laptop, it is weird because the "signal level" fields of all scanned access points are 0, so are the "noise level" fields. But on my Raspberry Pi which has Raspbian, I can see "quality", "signal level", and "noise level" fields have different readings. (This is not the primary problem, but if you know the answer please just breifly describe a little bit.)