Ubuntu – the “quality” of a Wi-Fi access point


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
                Protocol:IEEE 802.11bg
                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.)

Best Answer

  • According to the documentation for Wireless Extensions, the information provided to Wireless Tools comes from /proc/net/wireless, which are more clearly defined by:

    The basic difference between Quality - link and Quality - level is that the first indicate how good the reception is (for example the percentage of correctly received packets) and the second how strong the signal is. The Quality - level is some directly measurable data that is likely to have the same signification across devices.

    It appears that the Quality - link value is the one that other tools tend to reference most.

    It should also be pointed out that /proc/net/wireless is apparently a clone of /proc/net/dev which provides a different output, but you could in theory calculate the difference.

    Looking at the source briefly, it appears these values come from the driver. That could explain your problem with the Quality - level and Quality - noise values: your driver doesn't support Wireless Extensions, or doesn't support it well.

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