Relationship between Link quality and signal level

Dan Williams dcbw at redhat.com
Fri Dec 8 10:19:28 EST 2006


On Fri, 2006-12-08 at 14:34 +0000, David Goodenough wrote:
> Why would I get:- 
> 
>          Link Quality:0/92  Signal level:-69 dBm  Noise level:-90 dBm
>  
> from iwconfig?  Normally the link quality first number is roughly the
> difference between the signal level and the noise level, but here is
> reports 0.

This varies between drivers.  But the "correct" way to report link
quality is:

- Signal level: raw signal level either in dBm or RSSI from the card
- Link Quality: a subjective measure of how good the link is, including
signal strength, speed the link is operating at vs. capable speed,
packet loss, retries, etc.

Link Quality in correctly-written drivers should be a value in the range
[0, 100] inclusive representing a percentage between "worst" link and
"best" link.

Signal level is just the raw measure of RF energy received by the radio
on the card and is one component of link quality.

Link quality in Linux wireless drivers has never been extremely
reliable, though this is getting much better as time goes on.  The ipw
drivers are good examples of how to do link quality correctly.

> The other end of this link reports a more healthy link quality:-
> 
>           Link Quality:30/92  Signal level:-79 dBm  Noise level:-100 dBm
>  
> but (I presume because the first one says 0) the current rate is 1Mb/s
> 
> This link is running ad-hoc, these are the only two stations on this 
> network, and neither end can detect (using iwlist scan) any other users
> on this channel.  There is a clear line of site, and there are virtually
> no buildings in between to generate any non-802.11 interference.
> 
> Both ends are using Senao PCMCIA 802.11b cards, prism chipset, both
> at 1.8.0 firmware.

What kernel version and hostap driver version are you using?  The link
quality is calculated by the _driver_, not by the card or firmware, and
the different may be due to different driver versions.

Dan

> Any ideas welcome.
> 
> David
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