On Wed, Mar 26, 2008 at 10:07 PM, bruno randolf <bruno@xxxxxxxxxxxxx> wrote: > On Thursday 27 March 2008 07:59:11 Luis R. Rodriguez wrote: > > > + * @IEEE80211_HW_SIGNAL_UNSPEC: > > > + * Hardware can provide signal values but we don't know its units. > > > To be + * able to standardize between different devices we would like > > > linear + * values from 0-100. If possible please provide dB or dBm > > > instead. + * > > > + * @IEEE80211_HW_SIGNAL_DB: > > > + * Hardware gives signal values in dB, decibel difference from an > > > + * arbitrary, fixed reference. If possible please provide dBm > > > instead. + * > > > > Signal should given in either be in dBm or an unspecified value. Since > > we have "unspecified" not sure why we would have the "db" value. Can > > you clarify what the difference between "unspecified" and "db" would > > be? I don't think it makes sense to refer to signal with a "db" value, > > unless we want "singal" here to be able to mean SNR. > > the best thing to specify is dBm and all drivers should try to do that. it's > the only absolute value which makes sense and can be compared between > different drivers and devices. > > however as we know some drivers simply cannot provide that. instead they have > some form of RSSI - RSSI in general meaning nothing more than "received > signal strength indication", which could be *anything*: voltage, any numbers, > in dB or linear or whatever. RSSI does *not* in general mean SNR! for example > you might remember the values of prism54, they were somewhere around 200. or > prism2 cards which had something around 100 (it's been a while since i've > touched them, so that might have changed). as jean noted the values don't > even have to be linear - so i think we should remove that from the > definition. > > so in the "unspecified" case we really don't know much about the units, in the > worst case we can't even assume that 1/2 the value will mean 1/2 of the > received signal strength. it's very hard to work with this values, but it's > better than nothing. > > other cards might not have noise, so no absolute (dBm) measurement, but a form > of RSSI which is measured in dB (that is a logarithmic difference). in this > case we might not know which reference point the measurement is taken to, but > we know how it behaves: -3 dB means half of the signal strength. if you add a > 6dB gain antenna you can expect to see the signal reading to increase by 6dB. > it's not strictly SNR (because we don't know the noise) but quite close to > it. > > i think it's important to know the difference between this 3 cases i order to > make some sense of the values in userspace gui, roaming agents, mesh link > metrics, etc... Agreed, but can you give me an example of a card which does provide RSSI in db for values we don't know and that UNSPEC cannot handle? Is this what is assumed of the rt2x00 cards? Luis -- To unsubscribe from this list: send the line "unsubscribe linux-wireless" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
