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View Full Version : How to convert Wireless % to dBm?

jimj
2008-04-24, 21:17
I have a SB3 connected to SC 7.0. On my Squeezebox when I go to "Settings->Information->Player Information->Wireless Signal Strength" I can see my strength as a percentage. Doesn't anyone know how to convert the SB's percentage to dBm? For example 100% = -40 dBm, 90% = -50 dBm, etc.

TIA,
Jim

mikeselectricstuff
2008-04-25, 01:36
It may not be realistic to get a particularly meaningful figure, as a dBm figure would imply some degree of accuracy in the hardware that does the measurement.
In practice, component tolerances etc. mean that the figure may vary significantly from unit to unit for a given signal strength, and there is almost certainly a not-easily defineable transfer function between the percentage and a nominal dBm figure as the measuring hardware is only designed to provide a rough figure.

jimj
2008-04-25, 06:22
It may not be realistic to get a particularly meaningful figure, as a dBm figure would imply some degree of accuracy in the hardware that does the measurement.
In practice, component tolerances etc. mean that the figure may vary significantly from unit to unit for a given signal strength, and there is almost certainly a not-easily definable transfer function between the percentage and a nominal dBm figure as the measuring hardware is only designed to provide a rough figure.

I don't see the correlation between unit of measurement and degree of accuracy. Somewhere in the code it must convert from dBm to %. The percentage can't just magically appear, it must be calculated somewhere. At the very least I'd be interested to find out what the SB considers to be perfect signal strength.

That fact that wireless networking is primarily composed of black magic and variables out of anyone's control is irrelevant. I.e. I want to know if I set two SB's next to each other and they each get different signal strengths. And I will know that now, it's just presented as a percentage instead of dBm that I'm used to working with (i.e. unit of measurement doesn't matter).

Thanks,
Jim

Squonk
2008-04-25, 08:32

HTH

mikeselectricstuff
2008-04-25, 11:25
I don't see the correlation between unit of measurement and degree of accuracy. Somewhere in the code it must convert from dBm to %.

Not necessarily. There is no fundamental reason that the hardware needs to produce a figure even vaguely proportional to dBm.

That fact that wireless networking is primarily composed of black magic and variables out of anyone's control is irrelevant.
except that this makes measuring things in terms of real physical units like dBm is costly, and unnecessary for a wifi receiver

I.e. I want to know if I set two SB's next to each other and they each get different signal strengths. And I will know that now, it's just presented as a percentage instead of dBm that I'm used to working with (i.e. unit of measurement doesn't matter).
Thanks,
Jim
If the unit of measurement doesn't matter, what's wrong with percent?

The hardware will be producing a measuremnt value that has some relationship to the signal strength ( and/or possibly some secondary signal quality indicators like error rates). Expressing it as a percentage makes some sense as it is related to the range of values measurable with that hardware (although almost certainly not following linear, log or other 'nice' transfer function, nor very repeatable between units), possibly scaled to the limits of useable signal levels.
However as soon as you call it dBm, this implies a relationship to 'real' physical values that may be completely misleading, unrealistic or just plain nonsensical, especially if the value is derived from secondary signal-quality indicators.

jimj
2008-04-25, 11:53
Not necessarily. There is no fundamental reason that the hardware needs to produce a figure even vaguely proportional to dBm.

So the percentage measurement is just the RSSI referenced in Squonk's PDF? If so that makes me want to see dBm even more since RSSI has no universal meaning.

except that this makes measuring things in terms of real physical units like dBm is costly, and unnecessary for a wifi receiver

If the unit of measurement doesn't matter, what's wrong with percent?

I didn't realize there was a significant cost to producing a dBm measurement, that would be valid reason for not displaying it. There's nothing wrong with a percent per se, I would just like to see something more concrete/easily understandable.

For example I know that all wireless devices I've used have no problems at -60 dBm, I rarely see problems at -70 dBm, and things start to get sketchy nearing -80 dBm. My SB averages a 75% wireless signal strength, but what does that mean? Is 75% good? What percentage strength do I need to maintain a good connection with a SB? My knowledge of dBm does no good with a percentage that only has meaning relative to the Squeezebox. A dBm measurement allows you to perform a more apples to apples comparison between different wireless devices.

The hardware will be producing a measuremnt value that has some relationship to the signal strength ( and/or possibly some secondary signal quality indicators like error rates). Expressing it as a percentage makes some sense as it is related to the range of values measurable with that hardware (although almost certainly not following linear, log or other 'nice' transfer function, nor very repeatable between units), possibly scaled to the limits of useable signal levels.
However as soon as you call it dBm, this implies a relationship to 'real' physical values that may be completely misleading, unrealistic or just plain nonsensical, especially if the value is derived from secondary signal-quality indicators.

I guess I would hope that a dBm would have a real relationship to physical values that I could trust. I agree that untrustworthy dBm values would be worthless.

How do you interpret wireless signal strength? Do you know what percentage strength is considered bad, ok, good?

Thanks for your detailed reply, I always enjoy finding out how much I don't know. :)

jimj
2008-04-25, 12:02

HTH

Reading the referenced PDF more closely I see that 20% signal strength is the roam threshold, so my wild guess that anything above 50% would be fine for SB use. I'd be interested in getting more informed opinions on that though.

I also see that they have RSSI to dBm conversion tables for different manufacturers, so if I know the wireless chipset used by the SB I might still be able to come up with a dBm value. Doesn't anyone know what wireless chipset the SB uses?

Thanks,
Jim

Mark Lanctot
2008-04-28, 05:44
Doesn't anyone know what wireless chipset the SB uses?

Atheros.

Incidentally from reading forum posts, it seems like the reliable signal strength for the SB is about 50% as you guessed, below this and problems may occur.

jimj
2008-04-28, 22:40
In case anyone cares the Conversion for Atheros chipset from % to dBm (per the pdf above) is:
-Subtract 95 from RSSI to derive dBm
-Notice that this gives a dBm range of –35dBm at 100% and –95dBm at 0%.

Keep in mind that mikeselectricstuff (who knows a lot more about wireless than I do) said converting % to dBm probably isn't too accurate, so use this formula at your own risk. :)

Thanks for all the replies,
Jim