Industry standard for minimum Wifi signal strength?
Solution 1:
As far as I'm aware, there's no "Industry Standard" for "good signal quality".
A number of tools I have at my disposal seem to show that "Poor" quality is between -100 and -85dBm, "Good" quality between -85 and -60dBm, and "Excellent" between -60 and -40dBm.
The question is probably more one of antenna design, which is a bit too advanced for here, but might fit on ElectronicEngineering or Physics stacks.
I have seen some Access Points which deliberately limit the number of clients per AP, or per-radio, for multiple-radio APs. The radiated power from the radio correlates poorly with number of clients per radio/per access point.
I'm going to have a read of the 802.11 specification and see if there's anything specific I've missed.
edit
802.11 defines the following as available metrics of radio signal quality.
- channel load
- noise histogram
- Station statistics
- location configuration information (LCI)
- neighbor report
- link measurement
- transmit stream/category measurement
example:
4.3.8.5 Channel load The channel load request/report pair returns the channel utilization measurement as observed by the measuring STA (Station).
These are what are defined in the standard. What's not mentioned is how some vendors don't implement all the possible values for these parameters.
Solution 2:
Signal strength in DB by itself is pretty useless for determining number of users an AP can support.
There are just too many other factors that play a role in this.
To name a few (and I know I'm missing a whole bunch of things):
Some end-user devices have better antennas/more powerful radios than others so they can make do with less signal.
A slightly underpowered AP (in terms of CPU horsepower) may have trouble dealing with a larger number of concurrent clients, especially as bad signal strength (from the AP itself or from the client devices) increased the CPU needed to deal with error-handling/retries/packet-loss.
Also the CPU power needed for the encryption in use may vary greatly between protocols.
How many SSID's you will run over the AP. Does it have to do VPN from the AP to an (external) gateway (often used for guest-wifi).
Is there a WLAN controller that deals with roaming, authentication, VPN so the AP itself needs to do less?
And then you are not even considering interference from other technology using the same or nearby radio-frequencies.
In general a more powerful AP in terms of CPU can handle more clients.
The only thing you can say about signal strength in DB is that it establishes a lower limit but exactly what that limit should be is impossible to say. It just depends to much on your client equipment.
I have build Wifi setups in about a dozen big office complexes and 6 warehouse environments.
From personal experience I would consider -65 db the lower limit in a b/g setup and -55 in a n environment. Otherwise it's just too unreliable for low-power endpoint devices (many phones, consumer laptops).
If your customers are mainly business laptops 70 and 60 is usually still OK. (These usually come with better antennas.)
Oddly enough iPhones/iPads and most Android smartphones/tablets are in the underpowered phone category even though their prices might suggest they ought to be business grade equipment.