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I am studying a Wireless subject and I have a random question where we have two floors each floor has a size of 70x140 meters and WIFI specs are as follows,

# of Antennas, 3
RF Pwr (EIRP) in dBm 17
Antenna Gain in dBi 2
UPnP able/cert able
Frequency: 2.4GHz

I need to find if I need two Wireless router or one to cover the entire two floors.

Now I have calculated Power Budget as 19

But How can I calculate how much area would one Wireless router will cover ?

I don't the answer i just need the guidance so I can calculate it myself and draw the graph.

Can anyone please help me ?

Thanks

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Well, you have the "test/book answer" and you have real life.

In real life, you try it if you want to know the real answer, since modeling the actual effect of walls, floors, etc is still somewhat substandard (or would require more effort to get an accurate model than going and checking in real life takes.) This is a "wireless site survey" and without one you are flying blind.

"Free space path" only applies if the path is in free space, which is exceedingly rare except for outdoor WiFi serving playing fields or the like. It does not account for the floor/ceiling you must have if the question is about 2 floors. Most commercial building floor structures (such as concrete over steel sheets) are exceedingly hostile to WiFi propagation. If there are any walls on the those floors, (ie, it's not just a warehouse like space) the answer changes again.

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    I absolutely agree that a wireless site survey should be done to get the apparent best placement for the APs. Another good practice is to do a follow-up survey after the Wi-Fi is up and running to see if there are any blind spots or too much overlap so that AP placement and power levels may be adjusted for optimum coverage. The data generated by the two surveys should be saved for future reference. – Ron Maupin Oct 18 '15 at 17:18
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You need to know the free space path loss -- that is, the loss between the transmitting antenna and the receiving antenna. With that you can figure out the radius of the area.

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As stated above by Ron Trunk, you can calculate the radius of the wireless signal using free space path loss.

But its worth noting that this calculation doesnt account for obstacles (walls, metal plates, bookshelf, etc), for example at the same floor or the floor,ceiling of the below/above floor.

Another propagation model is Log distance path loss, which is an extension to free space path loss, and it tries to take account of common obstacles in the calculations. http://www.gaussianwaves.com/2013/09/log-distance-path-loss-or-log-normal-shadowing-model/

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