Signal strength around AP

[interfacedy]

Hello, Anyone can explain the picture of AP signal. The picture shows signals strength in different angles so that we can know how strong the signal is at different angle. Do you think the picture show signal strength at some distance away from the AP? For example, what dBm it is at 10 feet, or 20 feet away from the AP? This way we compare signal strength of each AP. Thanks

[Leo Laohoo]

---

@interfacedy wrote:

Do you think the picture show signal strength at some distance away from the AP? For example, what dBm it is at 10 feet, or 20 feet away from the AP? This way we compare signal strength of each AP. 

---

The picture is the radiation "footprint" of an AP in a horizontal- and vertical point-of-view (when the radios are at full power).  

[Dustin Anderson]

You will not find any measurement based on distance as that would be by antenna design and power settings. Unfortunately the only way to see how one will perform for you is to buy one and test it in your environment. 

APs by a specific company will usually perform similar, but there is no guarantee. If you have Ekahau, that can help to at least simulate as they get a lot of the variables from the companies to give a good representation of the expected signal and coverage.

[saravlak]

the signal strength, cell-size and its pattern and range varies per each setting and it depends on Antenna type & gain/coding-Datarate/Tx power/channel/environment.

[patoberli]

There is actually a system behind those images. It's a bit complicated and you find the full explanation here:

https://www.cisco.com/c/en/us/products/collateral/wireless/aironet-antennas-accessories/prod_white_paper0900aecd806a1a3e.html

In very short:

3-dB beamwidth. 
The 3-dB beamwidth (or half-power beamwidth) of an antenna is typically defined for each of the principal planes. The 3-dB beamwidth in each plane is defined as the angle between the points in the main lobe that are down from the maximum gain by 3 dB. This is illustrated in Figure 3. The 3-dB beamwidth in the plot in this figure is shown as the angle between the two blue lines in the polar plot. In this example, the 3-dB beamwidth in this plane is about 37 degrees. Antennas with wide beamwidths typically have low gain and antennas with narrow beamwidths tend to have higher gain. Remember that gain is a measure of how much of the power is radiated in a given direction. So an antenna that directs most of its energy into a narrow beam (at least in one plane) will have a higher gain.

[Scott Fella]

Here is my 2 cents..... your title, "Signal strength around AP" can only be determined from taking measurements.  There is no calculation for this at all besides using a tool.  Like what was mentioned, a sites survey and post site survey is the best way to understand the RF in that specific environment.  

The key items why there are no calculations are: attenuation.  How can you have a calc that will determine your signal strength from a given ap, without knowing the attenuation between the client and access point?  If your ap is in an open area vs office area vs freezer or basement room.  Warehouse that is open with no materials vs warehouse with racks with 50% material vs ware house with racks and full of materials.  

There are tools like Ekahau and AirMagnet that can provide a prediction, but that is really used for estimation and not 100%.  Take an ap that you have and place it in another room and then place it in the hallway where you have line of sight.  Place the ap in the floor above or below and see if the signal is the same when the ap is the same distance away.

[interfacedy]

Agree with you. Here is an example. when we go to home dep, we have a lot choices to buy different watt of light bulb. Some people like more powerful light bulb, and other dont based on their demand. So we want to know if Cisco AP or other vendor's have this kind indicator to show how powerful the AP has so that people can know the basic feature roughly? or we can say all APs have the similar signal strength to emit? Thanks

[patoberli]

Your comparison is very good. Because lampA with 10 Watts compared to lampB with 10 Watts will probably not shine the same brightness, because of differently used technology, or even radiation pattern from the light-filament (not sure for the correct English term, but this stuff that emits the light) which is differently arranged.
In other words, you need to test out which lamp actually fulfills your requirements. Same is valid for Wi-Fi. And that is where the antenna radiation patterns come into play and help you for the decision.
One big difference though, if you have 10 lamps in the same room, you have a very bright but working room. If you have 10 APs on the same frequency in the same room, you have probably a not working Wi-Fi!

[interfacedy]

I agree with you. But you are talking about comprehensive results, which is mixed results among the signal output that the AP emits and environment absorption and interference etc around the AP. What I am talking about is only the signal output that the AP emits. Of course, there are a lot kinds of light bulbs. Some light bulb is LED, and other of them are classic traditional one. We cannot compare the old light bulb with LED. But watt is very basic indicator with which we can compare the same kind of light bulb. Even if comparing LED with traditional one, we can multiply traditional one with coefficient “6” then we can know how bright LED light bulb is.

[Scott Fella]

We are not taking about lights here. If you want to compare wireless to light bulbs, then it’s like a dimmer switch. You can provide the max output or not. RRM controls the power which will not be static and always changing. You can always calculate the EIRP and you can find the formula for that.

[patoberli]

And there is one value, but it's only the electrical power, which doesn't say anything about the actual radiation pattern. In most countries a radio can have 100 mW (that is 0.100 Watts) of output power. More is typically forbidden by the country. Some countries also allow different amounts of mW based on the channel the AP is running on. 

But as already stated, wireless works a bit different, as you get noise and co-channel interference (and other things), which is why you typically run an AP at 25-75% of maximum power (something that a site survey will calculate for you).