AP TX Power, RSSI, and dBm Measurements

[kfarrington]

Hello all

I have an 1240AP on my desk and the power is set to level 1 (which is the maximum)

We are in region E

The antenna is air-ant4941 2.2 dBi Dipole

If you look at the documentation

http://www.cisco.com/en/US/docs/wireless/access_point/channels/lwapp/reference/guide/1240_chp.html

So should the max power output be 17 dBm (50MW) on the AP correct?

So when I run up netstumbler, and have my laptop right next to the AP, I get as RSSI of -20 (I assume that is dBm) ? A negative value?

Just a tad confused?

Many thx indeed,

Ken

[Scott Fella · Accepted solution]

The TX power of 17 dBi is 50mW. What you see on your laptop of a -20 dBm is a good signal. Cisco's recomendation for data is a max of -72 dBm and for voice it is -65dBm. You will notice this when you start walking away from your AP. So if you are planning on adding another ap, you would want your coverage to be bordering eithe -72 dBm or -65 dBm.

Basically a good signal is 0 to -72 this will keep your signal at 54mb if running 'G' or 11mb if running 'B'. This goes the same for 802.11a.

[kfarrington]

Many thx for that, but im really sorry, I still dont understand.

If my AP is 17dBm (a positive) why is my laptop -20, a negative?

Sorry if I am being a thicko :))

Cheers

Ken

[rochoa8aeg · Accepted solution]

Basically you need to look at it this way... Say you have a home stereo receiver, most work this way, and you increase the volume (make it louder), you will see your negative digital display digits go down instead of up so if you put the volume really loud the digits will read -30 not so loud, -25 getting louder, -20 and louder, -10 really loud, -5 annoy your neighbors loud. Same with wireless signal, the lower the negative value the better the reception, however notice how kfarrington mentions and how Cisco recommends -75 for optimal performance. Once you start getting -80, -85, -90 that indicates poor to very poor coverage.

But also keep in mind that a -20 even though it is excellent coverage does'nt mean its optimal. Its just like standing right next to someone and when they need to tell you something they YELL in your ear as opposed to just speaking in a low mild manner which you could hear comfartably.

[scottmac · Accepted solution]

It is because your antenna is putting out a certain level of signal (17dBm - deciBells relative to a milliwatt, one milliwatt) and the receiver is only picking up a percentage of that.

If you stood under a large fountain of water (or a waterfall)that is outputting 50 Gallons-per-second with a coffee cup that can hold .05 Gallon, how much of the water could you catch at any given time? Only a little right?

Your AP, with an omni-directional antenna is acting like a fountain; it produces a donut-shaped field, 360 degrees in circumference laterally, and ~60-75 degrees vertically (with the antenna vertical relative to the floor) using a total of 17dBm worth of power...

Your receiver only sees a small portion of the total radiated power, so that's what it reports (also measured in dBm - deciBells relative to one milliwatt).

If you want to extend the analogy, as you move away from the fountain, the water flow becomes more like rain, with a maximum of .025 Gallons-per-second ... the "power" has dissipated sufficiently that even with an efficient container to catch the "water," there is not enough water to fill the cup.

The bottom line is that you radiate only so much power from the transmitting system via the antenna, and that power is distributed in a pattern according to the physics of the antenna.

That is why a yagi antenna or a parabolic dish transmits further than an omni with the same power; the radiated pattern is (through some physics tricks) concentrated ... instead of a broad azimuth of coverage, the power is concentrated along the Z axis of a narrow beam

of energy.

Good Luck

Scott

[john.preves · Accepted solution]

Here is my .02-

I always explain it like this.

Signal and power exist on a number line. Remember those? Zero in the middle the negative numbers to the left and the positive numbers on the right.

-100....-70.....-40....-20..0...5....10..20

The left side represents the signal strength and the right side represents power. Until I saw ScottMac's above post I haven't had the words to explain why. The receiver only sees a small portion (thanks dude).

The noise floor (the drop dead point where there will be no signal at all anymore) of a Cisco radio depending on what data rate etc. you are at is roughly -95dBm. Higher datarates make the noise floor come closer to zero such as -90dBm for 11Mbps (I think..)

Anyway - in order for you to have a good link your signal measurement needs to be at least a pre-determined distance from the noise floor. As stated above Cisco recommends at least -72dBm for data and -65dBm for voice.

So back to your number line:

-90....-80....-70....-60....-50.......-20..0

________|_____|________________|

_____data___voice___________ you

There is roughly a 15 to 18dBm difference between the very end of the data and the end of the signal all together. There is anywhere between a 25 and 30 dBm difference for voice. The closer you get to zero the better your signal will be.

Obviously, your data and voice will work even better in the 50's or 40's. The trick to designing a system is to figure out how many and where these radios need to go. Manipulating the environment utilizing different antennas, power and channel settings, and even the building itself is what we all do for a living.

As for the number line...everytime you add three dB, you double the power. Everytime you subtract three dB, you have it. Go ahead and count forwards and backwards by three and the math works out. Or better yet look up a chart online.

Hope this helps-

[Rob Huffman]

Hey Guys,

Great discussion here! I love the varied views that are both accurate and entertaining (some humour in there as well with the "Rocking out the neighbor analogy" :)

5 points for each of you for sure!

Thanks again,

Rob

[kfarrington]

Hey Guys,

Can I just add my many thx to the excellent and most informative responses.

I can certainly relate to the anoying the neighbors with my stereo (well once in a while).

Please accept my thanks to all and this has really made me understand the point.

Great stuff, cheers, will mark these posts :))

Ken

[sdt]

hope you don't mind if I put in a plug for a commercial product. On our website there is a software app. which you can use to plot signal strength received by the AP from the client.

You can see (and plot) as you move away from the AP, that the signal strength becomes lower (more negative). I think if you work with the app you'll get an intuitive feel for signal strength and reporting. You can download a free version for 2 APs at

http://www.oberonwireless.com/dot11-monitor.php

let me know if this helps!

Scott