Wireless Adapter Frequency and dBm combination


(squishy) #1

Hi,
I been playing with the Tx-power of my wireless adapter AWUS1900 and I was wondering if someone would mind dumbing down what they mean and if I understand them properly,
Tx power is the transmission power the strength in which signal is transmitted but in monitor mode of the adapter it would be the larger the tx-power the larger the area covered?
and the frequency would be the channels that my adapter is able to listen, like having 5.xx it would be listening to band within a/g. right? and if im wrong kindly correct me.

My adapter tx-power originally had on monitor mode had:
Tx-power = 18 dBm
frequency = 2.4

then when I did,
iw wlan0 set txpower fixed 5000

Tx-power = 40 dBm
frequency = 2.4

then when I did,
iw reg set US

Tx-power = 40 dBm
frequency = 5.3

So, im my opinion when doing a wireless assessment or wardriving the higher the tx power that the adapter can handle and higher frequency it can operate is better. is that correct ?
Would love to hear the opinion of others.
thnx


(Hardeep Singh) #2

In monitor mode, the card simply sits and waits for the signal to reach itself, imagine smelling a fragrance without moving yourself. The fragrance is the signal reaching you. So it doesn’t matter how far YOU can send signals if you are only receiving what’s coming to you.

There’s a general rule of thumb. The card that can send signals to longer distances is usually also good at receiving signal from longer distances.

From physics perspective,
An antenna is a simple device that receives the electric current from the radio (tuned at a certain frequency) and the energy from the oscillating current in the circuit is released in the form of electromagnetic waves. Which we refer to as signals.

Similarly, when a signal hits the antenna at its terminal, it gets charged and it generates an alternating current in the circuit, which is further amplified by the radio (our chipset/brain) and converted into digital equivalent.

So an antenna which is capable of radiating/vibrating for higher frequencies is also equally sensitive for waves of higher frequency but maybe low power, and the signal gets amplified by the radio itself.


Channels are a small window of frequencies within which a card can operate.
Example: Channel 1 operates on frequency range 2401 to 2403 MHz. The window here is 22 MegaHertz. Similarly for 5 GHz the band size is 20 MHz.
Yes, for 5 GHz your card can listen to (802.11a/h/j/n/ac/ax) as only these are the standards that operate on 5 GHz.

While monitoring 5 GHz traffic at a certain frequency, traffic at other frequencies will be totally invisible to you.
Your card is dual radio, so you can other part of the card which operates at 2.4GHz.

Theoretically yes. You can increase effective range by using a directional antenna. Using the same rule of thumb I described above. if you can send signal to long distances, you can receives from larger distances too. Just that the signal reception will be highly targeted because antenna is directional.

I hope I am clear enough :grimacing:


(Hardeep Singh) #3

Also, Your tx-power didn’t go past 40 dBm because that’s the hoghest value allowed legally in your country.

If you want to play around you have 2 options.

  1. Edit the CRDA values in your kernel for desired output. Say, 2 watts if your card is capable of operating at such power
  2. Or simply use Cloud based WiFi hacking labs. You won’t be having any legal issues that way. Since no packet is ever transmitted into the air :sweat_smile:

It must be 30 dBm for 2.4 GHz. Please recheck. No country allows more than 30 dBm by default, unless we edit the kernel packages to forcefully operate at such power.

:warning: It can be very dangerous to biological system if you work at such power very near to you. Energy from the radiation can be absorbed by the cells of our body and can even break them. Probably of 5 GHz. as 40 dBm is equivalent of 10 Watts of wireless power!!


(squishy) #4

As always great explanation, thanks.
Good references for understanding about the WLAN channel can be found here and here.

And yep it does actually go to 40 dBm and 2.4 GHz when I try to set the tx power to 50 dBm, so it means my adapter doesn’t allow it to be set to 50 dBm but the regulatory restriction isn’t also preventing it from being at 40 dBm.

And as what i understand frequency wise my adapter supports ac/a/b/g/n so i dont really need to thinker on the frequency and for tx power as you mentioned it’s good to be on the safe side of the law and the max allowed in my country is 30 dBm from here and as you mentioned higher than that is not so good.

But for the sake of being curious there is this tutorial on how to bypass the restrictions and increase your tx power from here


(Hardeep Singh) #5

I think there’s a mistake. You might be confusing tx-power with signal-level in iwconfig output? can you show a screenshot where you are actually observing the card to be set at 40 dBm?

See, 40 dBm means 10 Watts of wireless power!

and AWUS1900AC card that you are using is USB powered. A USB port can operate at maximum 5 Volts. Even if we ignore the power dissipation then also it’s not enough for a USB port to power the alfa card at such level of current.

Formula:
p = v * i

 Power = voltage * current

10 W / 5 V = current (A)

So for a USB port it needs to provide 2 amperes of output current in order to power a 10W device. know that USB ports have a limit of 0.5A only. Which is 4 times less than what is required.
Even if you use 4 USB ports with your device, consider the power dissipation. You’d need 5 USB ports for your card to operate at 10 Watts!

Also, the card’s chipset needs to be powerful enough to operate at 10 watts. I personally haven’t seen a wifi card well beyond 2 watts. That’s the reason I strongly suspect there’s either a misinterpretation issue or something else.
Please provide screenshot of your claim.

Yes, they are basically editing the CRD Agent files for desired output. But make sure your card is capable of operating at the desired power.

More the signal power, farther it can reach. As the signal gets weaker over long distances due to interfering objects that absorbs the signal’s energy causing the signal to get weak.


(squishy) #6

Here is the pic, and yes thanx again for that info, it just shows how noob i am in this field, great to learn as always :slight_smile:
dBm


(Hardeep Singh) #7

Two questions

  1. Did you edit the CRDA earlier for 40dBm?
  2. What is your card’s output power?

(squishy) #8

I havent thinker on the CRDA for it to hit 40 dBm and from the AWUS1900 guide power output is +/- 2 dBm but honestly I have no idea :slight_smile:


(Hardeep Singh) #9

Maybe some chipset “feature” with new alfa cards used for pentesting :slight_smile: I have no idea how can it even accept 40 dBm of output. It must show an error because

  1. Maximum allowed value by FCC is 30 dBm.
  2. Card itself doesn’t support more than 2 Watts.

There’s something for sure that I have no idea about. I will update this thread if I discover anything worthwhile.

2 Watts*

dBm (Decibel milliwatts) is a ratio of output power compared to the antenna which isotropically radiates power. or you can say compared to the “ideal” antenna.

For every 3 dBm, power in Watts doubles.

So, for 1 Watts it’s 30 dBm. If you increase +3 dBm it will double the power in watts.

So 33 dBm for 2 Watts.