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Super Contributor I

Transmitted and received power on IAP

Good morning community,

 

While doing tests on my lab, I see the following on my IAP regarding to power:

 

Received AP power by laptop:

ssid_pot.PNG

Received client power by AP:

laptop_pot.PNG

 

The AP in transmitting 6 dBm in the 2.4GHz band, and my laptop is touching the AP, so the free space path loss should be near to zero. Taking into account mismatch polarization between AP and laptop, mismatch number of antennas and so on, the free space path loss shouldn't be very high anyway. Then:

 

1. Why the received power by laptop is so low if the AP is transmitting at 6 dBm. Around 40 dB of free space path loss?

 

2. The AP is receiving 63, does this mean -63 dBm? If so, the same question as 1, many free space path loss...

 

Many thanks in advance.

 

Re: Transmitted and received power on IAP

Client power is not tied to AP power. I'm not sure what you're trying to acertain here. I will say using laptop clients and graphs by simply touching them or removing free space is still not accurate as the tools themslves are not the most accurate if you're trying to characterize (in detail) RF power. 

 

Jerrod Howard
Sr. Technical Marketing Engineer
Super Contributor I

Re: Transmitted and received power on IAP

Hi Jerrod,

 

 

Thanks for your interest on this. Maybe it will be better with other words, I'll put my questions simpler:

 

1. Why the received power by the laptop is so low (-36 dBm) if the AP is transmitting at 6 dBm and they are touching each other.

 

2. In the second graph, does number 63 mean -63 dBm?

Aruba

Re: Transmitted and received power on IAP

-36dBm is a VERY strong signal for any WiFi receiver. And while I would agree that when they're this close, you should see an even stronger signal, the RSSI detector at the receive probably can't handle such a signal and may just show you the maximum it can do.

At 1m distance, the typical pathloss at 2.4GHz is around 40dB. That becomes just 20dB at 10cm, but the FSPL model really only applies in the far field. In the near field things behave differently.

I'm pretty sure the graph is showing SNR (signal to noise ratio), for which dB is the correct measure. an SNR of 63dB is (unrealistic and) great.

/Onno
Super Contributor I

Re: Transmitted and received power on IAP

Hi Onno,

 

Good explanation!

For the first part, I know -36 dBm is a very great signal for normal situations, but in this case the laptop and AP are kissing each other. Therefore as you said the RSSI detector at the receiver probably can't handle such a signal and may just show you the maximum it can do or handle.

 

For the second part you say the graph shows SNR, since dB is the correct unit. But the Instant GUI shows the AP transmit power in dB, when it should be dBm, look at this screenshot:

instantguipower.PNG

The Aruba Instant User Guide says for the Client Tab:

 

Signal—Current signal strength of the client, as detected by the IAP.

 

But it is not clear. Do you know a command that shows the signal strength received by the AP (not SNR)? I have been searching but I didn't found anyone.

 

Anyway searching old posts I have found one where Colin says "The numbers are the signal strength that the AP sees from the client. If the number is positive, it is SNR. If it is negative, it is dbm."

 

http://community.arubanetworks.com/t5/Wireless-Access/Viewing-client-signal-in-Instant/td-p/297000

 

Aruba

Re: Transmitted and received power on IAP

I'm aware that our software is not always using the right units for power (dBm), antenna gain (dBi) and power ratios (dB), and am still coninced that the graph you showed is actually an SNR plot. Positive numbers and a range from 0 to 70 would not make sense if this was power.

The easy way to find out is to see if a stronger signal (reduced distance) makes this go up or down. I bet it will go up, confirming this is SNR.

I'm not sure where you can see received signal strength.

/Onno
Super Contributor I

Re: Transmitted and received power on IAP

Hi Onno,

 

I have made a test moving away and moving the AP closer to the laptop, and when bringing the AP closer to the laptop the number went up, so it is SNR as you said:

laptopsnr.PNG

I guess I can calculate the signal strenght by knowing the noise floor, which is -96 dBm as showed by the GUI. This would lead the AP is receiving from laptop a signal strenght = SNR+Noise = 67-96 = -29 dBm, which also makes sense. Many thanks!

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