Wifi monitoring to adjust directional antenna

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DrownLies
DD-WRT Novice


Joined: 29 Apr 2019
Posts: 2

PostPosted: Wed May 01, 2019 10:35    Post subject: Wifi monitoring to adjust directional antenna Reply with quote
Good day,

I am trying to use my WRT54GL v1 to connect to a Hotspot in my proximity. To improve the signal strength I bought a directional antenna which I need to adjust now.
In the firmware 'DD-WRT v3.0-r38159 mini (01/02/19)' I am using, there are only limited possibilities to monitor signal strengths of nearby Hotspots. Under Status -> Wireless there are the options to use 'Site Survey' and 'Wiviz Survery' which both don't really fit my needs. What I would need is a tool that could continuously list the signal strength, so I can find the best rotation and position of my antenna. I struggle to find such a tool for DD-WRT on my router and the situation is made more difficult since the WRT54GL only has 4 MB of flash, 16 MB RAM and no USB port.
The 'wl' commands seemed like they could help but when using telnet on my Windows 10 laptop to connect to my router those commands either weren't recognized or didn't provide any output.

Do you know any way to continuously monitor the signal strength of nearby W-LAN? Any help is highly appreciated!
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d0ug
DD-WRT Guru


Joined: 31 Jul 2015
Posts: 761

PostPosted: Thu May 02, 2019 6:16    Post subject: Re: Wifi monitoring to adjust directional antenna Reply with quote
DrownLies wrote:
Good day,

I am trying to use my WRT54GL v1 to connect to a Hotspot in my proximity. To improve the signal strength I bought a directional antenna which I need to adjust now.
In the firmware 'DD-WRT v3.0-r38159 mini (01/02/19)' I am using, there are only limited possibilities to monitor signal strengths of nearby Hotspots. Under Status -> Wireless there are the options to use 'Site Survey' and 'Wiviz Survery' which both don't really fit my needs. What I would need is a tool that could continuously list the signal strength, so I can find the best rotation and position of my antenna. I struggle to find such a tool for DD-WRT on my router and the situation is made more difficult since the WRT54GL only has 4 MB of flash, 16 MB RAM and no USB port.
The 'wl' commands seemed like they could help but when using telnet on my Windows 10 laptop to connect to my router those commands either weren't recognized or didn't provide any output.

Do you know any way to continuously monitor the signal strength of nearby W-LAN? Any help is highly appreciated!


Your best bet is to get a USB wifi adapter that allows for external antenna hookup. An appropriate pigtail to connect the directional antenna to that USB adapter. Hook that antenna up to the wifi adatper then plug it into your PC/Laptop and use whatever wifi monitoring software you prefer to aim the antenna. Once its aimed tighten down all bolts on the antenna, disconnect it from the usb wifi adapter then hook it up to your router.
jxm
DD-WRT Guru


Joined: 23 Jul 2017
Posts: 711
Location: Brisbane, Australia

PostPosted: Thu May 02, 2019 6:44    Post subject: Reply with quote
In Site Survey, the column RSSI stands for Received Signal Strength Indicator. Connect your directional antenna to your router, then move it around and the RSSI value will change if the antenna is tuned to the correct wifi frequency. The highest value you find will be the direction of the strongest signal.

Cheers.
d0ug
DD-WRT Guru


Joined: 31 Jul 2015
Posts: 761

PostPosted: Thu May 02, 2019 7:38    Post subject: Reply with quote
jxm wrote:
In Site Survey, the column RSSI stands for Received Signal Strength Indicator. Connect your directional antenna to your router, then move it around and the RSSI value will change if the antenna is tuned to the correct wifi frequency. The highest value you find will be the direction of the strongest signal.

Cheers.


These are an option but have a refresh rate that is annoyingly slow for aiming an antenna.

Keep in mind the RSSI is usually indicated by a negative number. -40 RSSI is a better signal than -80 RSSI. If you are doing a link to a distant wifi AP you also want to keep about 20db SNR between your noise floor and RSSI. In a typical environment you'll probably have a noise floor around -90 to -105, so you are going to need a signal between -70 to -85 to maintain a reliable wireless link.

On the wifi tab you will see your noise floor listed as just noise and your RSSI listed as signal. SNR is just these two numbers subtracted and the SNR is what you want to keep greater than 20. Below 20 you are going to have dropouts or just poor performance.

Another way to think of these numbers is like trying to talk to your buddy in a crowded room where 100 other people are also trying have individual conversations. The noise floor is going to be the noise of all other other conversations going on around you and the RSSI is how well you can hear you buddy trying to talk to you. If there is not enough difference between the noise floor and RSSI then you can't make out what your buddy is trying to tell you in that crowded room and you start with the "what did you say?"

Also depending on the distances involved a directional antenna is not always going to fix the problem. A directional antenna at one end with just a normal wifi router sitting at the other end is like two people trying to have a conversation across a football field with only one of them having a megaphone. One side will be able to hear, but the other side won't. Ideally there should be directional antennas on both sides.

RSSI only shows how well your end can hear the remote end. It is NOT a link quality indicator. You should also be able to login to the remote wifi AP and see what it's RSSI is for how well it can hear you. Otherwise it is just a guessing game how well things are going to work. If with your directional antenna you have an SNR of 20 but the remote end with an omnidirectional antenna only has a SNR of 5, then your link quality is going to be quite poor.

If you are going to go with just a directional antenna at one end, then you probably want as big of a parabolic antenna as you can use at your end. The larger surface area of the parabolic grid can capture and reflect more signal into the actual antenna that sticks out in the front. Like one of these https://www.altelix.com/2-4-GHz-24-dBi-Grid-Antenna-p/ag24g24.htm

This is a similar misconception about boosting the signal at a wifi access point. Boosting the signal at an AP to more than about 500mw is not going to help your signal any. Most client devices transmit at a fixed 250-500mw. It doesn't matter if you crank your AP all the way up to 1 watt. If you client device is still transmitting back at 250mw it does not fix the problem. You device can hear your AP better, but the AP cannot hear your device any better.
DrownLies
DD-WRT Novice


Joined: 29 Apr 2019
Posts: 2

PostPosted: Thu May 02, 2019 15:10    Post subject: Re: Wifi monitoring to adjust directional antenna Reply with quote
Thirst of all thank you very much d0ug and jxm for your quick responses!

d0ug wrote:

Your best bet is to get a USB wifi adapter that allows for external antenna hookup. An appropriate pigtail to connect the directional antenna to that USB adapter. Hook that antenna up to the wifi adatper then plug it into your PC/Laptop and use whatever wifi monitoring software you prefer to aim the antenna. Once its aimed tighten down all bolts on the antenna, disconnect it from the usb wifi adapter then hook it up to your router.


That is a good idea if I can't monitor nearby APs with my router. For now, I would like to keep trying and search for alternatives however.

jxm wrote:
In Site Survey, the column RSSI stands for Received Signal Strength Indicator. Connect your directional antenna to your router, then move it around and the RSSI value will change if the antenna is tuned to the correct wifi frequency. The highest value you find will be the direction of the strongest signal.

Cheers.


D0ug said it perfectly: The refresh rate is very low.

Thank you for the explanation of Wi-Fi connections d0ugh. It is very helpful. Some parts of it however I didn't quite understand.

d0ug wrote:
Also depending on the distances involved a directional antenna is not always going to fix the problem. A directional antenna at one end with just a normal wifi router sitting at the other end is like two people trying to have a conversation across a football field with only one of them having a megaphone. One side will be able to hear, but the other side won't. Ideally there should be directional antennas on both sides.


Doesn't a directional antenna improve both sending and receiving of a specific signal?

d0ugh wrote:
RSSI only shows how well your end can hear the remote end. It is NOT a link quality indicator. You should also be able to login to the remote wifi AP and see what it's RSSI is for how well it can hear you. Otherwise it is just a guessing game how well things are going to work. If with your directional antenna you have an SNR of 20 but the remote end with an omnidirectional antenna only has a SNR of 5, then your link quality is going to be quite poor.


Sadly I don't have access to the remote AP. I am trying to connect to one of Telekom's Hotspots. Telekom is my mobile phone carrier, and they allow access to the Hotspots they have in collaboration with Fon. This also means that I will be connecting to the AP through multiple walls and my antenna will be inside. I understand that this solution is far from perfect but I don't have many alternatives.

d0ugh wrote:

If you are going to go with just a directional antenna at one end, then you probably want as big of a parabolic antenna as you can use at your end. The larger surface area of the parabolic grid can capture and reflect more signal into the actual antenna that sticks out in the front. Like one of these https://www.altelix.com/2-4-GHz-24-dBi-Grid-Antenna-p/ag24g24.htm


Thank you for the link. This antenna looks interesting. Is this antenna preferable because of the higher dBi? I am not quite sure if it would be appropriate for indoor use.

I found a possible solution for my problem in the German DD-WRT wiki:
https://wiki.dd-wrt.com/wiki/index.php/DD-WRT_Doku_(DE)#Antennen_ausrichten
Here they use a script to monitor the Wlan quality continuously. They use Telnet to run the script, but as I said before I have troubles using Telnet. Do you think this script could help?

Quote:
while [ 1 ];
do CH=`wl channel|grep target|cut -c16-`;
MAC=`wl assoclist|cut -d' ' -f2`;
RA=`wl rate|cut -d' ' -f3`;
RSSI=`wl rssi|cut -d' ' -f3`;
NOISE=`wl noise|cut -d' ' -f3`;
let SNR="$RSSI- $NOISE";
echo "MAC=$MAC Kanal $CH: Rate=$RA RSSI=$RSSI Noise=$NOISE SNR=$SNR";
sleep 1;
done
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