Improving SNR levels

Unanswered Question
Mar 14th, 2010

Hi all,

I have the following setup:

- point to point link over 2km

- 2 of the BR1310G-A-K9-R

- 2 Yagi AIR-ANT1949

I tried to allign the antennat few times but I still get pretty low values of the signal and SNR. Any ideas how can I improve the SNR ratio? I attach some screenshots with the values. I already checked the channels congestion and it seems like CH 1 is the best choice. How about power setting sof the antennas? Shall I leave the values as default?

Thanks in advance.

I have this problem too.
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Scott Fella Mon, 03/15/2010 - 19:07

One thing is when you align the antennas, you need to adjust both sides together while looking at the signal.  Take a look at your connectors... are they properly sealed and tight?  All these things can make a difference... Your rssi looks good, but it also looks like you are seeing more noise and that is why your modulation is dropping.  Take a look at this link if you haven't regarding alignment.

http://www.cisco.com/en/US/docs/wireless/access_point/1300/installation/guide/130h_c4.html#wp1047924

remi-reszka Mon, 03/15/2010 - 20:53

Hi,

Thanks a lot for your input. I will be at client's site tomorrow so when all checked I will give feedback.

Scott Fella Tue, 03/16/2010 - 05:41

Make sure there is coax seal on the connectors... even though it is a new install, you don't want to have the client call you in a year that the link is down and you find that the cable needs to be replaced.

remi-reszka Wed, 03/17/2010 - 07:39

Hi Scott and thanks again for your help. I have only 2 bridges in point2point configuration so guess here no need for antennas polarization change, actually I was going to do that today but I'll leave it then. We checked the connectors, realligned the antennas and now get around -61dB signal strength and SNR jumps like a crazy from 20dB to 12dB, what could be causing that? We installed the bridges and antennas very close to huge telecoms antennas, do you think that could be causing the low SNR values?

On the other hand, we have around 200 PCs connecting a server on the other side. I deployed WPA-TKIP encryption so we did some tests. WIth around 20 stations connected we have a very good and stable link. Those 20 PCs generate about 200Kbps traffic. Now, when we connect around 200 PCs that generate around 2.5Mbps the link saturates very quickly and responses to the server over the link come to 300-500ms and worse, as opose with the 20 PCs, the responses are about 2-3ms.

Another observation, when we disable the encryption and connect all the 200 PCs, the link seems to be stable and responses from the server are mainteined at around 30ms, it jums occasionaly to 60-70ms but accasionally.

Do you think low values of SNR could be causing this? What can we do in this situation? We cannot leave the client without any security on the wireless link.

Please respond today if you can, I am at the client's site already so I could use your expertise.

Thanks in advance,

Remi

Scott Fella Wed, 03/17/2010 - 08:03

Low SNR reading will always have an affect on your modulation.  Modulation will determine what is the maximum speed with less errors.  Can the telcom tower be causing your noise level to be high... it sure can.  Also if there is any 2.4ghz bridging that crosses your path, that can cause your noise level to be high.  What you can do is find the best channel you can get and maybe rotate the antenna polarization if you have other 2.4ghz in the area... this migh help. Your RSSI is pretty good, so its just the noise that is causing your link speed to drop.

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