d-link dwl-2200ap unstable snr

I am using a D-Link DWL-2200AP access point connected to a 250mW amplifier and a 15db omni antenna. Sitting in a building 25 feet away, netstumbler is getting a SNR of 45, but it drops to 13 every other second. Even when I remove the amplifier and antenna and leave it with its standard 5db omni, I'm still getting rather large jumps (10-20db) in my SNR occurring every few seconds. Is this a hardware problem or an interference problem? My access point is set to channel 11 and netstumbler does not pick up any other access points on or near that channel. Thanks in advance for any help.

David

Usually a sign of interference. Note that not just other access points can cause this; other sources include cordless phones, microwave ovens, digital video transmitters, and even your own AP signal reflecting off another surface.

Bad antenna cabling or poor antenna design can also often cause this, and in this case using an amp will often make it worse.

What does the throughput look like? Unless you start getting dropped data it proably won't hurt too much. I'd recommend ditching the amp unless you really need it though.

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Jeremy

Marlborough WiFi Project

Thanks for the response!

Why does the amp make it worse? This is a campus setup. We have the access point + antenna and amplifier in a central location. The amplifier provides the needed boost to get the signal out to faculty residences on campus.

Here's another data point:

Sitting in a close room in a building near the edge of signal coverage I get a stable 30-35 SNR, but if I move one room back I get an unstable signal jumping between 0-25 SNR (and rarely in between). I still get a working connection in the back room, but I'm definately loosing packets frequently (ping -t gives 2ms for about 5-15 times, then drops a packet).

The fact that I can get a stable 30-35 SNR in one room makes me agree that it is likely interference rather than a cabling issue. Additionally, as I said, even if I remove the antenna/amplifier, I get weird SNR jumps in the same locations.

The idea that reflections of the original signal is causing interference makes some sense, but I wouldn't know how to track that down and minimize it.

What is the best way to pinpoint items causing interference? A spectrum analyzer? Hiring a wireless company to come out?

Thanks.

If the antenna or cabling is dodgy it can cause some of the RF transmitted by the AP to be reflected back into it and therefore causes interference. Using an amp means higher initial output and therefore stronger reflections, resulting in more degration of signal quality.

In your case as the amp serves a good purpose I then I'd suggest keeping it. I just brought that up in my previous post as many people I have come accross who use them actually don't need to and create more problems for themselves in the process.

Back to the main issue, I agree with your points about obstruction or reflection being the probable cause. Note that materials such as concrete very quickly degrade performance if they are in the line of sight back to the AP. Reflections are a bit more difficult to detect and overcome.

A spectrum analyser is a good tool for helping track down sources of external interference and also how well you are receiving the access point itself. If you hire a wireless company make sure they have techs with some RF knowledge and gear, some companies simply send out a monkey with a laptop and NetStumbler.

If you think that the signal is being blocked by the actual building materials it may be worth investing in an extra access point to function as a repeater.

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Jeremy

Marlborough WiFi Project

Thanks for the info!