Wireless network problem

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Chris Rennert 21 years ago

Hello all,

I am having an issue with a warehouse management implementation. Here is the scenario. We have 4 Symbol ap4131 access points in our warehouse, and 18 PDT6800 handheld scanners. We have 2 of the AP's set at channel 1, and the other 2 at channel 11. This was recommended by Symbol to try and eliminate interference between the AP's. I also have

2 linksys 802.11G aps in the warehouse both at channel 8 , with hawking Hi-gain antennas.

The issue we are having is the PDT6800's are constantly dropping (randomly throughout the day), and unable to reconnect for just about exactly the same amount of time every single time it happens (just about

10 minutes). We have changed the type of handheld radios we used, as well as turned off all the other access points (besides the symbols.). I had asked about the 3Com card earlier this week in this group in hopes of someone helping me find a wireless nic for a laptop that can use promiscuous mode. I was also hoping someone might be familiar with a situation such as this, and make some recommendations as to how I would attack it. Truthfully I am at a loss at the moment. I did happen to have my laptop connected to one AP when the PDT6800 lost connection, but there was nothing there to lead me to believe that this would happen, and really I have no idea which direction to go from here.

It happens mostly to the AP's and PDT6800's in the back of the warehouse, where I have 2 aps within about 50' of each other, yet on different channels.

I apologize for being so vague, if there are any specific questions that will help you guys diagnose this situation please do not hesitate to ask, and I will do my best to relay the information to you.

thanks again,

Chris

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Jeff Liebermann 21 years ago

The non-overlapping channels are 1, 6, and 11. The radios on channel

8 and 11 will somewhat interefere with each other. I would move the Linksys access points from 8 to 6.

Look for microwave oven, 2.4GHz cordless phones, wireless security cameras, 2.4Ghz wireless mouse, Bluetooth phone base near the access points, microwave plastic pre-heater, 2.4Ghz sulphur lamp, Zigbee networks, municipal wireless, and other 2.4GHz wireless networks. There have also been reports of wireless doorbells, radio remote controls, and high power commerical VHF/UHF radios and paging xmitters causing problems.

Changed from what to what?

If the intereference is coming from a non-802.11 source, Netstumbler and the various passive sniffers will not detect anything. You might try using Kismet on Linux to do the sniffing as it will support most NDIS5 compatible cards. The easiest way is to use a "Live CD" such as:

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's a huge download but worth it if you're planning to need 802.11 hacking and sniffing tools. However, my guess(tm) is that it's not

802.11 related. The consistant 10 minutes sure sounds like a production microwave oven. You'll need a spectrum analyzer and someone experienced to interpret the display to find the source.

What does "this would happen" mean? Did the laptop stay connected or not when the PDT6800 lost the connection?

Is the back wall of the warehouse tranparent to RF? Or is the warehouse made of steel and/or contrete? (Is it coming from inside or outside?)

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Jeff Liebermann 21 years ago

Probably interference of some type. What's strange is the exactly 10 minute downtime. That doesn't sound like a cafeteria microwave oven. More like an industrial plastic molding shop with a microwave pre-heater. Microwave ovens are also used as dryers to remove the water from raw materials before processing. Look around for a nearby industrial plant that might use something like this (especially near the back wall).

That means it's probably fairly well shielded against outside influences. However, there are always windows to let in and out the RF. Any big windows at the back wall?

There are cheapo spectrum analyzers that might work. One is based on an old Proxim RangeLan2 7400 PCMCIA card. |

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This one is not as nice, but should work: |

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Here's one based on an ancient Teletronics PCMCIA card: |

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The problem is that these cheapo spectrum analyzers are not very sensitive or sweep very fast. Basically, they're difficult to use unless you exactly what you're doing. Still, it's better than spending big dollars on renting an Agilent spectrum analyzer.

There are also relatively low cost Wi-Fi spectrum analyzers from: |

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videos at the bottom of: |

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worth watching to see what it's like.

Groan. I was afraid that would happen.

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Chris Rennert 21 years ago

Jeff Thank you so much for all the great informatin, I apologize for not being complete. When the PDT6800's lost connection, so did my laptop.

You mentioned microwave ovens. As far as I know the closest one to our AP's that consistently drop are about 300+ feet away, and behind 3 separate walls. I will check and see if any of the employees possibly have a microwaves near one of their work benches.

The building a steel building.

Thank you again, I will definitely go to that link, and grab the software you have recommended, as well as look for someone in the area that can do spectrum analysis.

Thanks again, and I will keep you up to date!

Chris

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