Multipath Interference

Unanswered Question
Aug 29th, 2007

I've got an interesting one here...

I?ve got a customer whom I suspect is experiencing severe multipath interference and/or some type of EMI interference from large machinery. They are a metal machine shop with large drills, saws, and planers with overhead cranes moving back and forth to transport the metal from one location in the plant to the other. Oh and did I mention the walls were all sheet metal too? No plasma cutters or microwave type heating machines luckily...

They are running 1242 IOS access points using 12.3.8 code, using primarily the AIR-ANT3213 pillar mount antennas. They do have one area with two 1728 antennas mounted correctly. The clients are desktop PC?s using a mix of Linksys WRT54G card/antennas (not diversity) and Cisco PI21ABG cards/antennas. A passive survey was done previously to determine coverage and placement and coverage seems to be fine. In the logs I see a lot of clients dis-associating due to Max Retries Limits reached. The radio stats indicate very high numbers of CRC errors and high retries (hundreds every 5 minutes). Voice is not in use here. Ping tests show a lost packet every 20 or so tries. They are running an ERP application where if it loses enough data over time it shuts down. Email, web and simple file sharing seem to be fine.

I?ve run Air Magnet Laptop Analyzer and found higher signal to noise ratios than I would like (-50 to -60). The only thing it seems to notice is the high CRC errors and retries. The signal levels are constantly bouncing from -50?s to -60?s and are never steady. The signal distribution levels are bouncing all over as well, which from a quick look at Air Magnet?s knowledge base indicated possible RF or multipath interference. There is a high level of management traffic at the 1mbps data rate, but I suspect that is from either the script they are running to constantly ping the ERP server, or the frequent re-associations. I do not see any co-channel interference.

So far to combat this I?ve had the customer move the pillar mount antennas off of the metal I-Beams which Cisco recommends. I?ve experimented with power settings and found that I could turn it WAY down (was at 25mW) to 1mW which actually still provides plenty of coverage shockingly. Turned on Long Preamble and increased the Max Retries thresholds. Once I did this, there was some decrease in the CRC errors and retries, but I?m still not comfortable with it.

Does anyone else have any ideas to combat this? I?m considering trying a lower gain antenna, but reaching the antennas is a bit difficult without a lift. I?m even considering trying the 5ghz band as I believe it is a little less susceptible to interference. Not sure if the customer is willing to swallow that pill though.

Thanks in advance?

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Overall Rating: 4 (2 ratings)
scottmac Wed, 08/29/2007 - 14:23

Try cutting your RTS time down (~1/2) and check the effect.

Shorter segments = less to interfere with and less to re-transmit.

For noisy environments, it usually helps. LIke many other things, the correct setting, and how much it'll help will depend a lot on your application and the rest of the system setup.

Good Luck


dennischolmes Thu, 09/06/2007 - 16:24


Get away from the omnis man. Look to placing diversity patch antennae around the perimeters of the building. Only use the omnis where needed to fill coverage holes. What was your actual noise floor? I'm betting it was pretty high. The directional patches will help with that as well as help control RF propagation. Make sure to angle the patches down at least 10 to 15 degrees as this will more equally disperse the electrons. Make sure to try this in a small area first to validate the fix. You could also be having an asynchronous power issue between the clients and APs. Remember, clients are very low powered devices and can easily be overpowered by loud APs. This is especially true when using omnis as the noise floor rises significantly.

s.kerekes Thu, 09/06/2007 - 19:42

Did you end up resolving this? I am seeing similar issues in a warehouse type setting. APs are 1130s and are mounted to the metal ceiling beams (not my design). This setup includes 7920s connecting to a local CME box. The voice is clipping periodically 1- 5 seconds. Channel overlap does not seem to be an issue although we have identified a device (Symbol Scan Gun) in the adjacent building that, when scanning, causes the clipping as well. Any suggestions????

tdilley4938 Sun, 09/09/2007 - 23:14

How high are your antennas if you need a lift? Antenna type and placement?Proper antenna configuration is a critical factor in maximizing radio range. As a general rule, range increases in proportion to antenna height. However, do not place the antenna higher than necessary because the extra height also increases potential interference from other unlicensed radio systems. Turning your power down is also a step in the right direction.

jake.kappus Mon, 09/10/2007 - 06:42

Thanks for the responses everyone.

Here's what went down...

The customer wasn't interested in changing antennas. What I did was turn the power as far down as I could (1mW). Lucklilly the survey was excellent and coverage was not an issue after turning the power down that low. Then I also limited the client power to 2mW as much as I could. They had a mix of Cisco and Linksys client adapters so I'm not sure how many were actually limited.

I also changed the fragmentation threshold down to 1024 and the RTS down to 1024. This was a recommendation from the folks over at the CWNP boards. I could have gone lower I suppose, but I didn't want to push my luck there. I also maxed the Max Retries & RTS Retries thresholds for good measure.

After all of this, we monitored the situation for about a day. I ran random continuous pings from random machines for well over an hour and not a single ping dropped. CRC errors were still high, but retries were down. I'm guessing the CRC's are just something we're going to have to live with in this high multipath environment.

All in all, the wireless network was much more stable after these changes.

Again, thanks for the replies.


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