Washington Post article on 802.11 interference

by Matthew Gast

Related link: http://www.washingtonpost.com/wp-dyn/articles/A38700-2004Apr24.html

I was interviewed last week for this article, which is part of a bundle of articles on 802.11. Most of what I talked about found its way into print, but there were a few items I mentioned that didn't make the final cut that I'll list here. As long as I'm at it, I might as well toss in a few points that I didn't mention because they were beyond the scope of the article:

  • Major trade shows that have Internet connections dropped on to the floor will have problems with channel overlap. CeBIT is hardly unique--I've seen the same problems at Networld+Interop in Las Vegas. The best we can do is keep the power down to avoid overlaps between the APs, and be friendly by limiting transmit power to cover only the area you want.

  • There is no way for an 802.11 network to control client power settings. There is some standards work that would enable APs to send messages to clients to adjust power settings, but it's not done yet. If the client computers are set for maximum power, their range will be much greater than the low-power APs, and a single client may potentially interfere with several APs. (I did not mention this because it was beyond the scope of the story.)

  • I'm pleased to see the microwave oven mentioned only in passing. By definition, microwave ovens should be shielded pretty well because they need to keep the energy in the oven to cook the food. Three years ago, Rob Flickenger ran some tests, and found the microwave didn't really affect throughput. If your microwave is affecting throughput, you should replace it because it might pose a danger.

  • In suburban residential areas, homes are probably far enough apart that automatic channel assignment technologies can help out. Dynamic channel assignment is one of the new features of new high-end 802.11 equipment targeted at the corporate market. It may be only a matter of time before the capabilities filter down to lower-end devices.

  • We talked a lot about cordless phones. My general rule is that expensive cordless phones are better for non-interference with 802.11. Part of the reason is that the cheaper phones assume that they own the entire ISM band and modulate accordingly, using the entire 80 MHz of spectrum. Higher-end cordless phones use frequency-hopping technology, which is somewhat kinder to other users of the band. Anecdotally, the Siemens Gigaset doesn't seem to interfere with 802.11b networks. (The Gigaset is a multi-handset system, and they probably need to use fancier technology so the handsets don't interfere with each other.) However, some of the people I know with the Gigaset aren't particularly fond of it for other reasons.

  • Naturally, the easiest way to avoid having your cordless phone interfere with your 802.11 is to make your cordless phone an 802.11 station. If you subscribe to a VoIP service that uses SIP, you have several choices. I haven't yet subscribed to a VoIP service because I need to have a land line to support my DSL service, and long distance is very cheap these days. Between free night and weekend calls on my mobile phone and a cheap long distance provider, I just don't need to subscribe to an additional telephony service. What I'd like is to get an 802.11 cordless phone. I don't really care whether the phone is done as a SIP gateway appliance that I can plug into my phone, or an VoIP-to-phone line bridge I can plug into my home Ethernet.

  • One small correction: I think the high-tech neighborhood I referred to in the article was North Carolina, not Florida. For a previous Associated Press story, I referred the reporter to fellow O'Reilly author Chuck Musciano. The AP story refers to his neighborhood in North Carolina.

One other amusing note about the story. I built the network at my parents' house a bit more than a year ago. It's much simpler than my home network, so I find it amusing that their network was mentioned in the press before mine!

Do you have a cordless phone in the same frequency band as your 802.11 network? If so, what has your experience with interference been?