Q: What are the five top interferers in the Wi-Fi spectrum? - Rodney, San Francisco

The Wizards gaze deeply into their crystal ball and respond:

Luc Roy, Chantry Networks

The top five interferers of the Wi-Fi spectrum are:

1. Yourself; most interference is caused by us, either by the access point or the WLAN client NIC.  Some materials are prone to reflection, causing the RF signal to bounce off materials and returning back to us, creating a multipath phase cancellation of the original signal.  Reflective materials include metal, lead-based curtains and glass with heavy lead content.

2. Other WLAN networks; this is especially the case when sharing the office space or building with others.

3. 2.4 GHz wireless phone; this is especially the case at home.  Personally, I’ve kept my 900 MHz phone so I get the cleanest of air for my WLAN network.

4. Bluetooth; many people don’t realize that Bluetooth runs at 2.4 GHz.  Although not a long range signal, when close to an access point, such as in a hallway or in a conference room, beware of significant degradation of access point performance.

5. Microwaves and fluorescent lights.

Scott Haugdahl, WildPackets

It’s tough to come up with five common non-malicious, non-WiFi RF interferers that cause serious problems, so how about a shot at the top three?  Disclaimer:  There are many environmental factors that can change the order of this list, and I have experienced different results on different 802.11 channels. In order of “worst” to “not-so-bad” (in the 2.4 GHz 802.11 b/g band, not the 5 GHz 802.11a band):
 
1. Microwave ovens.  Microwave ovens do not “frequency hop” so keep them at least 10 to 20 feet away from any Wi-Fi devices.
2. 2.4 GHz cordless phones.
3. Bluetooth, especially the stronger Class I 100 meter devices.

Roger Durand, Propagate Networks

Microwave ovens are No. 1.  They provide a pulse form of inference and typically hammer the middle of the Wi-Fi 2.4 GHz band.  When they send their pulses, the majority of consumer ovens occupy 802.11 b/g channels 5, 6 and 7, but they may start their pulse on channels 1 or 11. 

The second biggest interference comes from older continuous wave frequency modulated cordless phones.  These are not the more recent frequency hopping variety.  These continuous wave cordless phones typically occupy channels 0 through 2.