Stop the interference!

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. 

The third biggest interferers are the frequency hopping spread spectrum cordless phones.  These phones jump all over the 2.4 GHz spectrum. They don’t even listen before they talk, and typically cause a large number of Wi-Fi data packets to get trashed.DSSS) and Orthogonal Frequency Division Multiplexing (OFDM) cordless phones.  These phones are in both 2.4 GHz and 5 GHz spectrums, and are a little nicer because both 802.11b/g and 802.11a access points can recognize the traffic and treat it somewhat like another Wi-Fi device.  The catch?  When these phones are handling a call, you can lose half of your bandwidth for the duration of the call.  If you have multiple handsets on a single base station, though, the second remote handset will permanently cut your Wi-Fi bandwidth in half.  

Fourth on the list are Direct Sequence Spread Spectrum (

The fifth interferer is typically Bluetooth.  It is minor if you only have one device, but it can become a nuisance if you have several devices all over the house.