Too Much Noise
The second cause falls into the category of RF-based interference. Though you might think of wireless LAN interference only in terms of 2.4GHz cordless phones and microwave ovens, WLAN equipment itself is becoming another growing - and perhaps dominant - category of RF "noise".
Every form of communication has to deal with two components: signal, which is the part that contains the desired information; and noise, which is everything else. Key attributes of radio receiver design are maximizing sensitivity to signal and minimizing sensitivity to noise.
As long as 802.11b / g products receive sufficient signal, the Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA) mechanism built into the protocol coordinates their communication. This access method - similar to CSMA/CD used in wired Ethernet - makes sure that only one product transmits at a time, so that the data is understood by all receivers.
But when the radio energy detected by a piece of WLAN gear can't be understood - even if that energy comes from valid WLAN equipment - it turns into noise. Wireless LAN gear does a remarkable job of differentiating between signal and noise, but not all products are created equal in this area.
If you're using 802.11b or 802.11g equipment, you probably know that your access point has eleven channels that it can be set to. You may not know, however, that only three of those channels should be used. The reason for this is illustrated in Figures 1 and 2.
Figure 1: 802.11b adjacent channel overlap
The yellow shaded area in Figure 1 represents the power from channel 2's signal that overlaps into channel 1's main lobe (the largest "hump" and also the frequency band that contains most of the signal's power). Since a significant amount of channel 2's main lobe overlaps into channel 1's main lobe (and vice versa), communication on both channels will suffer. (Note that this effect is the same for any two adjacent channels, not just Channels 1 and 2.)
Contrast this picture with the situation shown in Figure 2.
Figure 2: 802.11b "non-overlapping" channel overlap
TIP: See this section of our Atheros Super-G NeedToKnow - Part 1 for a full explanation of channel overlap
This figure has the same scale as Figure 1, but shows signals in the "non-overlapping" channels 1, 6 and 11. Since the power from each signal doesn't magically stop at the 22MHz 802.11b channel boundaries, there is still overlap between "non-overlapping" channels. But in this case, the yellow shaded area that represents channel 11's power that is overlapping into the main lobe of channel 6 is at least 30 dB lower (1/1000) than channel 11's peak power. For most well-designed radios, this 30dB difference between signal and "noise" is sufficient to ensure good rejection of the adjacent channels' signals (i.e. noise).
Use of overlapping channels isn't the only source of WLAN RF interference. As I'll explain later (What Doesn't Help), some of the techniques that your might be using to "ignore" other WLANs actually can hurt rather than help your own WLAN's performance!
The take-away from all this is that not only do you have to deal with possible problems from microwave ovens and 2.4GHz phones, but neighboring WLANs themselves can also become interference sources.
Tip: Wireless networking management company Cirond argues that there are actually four channels (1, 4, 8, 11) that can be used for 802.11b and g with virtually no performance penalty. I recommend you stick with using Channels 1, 6, and 11 since they are more likely to be used by neighboring WLANs.