(The following excerpt has been copied from: http://www.wireless-nets.com/resources/tutorials/minimize_802.11_interference_issues.html )
By Jim Geier, Independent Consultant, Wireless-Nets, Ltd.
Radio frequency (RF) interference can lead to disastrous problems on wireless LAN deployments. Many companies have gotten by without any troubles, but some have installations that don’t operate nearly as well as planned. The perils of interfering signals from external RF sources are often the culprit. As a result, it’s important that you’re fully aware of RF interference impacts and avoidance techniques.
Impacts of RF interference
As a basis for understanding the problems associated with RF interference in wireless LANs, let’s quickly review how 802.11 stations (client radios and access points) access the wireless (air) medium. Each 802.11 station only transmits packets when there is no other station transmitting. If another station happens to be sending a packet, the other stations will wait until the medium is free. The actual 802.11 medium access protocol is somewhat more complex, but this gives you enough of a starting basis.
RF interference involves the presence of unwanted, interfering RF signals that disrupt normal wireless operations. Because of the 802.11 medium access protocol, an interfering RF signal of sufficient amplitude and frequency can appear as a bogus 802.11 station transmitting a packet. This causes legitimate 802.11 stations to wait for indefinite periods of time before attempting to access the medium until the interfering signal goes away.
To make matters worse, RF interference doesn’t abide by the 802.11 protocols, so the interfering signal may start abruptly while a legitimate 802.11 station is in the process of transmitting a packet. If this occurs, the destination station will receive the packet with errors and not reply to the source station with an acknowledgement. In return, the source station will attempt retransmitting the packet, adding overhead on the network.
Of course this all leads to network latency and unhappy users. In some causes, 802.11 protocols will attempt to continue operation in the presence of RF interference by automatically switching to a lower data rate, which also slows the use of wireless applications. The worst case, which is fairly uncommon, is that the 802.11 stations will hold off until the interfering signal goes completely away, which could be minutes, hours, or days.
Sources of RF interference
With 2.4 GHz wireless LANs, there are several sources of interfering signals, including microwave ovens, cordless phones, Bluetooth enabled devices, FHSS wireless LANs, and neighboring wireless LANs. The most damaging of these are 2.4 GHz cordless phones that people use extensively in homes and businesses. If one of these phones is in use within the same room as a 2.4GHz (802.11b or 802.11g) wireless LAN, then expect poor wireless LAN performance when the phones are in operation.
Microwave operating within 10 feet or so of an access point may also cause 802.11b/g performance to drop. Of course the oven must be operating for the interference to occur, which may not happen very often depending on the usage of the oven. Bluetooth enabled devices, such as laptops and PDAs, will cause performance degradations if operating in close proximately to 802.11 stations, especially if the 802.11 station is relatively far (i.e., low signal levels) from the station that it’s communicating with. The presence of FHSS wireless LANs is rare, but when they’re present, expect serious interference to occur. Other wireless LANs, such as one that your neighbor may be operating, can cause interference unless you coordinate the selection of 802.11b/g channels.