WLAN Design
Interference

802.11 WLANs are unlicensed users of the spectrum, which allows customers to manage their own products and implementations. As unlicensed users they must accept interference from other unlicensed users and are not allowed to cause interference to licensed users. This makes WLANs scalable as well as easy to implement and administer. The downside of using an unlicensed spectrum is that other devices can also use the same frequencies and, therefore, impact each other. Other devices using 2.4 GHz or 5 GHz, such as cordless phones, can have an impact regardless of the SSID and WEP implementations. While SSID and WEP provide security for the WLAN data, the RF signal itself is still subject to interference. This is because it is a Layer 1 transmission. Interference can be avoided in most instances by selecting products that operate outside these ranges.

The impact is only going to happen if the third party devices use more than a minimal amount of RF. If a person were to just turn on another 2.4 GHz device, not much will happen and no real impact will occur. But if that third party device starts to use the 2.4 GHz Spectrum, then both systems will suffer performance degradation. This stems from the fact that WLAN products are based on carrier sense, multiple access with collision avoidance (CSMA/CA). Before a transmission is sent, the transmitter checks the airwaves to see if the channel is available for use. If a third party is using the spectrum, then the airwaves will not be available. The device will hold off until the RF becomes available. In a wired Ethernet network, this would be the same as running a constant broadcast frame over the wire, and it will have the same effect.