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.