Spectrum analysis may be compared and contrasted with site survey measurements made using utilities and tools based solely on 802.11 measurements.
A spectrum analyzer is a specialized piece of equipment that measures RF signal energy across a range of frequencies. Spectrum analysis can be used in harmony with 802.11 channel analysis to provide a complete picture of the radio transmission and reception characteristics of a particular environment.
To best understand how spectrum analysis measurements differ from 802.11 channel measurements it’s beneficial to first understand exactly what’s meant by “802.11 channel measurement.” Site survey and monitoring software that presents information obtained from an 802.11 chipset can (..and this should be self-evident.). only manipulate information obtained from the 802.11 chipset. Hence, tools like AirMagnet, AiroPeek and NetStumbler, along with vendor utilities that present signal strength and/or Signal-to-Noise ratios like the Cisco AiroNet utility (and other similar utilities) are limited to the 802.11 channel space. That is, they are only capable of measuring RF conditions that present themselves in such a way that the 802.11 radio receiver and chipset can discern them. These utilities can report the presence of other 802.11 devices and can measure the strength of a received 802.11 signal, but they can NOT measure background RF energy in the environment.
The fact that 802.11-based tools are incapable of directly measuring background RF energy means that they are incapable of reporting the existence of other (non-802.11) devices that may be operating in the same band as the 802.11 network. This other background energy may come from cordless phones, microwave ovens, and (perhaps most significantly) from the channel “bleed over” that is present in all 802.11 networks.
In the 802.11 realm one often talks about “Channel 1, 6 and 11” as being “non-overlapping.” This is not a completely accurate statement. In fact, the specifications for an 802.11 network define only a center frequency for each channel assignment and then specify the bandwidth (above and below that center frequency) where the signal must drop by 3 dB, and then again by another 3 dB, and so forth. What’s defined is technically known as a “spectral mask” and the implication is that any 802.11 transmitter can generate energy in any part of the 802.11 band. The real-world effect is that when a sufficient number of 802.11 transmitters are present in an environment the transmission band develops a baseline of background energy that can spread across the entire band. The result of this background energy is that an 802.11 receiver can become “confused” as to what is the intended signal and what is the unintended background signal. Technically this is referred to as a condition where “coherent background signals” cause a receiver to “lose lock”. The result is that data is lost, retransmissions cause effective data rates to drop, and users can lose connections.
802.11-based tools are incapable of detecting background energy and the aggregate effect of multiple, overlapping spectral masks. Only a spectrum analyzer can do this.