RF Spectrum Analysis
With wireless systems it is very difficult to predict the propagation of radio waves and detect the presence of interfering signals without the use of test equipment. Radio waves don’t travel the same distance in all directions — instead walls, doors, elevator shafts, people, and other obstacles offer varying degrees of attenuation, which cause the Radio Frequency (RF) radiation pattern to be irregular and unpredictable. In order to achieve optimal reliability and throughput for a Wi-Fi (802.11) wireless network it is necessary to detect and identify sources of interference that affect its performance.
There are many electronic devices that transmit RF energy into the airspace. WiFi operates in the 2.4x/5.x GHz Industrial, Scientific, and Medical (ISM) bands. These particular frequency ranges are public and their use does not require licensing by the FCC. As a result, these bands — especially the 2.4x GHz band — tend to get rather crowded. Within the ISM bands WiFi networks compete for the medium not only with 802.11 devices but also non-802.11 devices, such as microwave ovens, bluetooth devices, cordless phones, baby monitors, audio/video senders, wireless cameras, etc.
|In this figure the horizontal white lines show the boundaries of the 13, 22 MHz channels in the 2.4 GHz band (1-11 in North America, 1-13 in Europe). Notice how adjacent channels overlap with one another.|
Consider the 2.4x GHz ISM band — professional installers of wireless computer networks attempt to optimize their client’s Wi-Fi network by strategically choosing a subset of the 2.4x GHz band for use by their wireless network devices. In North America the 2.4x GHz range of 2.401 – 2.473 GHz is separated into 11, 22 MHz channels. Note that 2.473 GHz minus 2.401 GHz equals 72 MHz — yet 11 times 22 MHz is 242 MHz. So, how does 242 MHz fit into 72 MHz? The answer is the channels overlap. An installer can configure the wireless router or access point (AP) to use channel 6, and then all data communication will occur over the range of frequencies associated with channel 6 (i.e. 2.426 GHz – 2.448 GHz). But if other wireless devices (802.11 or non-802.11) are also transmitting over this range of frequencies then the wireless network will be affected. So, when installing a wireless network or troubleshooting a poorly-performing one, it is important to choose a channel that is not subject to interference from other devices. That is, you don’t want the wireless network to compete with other devices for the same range of frequencies.
RF spectrum analyzers are widely used within the wireless communication industry for analyzing the frequency spectrum of radio frequency signals and transmissions. They are the only instrument that allow one to “see” all RF emissions within a predefined range of frequencies. They are an invaluable tool when it comes to designing, developing and testing wireless communication systems. Turns out they have another very useful application — and that is detecting RF interference. To be clear, interference is a relative term. Your wireless security system does not consider its radio signal transmissions to be “interference” since these emissions are required for the system to operate. But from the perspective of a WiFi wireless network, since those emissions affect wireless transmissions within the WiFi network, rendering the WiFi network inoperable, then they are viewed as RF interference. An RF spectrum analyzer is the tool of choice for detecting all RF transmissions — both desirable and undesirable, and in the context of troubleshooting WiFi networks it is used to detect transmissions from non-802.11 devices that could affect the performance of the wireless network.
The output from an RF spectrum analyzer is referred to as a spectrum trace. Typically, this is a 2-dimensional display where the vertical axis (Y-axis) is the magnitude of a signal and the horizontal axis (X-axis) its frequency. If the spectrum trace reveals the presence of RF emission peaks from other wireless devices then you have two choices. Either you can attempt to hunt-down and eliminate the source of interference or configure the WiFi access point to use a channel that is not affected by the interferer.
Below are shown 4 examples of measurements taken with the AirSleuth RF spectrum analyzer. In each figure there are 3 items to take note of:
a. The maximum value on the Y-axis
b. The horizontal white lines that denote the boundaries of the 11, overlapping 802.11 channels
c. The location (i.e. frequency) and height (relative signal strength) of the major RF peaks
|The air space is relatively quiet and there are no wireless devices transmitting in the 2.4x GHz band. Notice the maximum value on the Y-axis has a signal strength of -90 dBm, which is very weak. The reason you see peaks (and the output isn’t totally flat) is this display is using automatic scaling, such that the largest peak (regardless of how small it really is) will fill-up the screen. We know this is what background noise looks like because the maximum value on the Y-axis is -90 dBm.|
|A microwave oven is running. Here you can see that it emits RF energy across the entire 2.4x GHz band.|
|A 2.4 GHz cordless phone has been powered on. Notice the height of the peak in the vicinity of channel 1 is around -60 dBm. This is strong enough to knock-out wireless networks configured to use WiFi channel 1. In addition, because of the way that channels overlap the first peak would probably also interfere with a wireless network using channel 2.|
|Shows the pattern of activity from a wireless network (configured to use WiFi channel 6) as it is actively transmitting a large stream of data.|