Narrowband
Narrowband RF is basically the opposite of spread spectrum technology. Narrowband signals, depending on output power, frequency width in the spectrum, and consistency, can intermittently interrupt or even disrupt the RF signals emitted from a spread spectrum device such as an access point. However, as its name suggests, narrowband signals do not disrupt RF signals across the entire RF band. Thus, if the narrowband signal is primarily disrupting the RF signals in channel 3, then you could, for example, use Channel 11, where you may not experience any interference at all. It is also likely that only a small portion of any given channel might be disrupted by narrowband interference. Typically, only a single carrier frequency (a 1 MHz increment in an 802.11b 22 MHz channel) would be disrupted due to narrowband interference. Given this type of interference, spread spectrum technologies will usually work around this problem without any additional administration or configuration.
To identify narrowband interference, you will need a spectrum analyzer, shown above in Figure 9.12. Spectrum analyzers are used to locate and measure narrowband RF signals, among other things. There are even handheld, digital spectrum analyzers available that cost approximately $3,000. That may seem like quite a bit of money to locate a narrowband interference source, but if that source is disabling your network, it might be well worth it.
As an alternative, some wireless LAN vendors have implemented a software spectrum analyzer into their client driver software. This software uses a FHSS PCMCIA card to scan the useable portion of the 2.4 GHz ISM band for RF signals. The software graphically displays all RF signals between 2.400 GHz and 2.4835 GHz, which gives the administrator a way of "seeing" the RF that is present in a given area. An example of the visual aid provided by such a spectrum analyzer is shown in Figure 9.13.
In order to remedy a narrowband RF interference problem, you must first find where the interference originates by using the spectrum analyzer. As you walk closer to the source of the RF signal, the RF signal on the display of your spectrum analyzer grows in amplitude (size). When the RF signal peaks on the screen, you have located its source. At this point, you can remove the source, shield it, or use your knowledge as a wireless network administrator to configure your wireless LAN to efficiently deal with the narrowband interference. Of course, there are several options within this last category, such as changing channels, changing spread spectrum technologies (DSSS to FHSS or 802.11b to 802.11a), and others that we will discuss in later sections.
All-band Interference
All-band interference is any signal that interferes with the RF band from one end of the radio spectrum to the other. All-band interference doesn't refer to interference only across the 2.4 GHz ISM band, but rather is the term used in any case where interference covers the entire range you're trying to use, regardless of frequency. Technologies like Bluetooth (which hops across the entire 2.4 GHz ISM band many times per second) can, and usually do, significantly interfere with 802.11 RF signals. Bluetooth is considered all-band interference for an 802.11 wireless network. In Figure 9.14 a sample screen shot of a spectrum analyzer recording all-band interference is shown.
A possible source of all-band interference that can be found in homes and offices is a microwave oven. Older, high-power microwave ovens can leak as much as one watt of power into the RF spectrum. One watt is not much leakage for a 1000-watt microwave oven, but considering the fact that one watt is many times as much power as is emitted from a typical access point, you can see what a significant impact it might have. It is not a given that a microwave oven will emit power across the entire 2.4 GHz band, but it is possible, depending on the type and condition of the microwave oven. A spectrum analyzer can detect this kind of problem.
When all-band interference is present, the best solution is to change to a different technology, such as moving from 802.11b (which uses the 2.4 GHz ISM band) to 802.11a (which uses the 5 GHz UNII bands). If changing technologies is not feasible due to cost or implementation problems, the next best solution is to find the source of the all-band interference and remove it from service, if possible. Finding the source of all-band interference is more difficult than finding the source of narrowband interference because you're not watching a single signal on the spectrum analyzer. Instead, you are looking at a range of signals, all with varying amplitudes. You will most likely need a highly directional antenna in order to locate the all-band interference source.
Weather
Severely adverse weather conditions can affect the performance of a wireless LAN. In general, common weather occurrences like rain, hail, snow, or fog do not have an adverse affect on wireless LANs. However, extreme occurrences of wind, fog, and perhaps smog can cause degradation or even downtime of your wireless LAN. A radome can be used to protect an antenna from the elements. If used, radomes must have a drain hole for condensation drainage. Yagi antennas without radomes are vulnerable to rain, as the raindrops will accumulate on the elements and detune the performance. The droplets actually make each element look longer than it really is. Ice accumulation on exposed elements can cause the same detuning effect as rain; however, it stays around longer. Radomes may also protect an antenna from falling objects such as ice falling from an overhead tree.
2.4 GHz signals may be attenuated by up to 0.05 dB/km (0.08 dB/mile) by torrential rain (4 inches/hr). Thick fog produces up to 0.02 dB/km (0.03 dB/mile) attenuation. At 5.8 GHz, torrential rain may produce up to 0.5 dB/km (0.8 dB/mile) attenuation, and thick fog up to 0.07 dB/km (0.11 dB/mile). Even though rain itself does not cause major propagation problems, rain will collect on the leaves of trees and will produce attenuation until it evaporates.
Wind
Wind does not affect radio waves or an RF signal, but it can affect the positioning of outdoor antennas. For example, consider a wireless point-to-point link that connects two buildings that are 12 miles apart. Taking into account the curvature of the Earth (Earth bulge), and having only a five-degree vertical and horizontal beam width on each antenna, the positioning of each antenna would have to be exact. A strong wind could easily move one or both antennas enough to completely degrade the signal between the two antennas. This effect is called "antenna wind loading", and is illustrated in Figure 9.15.
Other similarly extreme weather occurrences like tornadoes or hurricanes must also be considered. If you are implementing a wireless LAN in a geographic location where hurricanes or tornadoes occur frequently, you should certainly take that into account when setting up any type of outdoor wireless LAN. In such weather conditions, securing antennas, cables, and the like are all very important.
Stratification
When very thick fog or even smog settles (such as in a valley), the air within this fog becomes very still and begins to separate into layers. It is not the fog itself that causes the diffraction of RF signals, but the stratification of the air within the fog. When the RF signal goes through these layers, it is bent in the same fashion as visible light is bent as it moves from air into water.
Lightning
Lightning can affect wireless LANs in two ways. First, lightning can strike either a wireless LAN component such as an antenna or it may strike a nearby object. Lightning strikes of nearby objects can damage your wireless LAN components as if these components are not protected by a lightning arrestor. A second way that lightning affects wireless LANs is by charging the air through which the RF waves must travel after striking an object lying between the transmitter and receiver. The affect of lightning is similar to the way that the Aurora Borealis Northern Lights provide problems for RF television and radio transmissions.
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