Upgrading and Repairing Networks

Sources of Interference for Wireless Networks

Because wireless network products use radio waves for the "physical" transmission medium, you need to consider other devices that produce radio waves in the same spectrum that IEEE 802.11b devices use. For example, the most common device, which is present in the home, many offices, and many public places, is the microwave oven. Yes, these devices use radio waves to heat your food, and they have a metal grating surrounding them that is supposed to prevent microwave transmission from emanating outside the box. However, if that were true you wouldn't see those warnings saying you shouldn't be close to one if you have a pacemaker and there wouldn't be a market for inexpensive devices you can purchase to measure leakage from a microwave oven. Microwave ovens do leak microwave signals and these can interfere with IEEE 802.11 devices.

The good news is that microwave ovens aren't typically operating continuously. However, you still should consider them a source of interference that can dramatically slow wireless communications. Another source of interruption to wireless networks operating in the 2.4GHz radio spectrum is other consumer devices, such as 2.4GHz portable telephones, as well as camera devices that can be used to transmit video back to your PC. Consider this when deciding whether to use a wireless network that uses the same radio spectrum. Note that the newer 5.8GHz wireless telephones do not interfere with 802.11b, 802.11g, and Bluetooth devices because these operate in the 2.4GHz frequency band.

Wireless devices based on these two main standards can interfere with each other. It is beyond the scope of this chapter to discuss the heated debate going on about which technology is better adapted to avoid interference from another wireless device, be it 802.11a, b, g, or even Bluetooth. Each group of supporters can make arguments about the capability of their products to recover from interference, but at this point, there isn't enough data or testing to prove it one way or the other.

In a business environment, however, other sources of interference must be considered. For example, some companies use microwave lighting. In military installations, radar can cause interference. Magnetic resonance imaging (MRI) devices used in hospitals can interfere with wireless network products. When you get down to it, microwave technology is used in many industrial applications, so you might want to perform testing beforehand instead of just choosing a solution that appears on the surface to solve your networking problem. Interference can be mitigated, in some cases, by simply placing additional APs so that mobile devices are closer to an AP. The farther away from an AP or a home base unit, the weaker the signal and, thus, the greater the chance for interference. As an alternative, many recent APs are designed to use replaceable antennas. You can replace the normal antennas with high-gain omni-directional antennas designed to improve range in all directions, or replace them with directional antennas to improve range in a particular direction.