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Wireless Networking Handbook
(Publisher: Macmillan Computer Publishing)
Author(s): Jim Geier
ISBN: 156205631x
Publication Date: 09/01/96

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Long-Term Cost Savings

Companies reorganize, resulting in the movement of people, new floor plans, office partitions, and other renovations. These changes often require re-cabling the network, incurring both labor and material costs. In some cases, the re-cabling costs of organizational changes are substantial, especially with large enterprise networks. A reorganization rate of 15% each year can result in yearly reconfiguration expenses as high as $250,000 for networks having 6,000 interconnected devices. The advantage of wireless networking is again based on the lack of cable—you can move the network connection by simply relocating an employee’s PC.

Wireless Network Concerns

The benefits of a wireless network are certainly welcomed by companies and organizations. Network managers and engineers should be aware, however, of the following concerns that surround the implementation and use of wireless networking:

  Radio signal interference
  Power management
  System interoperability
  Network security
  Installation issues
  Health risks

Radio Signal Interference

The purpose of radio-based networks is to transmit and receive signals efficiently over airwaves. This process, though, makes these systems vulnerable to atmospheric noise and transmissions from other systems. In addition, these wireless networks could interfere with other radio wave equipment. As shown in figure 1.9, interference may be inward or outward.

Inward Interference

Most of us have experienced radio signal interference while talking on a wireless telephone, watching television, or listening to a radio. Someone close by might be communicating with another person via a short-wave radio system, causing harmonic frequencies that you can hear while listening to your favorite radio station. Or, a remote control car can cause static on a wireless phone while you are attempting to have a conversation. These types of interference might also disturb radio-based wireless networks in the form of inward interference.


Figure 1.9  Inward and outward interference.

A radio-based LAN, for example, can experience some inward interference either from the harmonics of transmitting systems or other products using ISM-band frequencies in the local area. Microwave ovens operate in the S band (2.4 GHz) that many wireless LANs transmit and receive. These signals result in delays to the user by either blocking transmissions from stations on the LAN or causing bit errors to occur in data being sent. These types of interference can limit the areas in which you can deploy a wireless network. As an illustration, when deploying a wireless network at a site located in Washington, D.C., along the Potomac River, a company occasionally experienced a great deal of delay from stations located on the side of the building facing the river. The implementation team found, through radio propagation tests, that a military base on the opposite side of the river was periodically transmitting a radio signal. The interfering signal was strong enough for the LAN stations to misinterpret it as data traffic, forcing the stations to wait an inefficient period of time.


NOTE:  

To make matters worse, most radio-based products operate within the public, license free, ISM bands. These products do not require users to obtain FCC licenses, which means the FCC does not manage the use of the products. If you experience interference within the ISM band resulting from another product operating within that band, you have no recourse. The FCC is not committed to step in and resolve the matter, leaving you with the choice of dealing with delays the interference causes or looking for a different technology to support your needs.


Interference with radio-based networks is not as bad as it might seem. The products using the ISM bands incorporate spread spectrum modulation that limits the amount of damage an interfering signal causes. The spread spectrum signal covers a wide amount of bandwidth, and a typical narrow bandwidth interference only affects a small part of the information signal, resulting in few or no errors. Thus, spread spectrum-type products are highly resistant to interference. Narrowband interference with signal-to-interference ratios of less than 10 dB does not usually affect a spread spectrum transmission. Wideband interference, however, can have damaging effects on any type of radio transmission. The primary source of wideband interference is domestic microwave ovens that operate in the 2.4 GHz band. The typical microwave oven operates at 50 pulses per second and sweeps through frequencies between 2400 and 2450 MHz, corrupting the wireless data signal if within 50 feet of the interfering source. Other interference may result from elevator motors, duplicating machines, theft protection equipment, and cordless phones.

Outward Interference

Inward interference is only half of the problem. The other half of the issue, outward interference, occurs when a wireless network’s signal disrupts other systems, such as adjacent wireless LANs, navigation equipment on aircraft, and so on. This disruption results in the loss of some or all of the system’s functionality. Interference is uncommon with ISM band products because they operate on such little power. The transmitting components must be very close and operating in the same bandwidth for either one to experience inward or outward interference.

Techniques for Reducing Interference

When dealing with interference, you will want to coordinate the operation of radio-based wireless network products with your company’s frequency management organization, if one exists. This will avoid potential interference problems. In fact, the coordination with frequency management officials is mandatory before operating radio-based wireless devices of any kind on a U.S. military base. The military does not follow the same frequency allocations issued by the FCC. The FCC deals with commercial sectors of the U.S., and the military has their own frequency management process. You must obtain special approvals from the government to operate ISM-based products on military bases because they may interfere with some of their systems. The approval process can take several months to complete.


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