Mobile Computing


Thinking of implementing a wireless LAN for a mobile computing application? You've probably heard an earful of potential problems related to wireless networks, such as RF interference and low data rates; as a result, you may be debating whether to go forward with a wireless system. Before making a decision, you need to understand how these potential problems can affect you.

For now, we'll concentrate on RF interference. But before we do, let's make sure everyone understands how wireless LAN devices decide when to transmit.

End-user devices, equipped with radio network cards, use a sensing protocol to share the air medium. Devices transmit data only when they detect that there are no others transmitting. For example, the radio card in a handheld PC that needs to send data first senses the air to determine whether another station is transmitting. If the card detects no activity, it transmits a data packet. If it detects signals from another transmitting station, however, the radio card waits until the other station finishes its transmission.

RF interference occurs when unwanted signals appear to be coming from a legitimate wireless LAN radio card or access point. When this occurs, the interfering signal blocks transmissions on the wireless LAN until it goes away. Even worse, interference that strikes a packet in transit results in errors and retransmissions. This all leads to unhappy users experiencing network delays.

In 10 years of implementing wireless LANs, I haven't seen many situations where interference causes a system to fail completely. However, the memory of one particular incident has always motivated me to investigate all facilities before installing a wireless LAN.

A few years ago, a company called me in as a consultant to help solve a serious wireless LAN problem. The company had installed a 200-user wireless LAN in a three-story hospital situated along a beautiful river. The portion of the wireless LAN located on the side of the building near the river would not operate at all, though other parts of the network ran smoothly. After performing RF signal tests, I discovered that interfering signals were coming from a large radio station located on the other side of the river. The signals were strong enough to block wireless LAN transmissions on the side of the building near the river. Building construction, though, attenuated the unwanted signals enough to enable the remaining part of the network to operate successfully.

Though a high-powered radio station would obviously interfere with your wireless LAN to a great extent, there are other potential sources of interference that you also need to watch out for. What can you do to minimize the presence of potential interference? Follow these tips:

  • Utilize a site survey tool to measure the performance of the fixed components of the wireless LAN.
  • Locate wireless LAN access points and radio-based end-user devices at least 20 feet away from microwave ovens.
  • Ensure that you restrict the use of RF-based test equipment to a shielded room or to an area that is at a reasonable distance from the wireless LAN; the distance should be determined by the test equipment's output power.
  • Be careful not to operate laptops and other devices containing Bluetooth connectivity near wireless LAN-based products -- there's too much potential for interference.
  • Tune the wireless LAN to operating channels that minimize interference from other nearby wireless LANs and sources of radio waves.
  • Add additional access points to strengthen the wireless LAN signals between radios and access points.

Still interested in implementing a wireless LAN? If you decide to push forward, be certain to investigate and resolve potential interference problems before purchasing and installing components!

Next time, we'll discuss the new higher data rates available with IEEE 802.11a-based wireless LAN products.

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