Mobile Computing

ITworld.com |  Networking

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.

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