Infrared Light–Based Wireless LANs
Infrared light is an alternative to using radio waves for wireless LAN connectivity. The wavelength
of infrared light is longer (lower in frequency) than the spectral colors, but much shorter
(higher in frequency) than radio waves. Under most lighting conditions, infrared light is invisible
to the naked eye. Infrared light LAN products operate around the 820 nanometer wavelength
because air offers the least attenuation at that point in the infrared spectrum In comparison to radio waves, infrared light offers higher degrees of security and performance.
These LANs are more secure because infrared light does not propagate through opaque
objects, such as walls, keeping the data signals contained within a room or building. Also,
common noise sources such as microwave ovens and radio transmitters will not interfere with
the light signal. In terms of performance, infrared light has a great deal of bandwidth, making
it possible to operate at very high data rates. However, infrared light is not as suitable as radio
waves for mobile applications because of its limited coverage.
You’ve probably been using a diffused infrared device for years—the television remote control,
which enables you to operate your TV from a distance without the use of wires. When you
press a button on the remote, a corresponding code modulates an infrared light signal that is
transmitted to the TV. The TV receives the code and performs the applicable function. This is
fairly simple, but infrared-based LANs are not much more complex. The main difference is
that LANs use infrared light at slightly higher power levels and use communications protocols
to transport data.
When using infrared light in a LAN, the ceiling can be a reflection point (see Figure 2.8). This
technique uses carrier sense protocols to share access to the ceiling. Imagine, for example, that
there is a room containing four people who can communicate only via flashlights. To send
information, they can encode letters that spell words using a system such as Morse code. If
someone wants to send information, he first looks at the ceiling to see if someone is currently
transmitting (shining light onto the ceiling). If there is a transmission taking place, the person
wanting to send the information waits until the other person stops sending the message. If no
one is transmitting, the source person will point his flashlight at the ceiling and turn the light
on and off, according to the code that represents the information being sent.