Page 53 - DCAP311_DCAP607_WIRELESS_NETWORKS

Unit 3: Radio Frequency and Light Signal Fundamentals

Self-Assessment Notes

Fill in the blanks:
1. ................................ communication is a method of transmitting information from one place
to another by sending pulses of light through an optical fiber.
2. Fiber optics is a medium for carrying information from one point to another in the form of
................................
3. ................................ transceivers are often used for portable or mobile use.
4. A frequency of electromagnetic radiation in the range at which radio signals are transmitted,
ranging from approximately 3 kilohertz to ................................ gigahertz.
5. The ................................ is frequently measured as the distance from one crest of the wave
to the next.
6. ................................ is used for many modern RF modulation algorithms

7. Tag collision and reader collision are common problems with ................................
8. ................................ collision occurs when numerous tags are present in a confined area
9. RFID ................................ collision results when the coverage area managed by one RFID
reader overlaps with the coverage area of another reader
10. ................................ are often used to correct the ISI.

3.3 Working of Light Signals

Light signals have been in use with communications systems for even longer than RF systems.
Lanterns would provide a source of light to use with sending codes between ships at sea hundreds
of years ago. Light guns are still in use today at many airports as a backup communication with
aircraft having malfunctioning radio gear.
Wireless networks that utilize light signals, however, are not as common as these that use radio
signals. Light signals generally satisfy needs for special applications, such as building-to-building
links and short-range personal-area networks. Some wireless LANs and inter-building products
use laser light to carry information between computers.
A light signal is analog in form and has a very high frequency that’s not regulated by the FCC.
Most wireless networks that use light for wireless signaling purposes utilize infrared light, which
has a wavelength of approximately 900 nanometers. This equates to 333,333 GHz, which is quite
a bit higher than RF signals and falls just below the visual range of humans.
Diffused and direct infrared are two main types of light transmission. Figure 3.1 illustrates these
two concepts. Diffused laser light is normally reflected off a wall or ceiling, and direct laser
is directly focused in a line-of-sight fashion. Most laser LANs utilize diffused infrared; inter-
building modems and PDAs use the direct infrared technique.
Infrared light has very high bandwidth; however, the diffusing technique severely attenuates the
signal and requires slow data transmissions (less than 1 Mbps) to avoid significant transmission
errors. In addition, this technique limits wireless component spacing to around 40 feet, mainly
because of the lower ceilings indoors and resulting signal path geometry. The advantage is
relatively easy installation with inexpensive components.
The direct infrared approach, commonly referred to as free-space optics, intensifies the light
signal power similarly to a directive radio signal antenna. This increases the range of low-power
laser systems to a mile or so at data rates up in the Gbps range.

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