Syngress - Eleventh Hour Network+ Exam N10-004 Study Guide (11 ...

Wireless Network Concepts 57
RF COMMUNICATIONS
RF characteristics and facts:
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Wireless networks use a special type of electric current known as RF.
RF is created by applying alternating current (AC) to an antenna to produce
an electromagnetic (EM) field.
Wireless networks use the EM field for communications, which is the region
of space that is influenced by EM radiation.
On wireless networks, amplitude decreases with distance, resulting in the
degradation of signal strength and the ability to communicate.
Radio waves are affected by the presence of obstructions and can be
reflected, refracted, diffracted, or scattered, depending on the properties
of the obstruction and its interaction with the radio waves, all of which
result in signal degradation.
The interference created by bounced radio waves is called multipath interference.
Common sources of multipath interference include metal doors,
metal roofs, water, metal vertical blinds, and any other source that is highly
reflective to radio waves.
EM fields are also prone to interference and signal degradation by the presence
of other EM fields, such as interference produced by cordless phones,
microwave ovens, and a wide range of devices that use the same bands.
To mitigate the effects of interference from these devices and other sources
of electromagnetic interference, RF-based wireless networks employ spread
spectrum technologies. Wireless networks use a “spectrum” of frequencies
for communication.
SPREAD SPECTRUM TECHNOLOGY
Spread spectrum technology characteristics and facts:
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Spread spectrum defines methods for wireless devices to send a number
of narrowband frequencies over a range of frequencies simultaneously for
communication.
The narrowband frequencies used between devices change according to a
random-appearing, but defined pattern, allowing individual frequencies to
contain parts of the transmission.
Crunch Time
Two methods of synchronizing wireless devices are as
follows:
■ Frequency hopping spread spectrum (FHSS) FHSS
works by quickly moving from one frequency to
another, according to a pseudorandom pattern.
The frequency range used by the frequency hop is
relatively large (83.5 MHz), providing excellent protection
from interference. The amount of time spent
on any given frequency is known as dwell time,
and the amount of time it takes to move from one
frequency to another is known as hop time. Wireless
networks that use FHSS include HomeRF and
Bluetooth.