Information Technology Reference
In-Depth Information
A
B
No signal
Obstruction
Good signal
Transmitter
FIGURE 13.2
Line-of-sight operation: Radio-frequency signals at ultra-high frequencies are blocked by
obstructions.
Line-of-Sight Concepts
One of the most important rules in the operation of wireless LANs is that the technol-
ogy operates only in a straight line, with a relatively unencumbered path. This is called
line-of-sight
operation, because you can usually operate the connection satisfactorily
between two points if you can see between them, within obvious range limits (Fig. 13.2).
In the upper frequencies of radio transmission, such as those bands used for
WLANs, the electromagnetic waves tend to be incapable of penetrating dense objects,
particularly those with any surface conductivity. In other words, air and glass are no
problem, steel and other metals are, and everything else in between is…well, in between.
Many materials will cause an attenuation of the signal. Plasterboard and plas-
ter are mild attenuators; cement and cinderblock walls are bad, and steel enclosures
are worse. The use of foil-covered wallboard, metal wall studs, reinforcing bars, or
mesh greatly increases the attenuation of the wireless signal.
Reflections of a signal may occur because of highly conductive objects in the
area of either the transmitter or the receiver. For example, a signal might bounce off
a metal door, a steel file cabinet, or even a parked truck. If the signal travels to the
receiver both via a direct and a reflected path, an interference problem called
multi-
path interference
can occur. If the two signals are of similar strengths and arrive at
the receiver out of phase, it may be impossible to recover a usable signal. We com-
monly see this phenomenon on a broadcast television signal as a ghost image when
the antenna is misaligned or when large reflective objects are nearby.
Antenna Operation
Antennas operate by capturing the transmitted electromagnetic field and converting
the energy to an electrical signal of the same frequency. All of the information that
