User Manual

www.moxa.com [e-mail] 10
2009 Industrial Wireless Guidebook
Understanding Industrial WLAN – IEEE 802.11
2
Electromagnetic Waves
To understand how energy is transferred through the air, we need to review basic electromagnetic theories.
Electromagnetic (EM) waves are formed by alternating current rapidly changing direction on a conductive
material. The rapid oscillation of electric and magnetic fields around the conductor projects electromagnetic
waves into the air (see the figure below). In order for current to be radiated into the air in the form of
electromagnetic waves, a few factors are critical, namely, the length of the conductor and frequency of the AC
current. Higher frequency reduces the requirement for conductor length.
The conductors are called antennas. Antennas
transform electric energy into EM waves during
transmission and turns EM waves into electric energy
during reception. The size and length of the antenna
is directly proportional to its desired transmission/
reception frequency. As shown in the figure to the
right, electromagnetic waves are radiated from a
directional antenna in a parabolic shape.
As EM waves propagate through the air, they will experience different types of alterations as they are
intercepted by different obstacles. Obstacles in the signal path introduce the following alteration to the signals:
Chapter 2
Understanding Industrial WLAN – IEEE 802.11
Wireless Communication
In a wireless environment, the communication medium is air. Radio waves carrying data propagate
from point to point through free space. Due to the characteristics of this unguided medium, wireless
communication calls for a very different set of knowledge and skills than traditional wired communication
systems. Getting the most out of your wireless environment requires a basic understanding of the
following scientific principles that govern wireless communications.
2.1 IEEE 802.11 Basics