c02.indd 08/22/2014 Page 32 In addition to understanding the OSI model and basic net-
working concepts, you must broaden your understanding of many other networking technologies in order to properly design, deploy, and administer an 802.11 wireless network. For instance, when administer- ing an Ethernet network, you typically need a comprehension of TCP/IP, bridging, switch- ing, and routing. The skills to manage an Ethernet network will also aid you as a WLAN administrator because most 802.11 wireless networks act as “portals” into wired networks.
The IEEE defi nes the 802.11 communications at the Physical layer and the MAC sublayer of the Data-Link layer.
To fully understand the 802.11 technology, you need to have a clear concept of how wireless works at the fi rst layer of the OSI model, and at the heart of the Physical layer is radio frequency (RF) communications.
In a wired LAN, the signal is confi ned neatly inside the wire, and the resulting behaviors are anticipated. However, just the opposite is true for a wireless LAN. Although the laws of physics apply, RF signals move through the air in a sometimes unpredictable manner.
Because RF signals are not saddled inside an Ethernet wire, you should always try to envi- sion a wireless LAN as an “ever changing” network.
Does this mean that you must be an RF engineer from Stanford University to perform a WLAN site survey or monitor a Wi-Fi network? Of course not, but if you have a good grasp of the RF characteristics and behaviors defi ned in this chapter, your skills as a wire- less network administrator will be ahead of the curve. Why does a wireless network per- form differently in an auditorium full of people than it does inside an empty auditorium?
Why does the range of a 5 GHz radio transmitter seem shorter than the range of a 2.4 GHz radio transmitter? These are the types of questions that can be answered with some basic knowledge of how RF signals work and perform.
Wired communications travel across what is known as bounded medium.
Bounded medium contains or confines the signal (small amounts of signal leakage can occur). Wireless communications travel across what is known as unbounded medium. Unbounded medium does not contain the signal, which is free to radiate into the surrounding environment in all directions (unless restricted or redirected by some outside influence).
In this chapter, we fi rst defi ne what an RF signal does. Then, we discuss both the prop- erties and the behaviors of RF.
What Is a Radio Frequency Signal?
This book is by no means intended to be a comprehensive guide to the laws of physics, which is the science of motion and matter. However, a basic understanding of some of the concepts of physics as they relate to radio frequency (RF) is important for even an entry- level wireless networking professional.
The electromagnetic (EM) spectrum, which is usually simply referred to as spectrum, is the range of all possible electromagnetic radiation. This radiation exists as self-propagating electromagnetic waves that can move through matter or space. Examples of electromag- netic waves include gamma rays, X-rays, visible light, and radio waves. Radio waves are electromagnetic waves occurring on the radio frequency portion of the electromagnetic spectrum, as pictured in Figure 2.1.
F I G U R E 2 .1 Electromagnetic spectrum
Radio Microwaves Infrared Visible Ultraviolet X-ray Gamma Ray Low Frequency
Long Wavelength Short Wavelength
High Frequency
An RF signal starts out as an electrical alternating current (AC) signal that is originally generated by a transmitter. This AC signal is sent through a copper conductor (typically a coaxial cable) and radiated out of an antenna element in the form of an electromagnetic wave. This electromagnetic wave is the wireless signal. Changes of electron fl ow in an antenna, otherwise known as current, produce changes in the electromagnetic fi elds around the antenna.
An alternating current is an electrical current with a magnitude and direction that varies cyclically, as opposed to direct current, the direction of which stays in a constant form. The shape and form of the AC signal—defi ned as the waveform—is what is known as a sine wave, as shown in Figure 2.2. Sine wave patterns can also be seen in light, sound, and the ocean. The fl uctuation of voltage in an AC current is known as cycling,
34 Chapter 2 ■ Radio Frequency Fundamentals
c02.indd 08/22/2014 Page 34 F I G U R E 2 . 2 A sine wave
An RF electromagnetic signal radiates away from the antenna in a continuous pattern that is governed by certain properties such as wavelength, frequency, amplitude, and phase.
Additionally, electromagnetic signals can travel through mediums of different materials or travel in a perfect vacuum. When an RF signal travels through a vacuum, it moves at the speed of light, which is 299,792,458 meters per second, or 186,000 miles per second.
To simplify mathematical calculations that use the speed of light, it is com- mon to approximate the value by rounding it up to 300,000,000 meters per second. Any references to the speed of light in this book will use the approximate value.
RF electromagnetic signals travel using a variety or combination of movement behaviors.
These movement behaviors are referred to as propagation behaviors. We discuss some of these propagation behaviors, including absorption, refl ection, scattering, refraction, diffrac- tion, amplifi cation, and attenuation, later in this chapter.
Radio Frequency Characteristics
These characteristics, defi ned by the laws of physics, exist in every RF signal: