To put it simply, data communication is the transferring of information between computers. No matter what form of communication is being used, there are many components that are required to achieve a successful communication.
Before we look at some of the individual components, let’s initially keep things simple and look at the three basic requirements for successful communications:
- Two or more devices want to communicate.
- There must be a medium, a means, or a method for them to use to communicate.
- There must be a set of rules for them to use when they communicate.
These three basic requirements are the same for all forms of communication, whether a group of people are having a conversation at a dinner party, two computers are transmitting data via a dial-up modem, or many computers are communicating via a wireless network.
The existence of a computer network essentially implies that the first requirement is met. If we did not have two or more devices that wanted to share data, we wouldn’t need to create the network in the first place.
The CWNA certification program also assumes this and is therefore rarely if ever concerned specifically with the data itself. It is assumed that we have data, and our concern is to transmit it.
We will focus on the second requirement for successful communications, the medium, means, or method to communicate. We will cover the components of radio frequency (RF), which make up what we refer to as the medium for wireless communications.
Here we will be concerned with the transmission of the RF signal and the role of each of the devices and components along the transmission path. We will also look at how each device or component affects the transmission.
In “Radio Frequency Fundamentals,” we discuss that there are many RF behaviors that affect the signal as it leaves the transmitter and travels toward the receiver. As the signal moves through the different components and through the air, its power changes.
Some components increase the power of the signal (gain), while other components decrease the power (loss). We will discuss how to quantify and measure the power of the waves and calculate how the waves are affected by both internal and external influences.
Through these calculations, you will be able to accurately determine whether you will have the means to communicate between devices. Many components contribute to the successful transmission and reception of an RF signal.
Figure 1 shows the key components that will be covered. In addition to understanding the function of the components, it is important to understand how the strength of the signal is specifically affected by each of the components.
The transmitter is the initial component in the creation of the wireless medium. The computer hands the data off to the transmitter, and it is the transmitter’s job to begin the RF communication.
When the transmitter receives the data, it will begin generating an alternating current (AC) signal. This AC signal determines the frequency of the transmission. For an 802.11, 802.11b, or 802.11g transmission, the AC signal will oscillate around 2.4 billion times per second.
For an 802.11a transmission, the AC signal will oscillate around 5 billion times per second. This oscillation determines the frequency of the radio wave. The transmitter will take the data provided and modify the AC signal using a modulation technique to encode the data into the signal.
This modulated AC signal is now a carrier signal, containing the data to be transmitted. The carrier signal is then transported either directly to the antenna or through a cable to the antenna.
In addition to generating a signal at a specific frequency, the transmitter is responsible for determining the amplitude, or what is more commonly referred to as the power level, of the signal.
The higher the amplitude of the wave, the more powerful the wave is and the further it will travel. The power levels that the transmitter is allowed to generate are determined by the local regulatory body, such as the Federal Communications Commission (FCC) in the United States.
An antenna provides two functions in a communication system. When connected to the transmitter, it collects the AC signal that it receives from the transmitter and directs, or radiates, the RF waves away from the antenna in a pattern specific to the antenna type.
When connected to the receiver, it takes the RF waves that it receives through the air and directs the AC signal to the receiver. The receiver converts the AC signal to bits and bytes. The signal that is received is much less than the signal that is generated.
This signal loss is analogous to two people trying to talk to each other from opposite ends of a football field. Due to distance alone (free space), the yelling from one end of the field may be heard as barely louder than a whisper on the other end.
The signal of an antenna is usually compared or referenced to an isotropic radiator. An isotropic radiator is a point source that radiates signal equally in all directions. The sun is probably one of the best examples of an isotropic radiator.
It generates equal amounts of energy in all directions. Unfortunately, it is not possible to manufacture an antenna that is a perfect isotropic radiator. The structure of the antenna itself influences the output of the antenna, similar to the way the structure of a lightbulb affects the bulb’s ability to emit light equally in all directions.
There are two ways to increase the power output from an antenna. The first is to generate more power at the transmitter. The other is to direct, or focus, the RF signal that is radiating from the antenna.
This is similar to how you can focus light from a flashlight. If you remove the lens from the flashlight, the bulb is typically not very bright and radiates in almost all directions. To make the light brighter, you could use more powerful batteries, or you could put the lens back on.
The lens is not actually creating more light. It is focusing the light that was radiating in all different directions into a narrow area. Some antennas radiate waves as the bulb without the lens does, while some radiate focused waves as the flashlight with the lens does.
The receiver is the final component in the wireless medium. The receiver takes the carrier signal that is received from the antenna and translates the modulated signals into 1s and 0s. It then takes this data and passes it to the computer to be processed. The job of the receiver is not always an easy one.
The signal that is received is a much less powerful signal than what was transmitted due to the distance it has traveled and the effects of free space path loss. The signal is also often altered due to interference from other RF sources and multipath.
Intentional Radiator (IR)
The FCC Code of Federal Regulations (CFR) Part 15 defines an intentional radiator (IR) as “a device that intentionally generates and emits radio frequency energy by radiation or induction.”
Basically, it’s something that is specifically designed to generate RF as opposed to something that generates RF as a byproduct of its main function, such as a motor that incidentally generates RF noise. Regulatory bodies such as the FCC limit the amount of power that is allowed to be generated by an IR.
The IR consists of all the components from the transmitter to the antenna but not including the antenna, as seen in Figure 1. The power output of the IR is thus the sum of all the components from the transmitter to the antenna, again not including the antenna.
The components making up the IR include the transmitter, all cables and connectors, and any other equipment (grounding, lightning arrestors, amplifiers, attenuators, etc.) between the transmitter and the antenna.
The power of the IR is measured at the connecter that provides the input to the antenna. Since this is the point where the IR is measured and regulated, we often refer to this point alone as the IR.
Using the flashlight analogy, the IR is all of the components up to the lightbulb socket but not the bulb and lens. This is the raw power, or signal, that is provided, and now the bulb and lens can focus the signal.
Equivalent Isotropically Radiated Power (EIRP)
Equivalent isotropically radiated power (EIRP) is the highest RF signal strength that is transmitted from a particular antenna. To understand this better, think of our flashlight example for a moment.
Let’s assume that the bulb without the lens generates 1 watt of power. When you put the lens on the flashlight, it focuses that 1 watt of light. If you were to look at the light now, it would appear much brighter.
If you were to measure the brightest point of the light that was being generated by the flashlight, due to the effects of the lens, it may be equal to the brightness of an 8 watt bulb. So by focusing the light, you are able to make the equivalent isotropically radiated power of the focused bulb equal to 8 watts.
As you know, antennas are capable of focusing or directing RF energy. This focusing capability can make the effective output of the antenna much greater than the signal entering the antenna. Because of this ability to amplify the output of the RF signal, regulatory bodies such as the FCC limit the amount of EIRP from an antenna.