Wireless Networking Basics 1: About Analog and Digital Transmissions

So you’re walking around with your laptop or that nifty new tablet you got last week. You’re looking for a good Wi-Fi hotspot to connect with. Have you ever wondered how your portable devices recognize that there’s a wireless network in the area? Well, it’s a pretty complicated topic, so I’ll try to break it down and start off with a rundown of Analog and Digital transmissions.

Imagine standing in your yard with a garden hose. If you’re using it to put out a campfire or water your garden, you might move the hose up and down, creating a continuous series of waves. These waves will remain uninterrupted as long as the water stays on. These waves are a good analogy for or analog signals.

Analog signals are used for your telephone (to transmit your voice), dial-up Internet (in which computers need modems to translate the signals into a digital format), radios, and televisions. Analog signals are continuous, like the uninterrupted stream of water from the garden hose, and only frequency and wavelength distinguishes one wave from the next.

If you were to put your thumb over the end of the hose and then take it off (or if someone turned the hose off and turned it back on), the water would become discontinuous, or stop flowing and start again. Digital signals consist of data transmitted with a series of starts and stops. Morse code is an example of a digital signal, since it consists of a series of dots and dashes. When computers “talk” to each other, they use binary code over a digital signal.

Wireless networks rely on data traveling on radio waves. These waves are a part of the electromagnetic spectrum, like visible light or infrared (heat) waves, which requires no special medium for movement.

Visible light and heat waves do have their limitations. Light, for example, cannot penetrate objects like walls or ceilings and surrounding objects absorb heat waves. Radio waves do not have these limitations and can also travel long distances.

Wordnet’s definition of “Frequency” is “the number of occurrences within a given time period”. As it relates to radio frequencies, frequency refers to the number of times a wave completes its cycle within a given period of time, usually a second. This measurement is called hertz (Hz), or cycles per second. A Kilohertz is a thousand hertz, a megahertz is a million hertz, and so on. 1000 cycles per second would typically be written as 1 KHz, or one kilohertz.

Cycles are illustrated by an up-and-down wave called an oscillating signal or sine wave. The wave starts at zero, moves up to its maximum voltage, and then down to its minimum voltage. Then, it returns to zero, thus completing one cycle.

The frequency can be changed. Back to our garden hose analogy again. (Kind of unoriginal, I know, but it works.) The pressure on the water line would represent the electrical current’s voltage (V), which is measured in volts. The water flow can represent the current and is measured in amperes, also called amps. If you bought a hose with an increased diameter, you would reduce the resistance to the water flow, meaning the water can flow through the hose faster.

Resistance, also called impedance, is measured in ohms. German physicist George Ohm first proposed the formula for voltage, V=I*R, or voltage equals Current times Resistance. Electrical power is calculated by taking the voltage and multiplying it by the current, and is measured in watts.

Radio wave frequency is changed by changing the voltage or electrical pressure on a wire. When electricity passes through a wire, it creates a magnetic field around the wire and this field radiates radio waves. Radio transmissions use the radio waves as a carrier signal.

Radio transmissions are merely carrier signals that, by themselves, carry no data. Modulation, or changes to the signal, can be made to carry the information. Changes can be made to the height, frequency, or relative starting point of the signal. These changes can be made with either analog or digital signals.

Amplitude Modulation, or AM, involves changes to the height of the carrier wave. An increase in height represents a 1 bit, while a decrease represents a 0 bit. The image demonstrates what this would look like on a sine chart. If you’ve ever tuned into an AM station on the radio, you’ll notice that you can pull the station from a greater range than FM, but it’s more vulnerable to electromagnetic interference from nearby power wires.

Frequency Modulation, or FM, changes the frequency of the waves. An increase in frequency represents a 1 bit, while a decrease represents a 0 bit. It can punch through the electromagnetic interference from nearby power sources, but tends to have a shorter range.

Phase Modulation, or PM, changes the starting point in a signal. These changes take place only when the binary bits being transmitted change from a 1 to a 0 and vice versa.

Digital Modulation

Most wireless systems use digital modulation. Advantages of this include better use of the bandwidth, less power used to transmit data, and better performance when there is interference.

Amplitude shift keying (ASK) is similar to AM analog, in that changes in the carrier’s height represent a 1 or 0 bit. Instead of both a 1 and 0 bit having a carrier signal, however, the 1 bit has a carrier signal while the 0 bit does not with ASK.

Frequency shift keying (FSK) changes the frequency of the carrier signal. Because it is sending a binary signal, the carrier signal starts and stops.

Phase shift keying (PSK) is similar to phase modulation. The difference is that the carrier signal starts and stops.

Because radio transmissions speed through the air rather than through a cable, they can be affected by their surroundings. Walls, doors, metal structures, and even the weather can have an effect on performance. As a result, the administrator of a wireless network will usually consider environmental factors when troubleshooting a network.

Gain is defined as the positive difference in amplitude between two signals. Amplification is often used synonymously with gain; however, gain is technically a measure of amplification. This can occur intentionally, by boosting the power supply to the signal, or unintentionally, when an RF signal bounces off objects like trees or walls and combines with the original signal to amplify it.

Loss, also called attenuation, is the negative difference in amplitude between to signals. Like gain, this can be intentional (somebody decreases the electrical signal) or unintentional (the signal gets absorbed by surrounding objects). Causes of loss can include, though is not limited to, reflection of the signal (metal objects can cause it to bounce back toward its source), scattering (rain can scatter a signal), or refraction (the signal hits cold damp air after moving through warmer, drier air).

Radio frequency power can be measured in milliwatts (mW) or decibels. A milliwatt is 1/1000 of a Watt and is used for linear scale measurements. Answers.com defines a linear scale as “A scale in which the divisions are uniformly spaced.” These scales generally start at zero; one example is a speedometer in a car. The loss or gain in radio frequency power can be measured by the number of milliwatts being transmitted. However, this does not necessarily reveal the relation to the whole.

To determine the gain or loss in radio frequency power in relation to the whole, most professionals in the field use decibels (dB) and use the 10’s and 3’s Rules of RF Math.