EC312 Lesson 11: Communications Systems, EM Spectrum, & Signals

Objectives:(a) Understand the basic components of a communications channel and the impact of using free space as a

medium for communication as it relates to security.

(b) Identify which portions of the electromagnetic spectrum are used for specific purposes and why these bands (BW) correspond to specific function (HF, VHF, UHF, microwaves , light ,UV.)

(c) Define the basic properties of a periodic electromagnetic signal (period, frequency, wavelength, phase, and amplitude) and describe their mathematical relationship.

(d) Understand the difference between the time domain and frequency domain representation of a signal.

This lesson marks the beginning of the next section of EC312. In Unit 1, the Host Section, we examined how data is stored and accessed in memory at the machine level and examined the resulting threats against specific computers, focusing on the buffer overflow attack. In Unit 2, the Wireless Section, we will gain an appreciation for the many considerations of communicating between computers and other hardware in a wireless environment - where we don’t have physical connections to every computer, router, etc. In Unit 3, the Networks Section, we will concentrate on understanding how a specific wired network works, (the Controller Area Network) first at the physical layer and then at the Data Link layer, and how an underlying network is just as important and vulnerable as a host computer

(Credit: Dane Brown)

EC 312

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I. Communications Systems and the Electromagnetic Spectrum “Cyber” is defined by National Security Presidential Directive 54 Homeland Security Presidential

Directive 23 (NSPD-54/HSPD-23) as a “global domain within the information environment consisting of an interdependent network of information technology infrastructures, including the internet, telecommunications networks, computer systems, and embedded processors and controllers… Common usage of the term also refers to the virtual environment of information and interactions between people.” In other words, in order to understand the potential impacts of Cyber we can’t just focus on a computer or even a network; we need to consider all aspects of the information environment. To understand this “environment” and fully appreciate cyber, you have you look at the “big picture” of how the machines and networks communicate globally. A good example is depicted below:

(From Wikimedia Commons: This work is in the public domain in the United States because it is a work prepared by an officer or employee of the United States Government as part of that person’s official duties under the terms of Title 17, Chapter 1, Section 105 of the US Code)

1. Communication SystemsPurpose: to transmit information over a distanceThis “Information” could be: Audio (speech, music)

VideoSensor data (temperature, pressure)Data (digital information)

We’ll come back to this!

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A general model of all communications systemsFrenzel, Principles of Electronic Communication Systems, McGraw Hill, 2008

Basic components: Transmitter – converts information into an electronic form suitable for the channel Channel – the physical medium through which an electronic signal travels

o wireo fiber-optico free space (i.e., “wireless” or “radio”)o water (sonar)

Receiver – converts the transmitted signal back to a usable form(Transceiver – combination tx/rx)

Significant, although undesired, component: NOISE - random energy that enters the system and interferes with the transmitted message. Noise degrades or interferes with transmitted information. Noise refers to the corruption of the original information signal and is one of the fundamental limiting factors in all communication systems. Noise could be unintentional interference (atmosphere), intentional (jamming)… More on this in the next lessons.

For this part of the course, we will focus on communications systems in which our medium or channel is FREE-SPACE! Signals that propagate in free space are often referred to as “wireless” or “over-the-air” signals, and all signals in free space are part of the Electromagnetic Spectrum. With wireless routers and satellites part of almost every network, especially in military applications, cyber and the electromagnetic spectrum form a single, continuous environment.

2. Electromagnetic SpectrumThe electromagnetic spectrum is the range of all possible frequencies of electromagnetic waves. The spectrum is broken into regions/ranges and classified by frequency and wavelength:

Frequency (f ) is the number of cycles per second of a wave measured in hertz (Hz).Wavelength ( ) is the length of one cycle of a transmitted wave measured in meters.

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Recall that the speed of wave propagation is given by , or, rearranging . For electromagnetic waves traveling in air (or vacuum), c is roughly 3 x 108 m/s.

Frenzel, Principles of Electronic Communication Systems, McGraw Hill, 2008

This should give you an idea of the relationship between wavelength size and frequency throughout the Electromagnetic Spectrum:

(From Wikimedia Commons)

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You should be familiar with the frequency ranges for spectrum uses ranging from ELF to EHF.

Extremely low frequency (ELF) 30 to 300 Hz. Power line frequencies and low and of human audio. Voice frequency (VF) 300 to 3000 Hz. Normal range of human speech. Very low frequency (VLF) 3 to 30 kHz. Used for communications with submerged submarines. Low frequency (LF) 30 to 300 kHz. Long range radio navigation. Medium frequency (MF) 300 to 3000 kHz. AM radio and long range communication. High frequency (HF) 3 to 30 MHz. Known as “short wave”, used by two-way radio. Very high frequency (VHF) 30 to 300 MHz. Radio communications and FM radio. Ultra high frequency (UHF) 300 to 3000 MHz. TV, military and cell phones. Super high frequency (SHF) 3 to 30 GHz. Microwave. Satellite communications and radar. Extremely high frequency (EHF) 30 GHz to 300 GHz. Satellite communications.

Example

What is the wavelength of an FM radio station whose frequency is 101.1 MHz?

Solution:

Example

What is the frequency of a signal whose wavelength is 8 cm?

Solution:

Bandwidth : Bandwidth is that portion of the frequency spectrum occupied by a signal. It is the difference between the upper and lower frequency limits of the signal. Typical bandwidths:

AM Station – 10 kHz FM Station – 180 kHz TV Station – 6 MHz

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Federal Communications Commission (FCC)

There are a finite number of frequencies in the Electromagnetic Spectrum, and everyone wants a piece. The FCC was established by the Communications Act of 1934 to regulate interstate and foreign communication. The FCC:

Allocates bands of frequencies Sets limitations on broadcast power Monitors broadcasts to detect unlicensed operations and technical violations Auctions spectrum usage

II. Signals as a Function of Time and Frequency

Recall that the purpose of a Communications System is to transmit information over a distance.

Thus far, we’ve covered that during this section of the course we’re going to focus on free space as our channel or medium, which means we’re considering the electromagnetic spectrum. Why do we care?

FCC Radio Frequency Allocation Table

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ADM Greenert points out that we want to control information… and we know that information can be in the form of audio, video, digital data… We transmit our information in the form of a signal.

1. SignalsA signal is a function that conveys information. Signals are considered either analog or digital.

Analog SignalsAn analog signal is a continuously varying such as in voice or music.

(a) Sine wave “tone.” (b) Voice. (c) Video (TV) signal.

Digital SignalsA digital signal changes in discrete increments and represent digital (binary) information such as that used by computers.

(a) Telegraph (Morse code). (b) Continuous-wave (CW) code. (c) Serial binary code.

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Simple example: tuning fork (the noise it produces is its signal)

You can hear the tuning fork, but if you were to look at it graphically as a function of voltage over time, you would see a something that looks like this:

0 0.005 0.01 0.015 0.02 0.025-0.15

-0.1

-0.05

0

0.05

0.1

0.15

Time (sec)

Vol

tage

(V)

2. Time Domain (Sine Wave)

Mathematically:

Amplitude (Vm) – distance from average to peak

Period (T) – time (in seconds) to complete one cycle

Frequency (f) - number of cycles in one second

Phase ()– Lateral shift wrt reference axis

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The sine wave is one way to represent the sound the tuning fork makes as a function of time. This is referred to as “time domain.” This signal can also be represented as by using its frequency, or in the “frequency domain.”

3. Frequency Domain (Frequency Spectrum) – display of signal amplitude vs. frequency.The frequency domain refers to signals with respect to frequency (instead of time). A plot in the frequency domain displays signal amplitude versus frequency. Since our tuning fork is a very simple tone with a single frequency component of 440 Hz, the frequency domain plot looks like this:

0 500 1000 1500 2000 2500 3000 3500 40000

0.05

0.1

0.15

0.2

0.25

0.3

0.35

Frequency (Hz)

Vol

tage

(V)

If the tuning fork played a lower or higher tone, the “spike” on the plot would shift to the lower or higher frequency. Both the sine wave and the frequency display represent the tone of the tuning fork – they’re just different ways to identify the same signal.

FREQUENCY VS. WAVELENGTH DEMONSTRATIONUsing O-Scope, Frequency Generator, & speaker

Set frequency to 440 Hz, display on scope, turn on speaker volume As frequency increases, what happens?

o Sound gets higher (& louder)o Sine wave gets “closer” togethero Wavelength gets shorter

As frequency decreases, the opposite happenso Voice frequency range is 300 Hz – 3 kHz – can we hear higher? Lower?

That demonstrates the relationship between frequency and wavelength for a simple signal, which we need to understand before learning how to manipulate signals. The tuning fork is as simple as it gets, but any information can be conveyed by a signal – like music!

In Windows Media Player:

o Enter “Now Playing” mode by selecting icon in the bottom right corner of window.o Right-click anywhere in window, select “Visualizations” -> “Bars & Waves”

“Scope” shows a time domain Oscilloscope-like display “Bars” shows frequency domain content

LCDR Jennie Wood, CDR Hewitt Hymas

Help us improve these notes! Send comments, corrections and clarifications to vincent@usna.edu

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