1. Chapter 8 Fundamentals of Communications
Information Technology in Theory
By Pelin Aksoy and Laura DeNardis
Chapter 8 Fundamentals of Communications

2. Information Technology in Theory
Objectives
Understand how binary streams are physically generated
Learn how carriers are modulated to carry the binary streams
Understand important transmission concepts, including attenuation, bandwidth, channel capacity, and multiplexing
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3. Objectives (continued)
Learn the properties of different types of transmission media
Identify sources of transmission errors and learn about error detection and correction techniques for digital transmission systems
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4. Information Technology in Theory
Electrical Signaling
Electrical signals transmitted through conducting materials, such as metal wires, effectively transmit both analog and digital information
Metallic conductors, such as copper wires, comprise atoms with loosely attached electrons, or negatively charged particles, around their nuclei
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5. Electrical Signaling (continued)
When a voltage/potential difference from an electrical source, such as a battery, is introduced between the two ends of a conductor, the electrons are stimulated to move within the metal from one atom to another
The relationship between voltage (V), current (I), and resistance (R) is defined by Ohm’s Law: V = IR
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6. Electrical Signaling (continued)
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7. Analog and Digital Signaling
When analog signals are transmitted across a conductor, a continuous voltage difference proportional to the amplitude of the analog signal is applied at the input of the communications circuit
The current flowing through the circuit is proportional to the applied voltage according to Ohm’s law, as there is a resistance associated with the circuit
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8. Analog and Digital Signaling (continued)
One method for sending digital information across a communication system is called binary signaling
An alternative method to sending digital information across a communication system is 4-ary signaling
The 4-ary signaling may be generalized to M-ary signaling
The data rate (D) in bits per second for M-ary transmission can be calculated by the following: D = R log2M
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9. Analog and Digital Signaling (continued)
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10. Analog and Digital Signaling (continued)
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11. Analog and Digital Signaling (continued)
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12. Analog and Digital Signaling (continued)
Problem: Calculate the data rate for a communication system that employs 8-ary signaling if the signal transmission rate is 1000 signals per second
R = 1000 signals per second
M = 8
According to the equation:
D = R log2M = 1000 log28 = 1000 × 3 = 3000 bps = 3 Kbps
The data rate is therefore 3 Kbps
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13. Radio Wave Communications
Besides electrical energy transmitted over conductors, electromagnetic (EM) energy transmitted over air or a vacuum is also commonly used for analog and digital communications
EM energy travels in the form of EM waves, such as radio waves, light waves (infrared, visible light, ultraviolet), x-rays, and gamma rays
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14. Radio Wave Communications (continued)
The EM waves are used as carriers to wirelessly carry both analog and digital information by altering certain properties of the wave in proportion to the information signal
These waves vary in a sinusoidal manner, taking on a range of frequencies
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15. Radio Wave Communications (continued)
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16. The Electromagnetic Spectrum
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17. The Electromagnetic Spectrum (continued)
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18. The Electromagnetic Spectrum (continued)
Besides frequency, EM waves can also be characterized in terms of their wavelength (λ)
λ=c/f
EM energy can occur naturally or be generated artificially
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19. Information Technology in Theory
The Radio Spectrum
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20. The Radio Spectrum (continued)
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21. The Radio Spectrum (continued)
The Federal Communications Commission (FCC) allocates radio frequencies in the United States, and most countries have a corresponding organization that assigns frequencies
Cordless telephones, walkie-talkies, and wireless network adapters are examples of other systems that have designated sets of operating frequencies but do not require permission for channel use
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22. Modulation/Demodulation
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23. Analog Modulation Techniques
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24. Analog Modulation Techniques (continued)
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25. Analog Modulation Techniques (continued)
FM and PM have higher fidelity than AM
AM is more vulnerable to noise, but systems that employ AM typically consume less power and have a wider coverage area
FM and PM are also more expensive to implement because they require a slightly more complex demodulator
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26. Digital Modulation Techniques
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27. Digital Modulation Techniques (continued)
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28. Digital Modulation Techniques (continued)
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29. Digital Modulation Techniques (continued)
The quality of FSK and PSK exceeds that of ASK, although ASK consumes less energy
ASK is commonly used in fiber-optic communication systems, and PSK is commonly used in satellite communications, space exploration, modems, and computer networking
FSK is regularly used in facsimile machines to transmit digital information across telephone lines
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30. Light-Wave Communications
Light-wave communication systems frequently use infrared, as in fiber-optic communication, and infrared/visible light, as in free-space optical communications, for carrying information
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31. Light-Wave Communications (continued)
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32. Light-Wave Communications (continued)
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33. Information Technology in Theory
Attenuation
When signals travel through any transmission medium, including fiber-optic cable, copper wire, or free space, they lose energy
The loss of energy, called attenuation, is a significant factor that affects the quality and distance of communications
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34. Attenuation (continued)
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35. Attenuation (continued)
Attenuation is measured in decibels (dB), and each transmission medium has its own attenuation figure, which is measured in dB per unit length
In long-haul communication systems, electronic devices called repeaters serve as amplifiers, and are placed at certain intervals to amplify weak signals and relay them along the transmission line
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36. Attenuation (continued)
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37. Information Technology in Theory
Bandwidth
The primary factor that generally governs the choice of transmission media is its bandwidth
In digital systems, the maximum number of bits per second (channel capacity, or C) that can reliably be carried over a channel depends on the bandwidth B (expressed in Hz) of the channel and a unitless ratio called the signal-to-noise ratio (SNR)
The formula is: C = B log2(1+SNR)
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38. Bandwidth (continued)
Sources of noise in communication systems are numerous
An important type of noise is called thermal noise; it arises from random agitation of electrons of the conductor material due to heat
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39. Information Technology in Theory
Multiplexing
A single line can simultaneously transmit multiple information-carrying signals using a technique called multiplexing
Multiplexing signals over a single transmission line uses one of several possible techniques:
Time division multiplexing (TDM)
Frequency division multiplexing (FDM)
Statistical multiplexing
Wavelength division multiplexing (WDM)
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40. Multiplexing (continued)
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41. Time Division Multiplexing
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42. Copper Transmission Media
Twisted pair
Unshielded Twisted Pair (UTP)
Shielded Twisted Pair (STP)
Coaxial Cable
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43. Copper Transmission Media (continued)
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44. Information Technology in Theory
Twisted Pair
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45. Twisted Pair (continued)
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46. Information Technology in Theory
Coaxial Cable
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47. Managing Errors in Digital Communication Systems
Whenever digital information is sent across any communication channel, be it twisted pair, coax, air, or optical fiber, there is always a possibility that some bits will arrive at their destination with errors
The incorrect detection of a binary digit is called an error
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48. Managing Errors in Digital Communication Systems (continued)
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49. Managing Errors in Digital Communication Systems (continued)
By encoding a bit stream prior to transmission using an assortment of techniques called error-control coding (ECC), the receiver can detect and sometimes even correct errors that may occur at the receiver
Some of these codes include:
Block codes
Convolutional codes
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50. Information Technology in Theory
Block Codes
Single parity checking
Rectangular coding
Cyclic redundancy checking (CRC)
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51. Single Parity Checking
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52. Information Technology in Theory
Rectangular Coding
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53. Cyclic Redundancy Checking
CRC is slightly more complex than parity checking or rectangular coding and is based on appending a stream of bits to the end of a data block
The appended bits are generated by performing a simple mathematical operation on the original bit stream
CRC is also used for verifying data integrity within the area of computer forensics
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54. Information Technology in Theory
Convolutional Codes
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55. Digital Communications Scenario
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56. Digital Communications Scenario (continued)
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57. Digital Communications Scenario (continued)
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58. Information Technology in Theory
Summary
Binary signaling schemes communicate digital information over a transmission system by corresponding a 0 or a 1 to one of two discrete voltage values
Signaling schemes may be extended to M-ary signaling, whereby log2M groups of bits may be sent at one time using M different signaling levels
The capacity of a channel, expressed in bits per second, depends on the bandwidth and the signal-to-noise ratio of the channel
Modulation techniques superimpose information signals onto a carrier wave, such as a radio wave or a light wave, by varying some of its properties, such as the wave’s amplitude, frequency, and phase
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59. Information Technology in Theory
Summary (continued)
Attenuation (measured in decibels) is the loss of energy occurring over a transmission line; it is a major factor that affects the quality and distance of communications
Multiplexing techniques enable a single transmission line to simultaneously transmit multiple information-carrying signals
Sources of transmission errors include electromagnetic interference, distortion, systems failures, and atmospheric conditions such as lightning and rain
By encoding bit streams prior to transmission using error control coding (ECC) techniques, a receiver can detect and sometimes even correct errors that may occur at the receiver of a communication system
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