Fluent Essays

1. After reading Chapters 2 and 3 in the Dumas textbook, give a technical explanation as to why AM radio stations are much more susceptible to noise and interference compared to FM radio stations. 2. Modern technology has given us satellite radio. With 100+ stations and availability across the country, it has many benefits. However, there are other social implications such as local businesses losing an advertising outlet. HD Radio is a local high quality alternative, but doesnt have nearly the reach. Give your opinion on either technology. If you use either technology, give your impressions. Chapter 2: The Modern Signal Carriers: Electricity, Light, Media, and Impairments Principles of Computer Networks and Communications M. Barry Dumas and Morris Schwartz Principles of Computer Networks and Communications Chapter 2 Objectives Describe properties of electricity and electrical media Describe signal impairments in electrical transmission Differentiate between guided and unguided media, and list types of each Explain the role of light in communications, including sources, media, and transmission method Principles of Computer Networks and Communications Chapter 2 Overview Electricity and electromagnetic waves: Carry data as signals that propagate through a medium (physical path) Consist of one or more types of transmission media, that is: Bounded—confined by cables Unbounded—through air or space Are connected by switching and other equipment Principles of Computer Networks and Communications Chapter 2 Properties of Electricity and Electrical Media Ampere (Amp) Magnitude of electrical current Named after André Marie Ampère (French) Volt (v) Measure of electrical potential or pressure Named after Alessandro Giuseppe Volta (Italian) Ohm (ohm) Measure of resistance to current flow Named after Georg Simon Ohm (Greek) Principles of Computer Networks and Communications Chapter 2 Properties of Electricity and Electrical Media “Electricity consists of a flow of electrons called a current whose magnitude is measured in amperes, and strength (pressure) in volts.” 1 Volt = Electrical pressure required to move 1 AMP of current through 1 OHM of resistance Principles of Computer Networks and Communications Chapter 2 Properties of Electricity and Electrical Media “Electricity consists of a flow of electrons called a current whose magnitude is measured in amperes, and strength (pressure) in volts.” Conduction—the process of electron flow Conductors accommodate electrical flow: Copper Aluminum Principles of Computer Networks and Communications Chapter 2 Properties of Electricity and Electrical Media “Electricity consists of a flow of electrons called a current whose magnitude is measured in amperes, and strength (pressure) in volts.” Resistance—opposition to electron flow Insulators resist electrical flow: Rubber Plastic Air Principles of Computer Networks and Communications Chapter 2 Properties of Electricity and Electrical Media “Electricity consists of a flow of electrons called a current whose magnitude is measured in amperes, and strength (pressure) in volts.” Semi-conductors Usually act as insulators but can also behave as conductors Basis for computer chips Principles of Computer Networks and Communications Chapter 2 Properties of Electricity and Electrical Media—Current Current is the flow of electrons (electricity) Alternating Current (AC) Continuously changing direction and magnitude at a regular rate Provided by utility companies Most relevant to communications Direct Current (DC) Current in batteries Principles of Computer Networks and Communications Chapter 2 Properties of Electricity and Electrical Media—Current Cycle: Traces a sine wave pattern One complete journey from 0 through positive and negative strength and back to 0 Measured in Hz (cycles per second) Evident in both electricity and light waves Principles of Computer Networks and Communications Chapter 2 Properties of Electricity and Electrical Media—Radiation How does a signal travel through air and space? Radiation Varying current through a wire produces magnetic and electrical forces (electromagnetic waves) Electromagnetic waves radiate from the wire and mimic the pattern of change of the current in the wire Induction Radiated waves from the original wire create a current flow in a second wire that mimics the pattern of the current in the original wire, creating another electromagnetic wave Principles of Computer Networks and Communications Chapter 2 Properties of Electricity and Electrical Media—Radiation Some notes about radiated energy: Power of radiated energy depends on the power of the current that creates it The stronger the original energy, the greater the current Power attenuates (drops off) as it travels The farther the current goes in a wire, the weaker it gets Radiated waves disperse (spread out) as they travel Spreading the waves dilutes the wave power Induced current is always weaker than the current that induced it! Principles of Computer Networks and Communications Chapter 2 Properties of Electricity and Electrical Media—Radiation What purpose do we want our wire to serve? Wire carrying signals within our own network Objectives: Conserve signal energy (minimize radiation) Protect our signals from currents induced by other wires Principles of Computer Networks and Communications Chapter 2 Properties of Electricity and Electrical Media—Radiation What purpose do we want our wire to serve? Wire transmitting as an antenna Objective: Radiate as much signal energy as possible Principles of Computer Networks and Communications Chapter 2 Properties of Electricity and Electrical Media—Radiation What purpose do we want our wire to serve? Wire receiving as an antenna Objective: Absorb as much as the radiated signals as possible Principles of Computer Networks and Communications Chapter 2 Properties of Electricity and Electrical Media—Pioneers Michael Faraday (British physicist, chemist) Built on earlier work of Hans Christian Oersted Discovered phenomenon of electromagnetic induction James Maxwell (Scottish mathematician, scientist) Discovered propagation speed of an electromagnetic field is equal to speed of light Discovered light is a form of electromagnetic radiation Heinrich Hertz (German physicist) Discovered electricity could be propagated as electromagnetic waves Principles of Computer Networks and Communications Chapter 2 Properties of Electricity and Electrical Media—Waves Wave: “Regularly recurring pattern that moves away from the force that creates it” Principles of Computer Networks and Communications Chapter 2 Properties of Electricity and Electrical Media—Waves Period or, cycle Time it takes a sine wave to trace one complete pattern Periodic Waves are waves with period patterns that repeat over time T Figure 2.1 T Principles of Computer Networks and Communications Chapter 2 Properties of Electricity and Electrical Media—Waves Period cycle (sec) Time it takes a sine wave to trace one complete pattern Periodic waves are waves with period patterns that repeat over time Frequency (Hz) Number of times pattern repeats in 1 second Inversely proportional to period Wavelength Distance a wave travels in one cycle T λ f = 1 / T Figure 2-2 Principles of Computer Networks and Communications Chapter 2 Properties of Electricity and Electrical Media—Waves Wavelength calculation λ = νm T Wavelength Velocity of light through the medium One wave period (seconds) Principles of Computer Networks and Communications Chapter 2 Properties of Electricity and Electrical Media—Waves Wavelength calculation in comms λ = Wavelength Velocity of light through the medium Wave frequency (cycles/second) νm 1 f Principles of Computer Networks and Communications Chapter 2 Properties of Electricity and Electrical Media—Waves Speed of Light: “In a vacuum, all electromagnetic radiation travels at the speed of light.” In other media—that is, not in a vacuum— electromagnetic radiation will travel slower 300,000 (km/sec) 186,000 (miles/sec) Principles of Computer Networks and Communications Chapter 2 Signal Impairments in Electrical Transmission Noise Unwelcome energy in the transmission media Distortion Unwanted changes in signal shapes Caused by interactions between signals and media Attenuation Signal distortion from energy lost as the signal travels Caused by resistance of the medium to electrical flow Limiting factor of network cable length Principles of Computer Networks and Communications Chapter 2 Signal Impairments in Electrical Transmission [cont.] Thermal Noise Background noise, white noise, Gaussian noise, hiss Unwanted energy in the transmission line Cannot be eliminated! Principles of Computer Networks and Communications Chapter 2 Signal Impairments in Electrical Transmission [cont.] Electromagnetic Interference (EMI) Unwanted energy induced by radiation from an external source Affects wireless signals Crosstalk Energy induced in a wire by signals from another wire Impulse noise (“spikes”) Unpredictable! Large, sudden power surge Usually very short duration Principles of Computer Networks and Communications Chapter 2 Signal Impairments in Electrical Transmission [cont.] Delay distortion From the way wires affect signal velocity Frequency components of signals arrive at the receiver at different times Limiting factor of network cable length Intermodulation distortion From non-linearity in a comms system Harmonics (multiples of original signals) appear that were not present in the original signal, making it difficult to distinguish the original from the noise Principles of Computer Networks and Communications Chapter 2 Common Guided Electrical Media Twisted pair Most commonly used 1-wire carries signal; other wire carries ground Wires are insulated and twisted Number of twists per inch = twist rate Coaxial Two conductors are concentric (not twisted) Wire conductor running down the center of the cable is surrounded by conducting braided metal or foil protected by an outer jacket Principles of Computer Networks and Communications Chapter 2 Common Guided Electrical Media Twisted pair—why twist? Minimize prospect of parallel wires: Induced currents are weakest where wires are not parallel Twisting reduces crosstalk from external radiation The greater the twist rate difference between pairs, the less the intra-cable crosstalk Principles of Computer Networks and Communications Chapter 2 Common Guided Electrical Media Types of twisted pair UTP—Unshielded Twisted Pair Most common Widely used for telephone connections and Ethernet LANs STP—Shielded Twisted Pair Added conductive shielding reduces external noise Wire mesh or foil wrapped around twisted pair Works in 2 directions: Stops external EMI from distorting signals Prevents internal EMI from distorting signals in other cables Widely used in token ring LANs Principles of Computer Networks and Communications Chapter 2 Common Guided Electrical Media Advantages of coaxial vs. twisted pair Much greater capacity Relatively immune from external interference Disadvantages of coaxial vs. twisted pair More expensive Bulkier Difficulty to modify Difficult install around sharp bends Principles of Computer Networks and Communications Chapter 2 Common Guided Electrical Media Backbone: “A high-capacity common link to which networks and communications devices are attached. ” A backbone must have significantly greater capacity than the networks it connects Principles of Computer Networks and Communications Chapter 2 Unguided Media and Antennas All unguided media use antennas for transmission and receipt of signals Anything that conducts electricity can be the transmitter or the recipient of induced radiation There are 3 electromagnetic radiation (EMR) groupings relevant to communications Infrared light Microwaves Radio waves Principles of Computer Networks and Communications Chapter 2 EMR Frequency Bands for Communications Category Frequency (Hz) Wavelength (m) Type Visible 7.5 x 1014 4 x 1014 7 x 10-7 to 4 x 10-7 Line of sight Microwave 3 x 1011 3 x 109 10-1 to 10-3 Line of sight Infrared 4 x 1014 3 x 1011 10-1 to 7 x 10-7 Line of sight Radio < 3 x 109 10-1 and greater Omni-directional Principles of Computer Networks and Communications Chapter 2 EMR Frequency Bands for Communications Category Frequency (Hz) Wavelength (m) Type Visible 7.5 x 1014 4 x 1014 7 x 10-7 to 4 x 10-7 Line of sight Microwave 3 x 1011 3 x 109 10-1 to 10-3 Line of sight Infrared 4 x 1014 3 x 1011 10-1 to 7 x 10-7 Line of sight Radio < 3 x 109 10-1 and greater Omni-directional The higher the frequency of the EMR the more directional/more focused Principles of Computer Networks and Communications Chapter 2 EMR Frequency Bands for Communications Category Frequency (Hz) Wavelength (m) Type Visible 7.5 x 1014 4 x 1014 7 x 10-7 to 4 x 10-7 Line of sight Microwave 3 x 1011 3 x 109 10-1 to 10-3 Line of sight Infrared 4 x 1014 3 x 1011 10-1 to 7 x 10-7 Line of sight Radio < 3 x 109 10-1 and greater Omni-directional Lower frequency EMR is omni-directional, propagating in all directions at once Principles of Computer Networks and Communications Chapter 2 Unguided Media and Antennas— Electromagnetic Radiation (EMR) Ways to communicate without using line of sight: Depending on the material, EMR can Pass through [IR—TV remotes] Be refracted [RF—cell phones] Be diffracted [RF—cell phones] Be reflected [RF—cell phones] Principles of Computer Networks and Communications Chapter 2 The Basic Nature of Light Light behaves: As a particle (quantum optics) With motions like waves of energy (wave optics) Light will change direction when it encounters another medium When sunlight strikes the surface of a lake, it becomes reflected or refracted (bent) Principles of Computer Networks and Communications Chapter 2 The Basic Nature of Light What is diffraction? “When an electromagnetic signal hits the edge of an object that is large compared to the signal wavelength, the signal propagates in many directions, with the edge as the apparent source.” Light diffraction Wave phenomena Has direct application in communication by light Separates a light beam into its component wavelengths Each wavelength can carry information independently and simultaneously Principles of Computer Networks and Communications Chapter 2 Common Media for Use with Light Optical fiber Highly refined pure silica Very low attenuation (“half power point” —point of travel where a signal has lost half of its original power) Principles of Computer Networks and Communications Chapter 2 Common Media for Use with Light Optical Fiber Cable Core—signal carrying fiber runs through the cable Cladding—surrounds the core; keeps light from reflecting Coating—covers cladding; absorbs light escaping the core Typically, hundreds and even thousands of fibers are bundled together Figure 2.5 Figure 2.4 Principles of Computer Networks and Communications Chapter 2 Light Sources for Computer Communications All communications systems have 3 related components: Signal source Medium to conduct the signals Receiver to accept the information For optical communication systems: LEDs and Lasers Light Detector Optical Fiber Principles of Computer Networks and Communications Chapter 2 Lighting up the Core Source light rays can enter the core from 3 ways: (Ideal) light rays point straight through the core Refraction angle is 90o+ at core/cladding interface (Worst) refraction angle is < 90o; light is absorbed by coating Types of optical fiber Multimode Step index—subject to absorption; suitable for short distances Graded index—partial solution to zigzag cable problem Single mode—long distance/high-speed comms Principles of Computer Networks and Communications Chapter 2 Signal Impairments in Light Transmission Absorption Impurities in the fiber during manufacturer Shorter wavelengths have more absorption Scattering Small contaminants and density differences in the core Bends Macro-bending—can be seen; cable is too severely bent Micro-bending—mishandling; kinks in the cable Coupling Splicing cables and attaching cable to connectors Principles of Computer Networks and Communications Chapter 3: Signal Fundamentals Principles of Computer Networks and Communications M. Barry Dumas and Morris Schwartz Principles of Computer Networks and Communications Chapter 3 Objectives Differentiate between signals and information Describe characteristics, strengths, and weaknesses of analog and digital signals Understand the relationship between signals and sine waves Understand the role of noise in a system Differentiate between signal amplification and signal regeneration Describe the elements that are involved in and the methods for determining system bandwidth Principles of Computer Networks and Communications Chapter 3 Overview There are 2 basic forms of information: Analog Produced by real world events (e.g., voice, music) Can take on infinite values created from the event Digital Produced by computers Only two values: 1 or 0 There are also 2 basic forms of signals (see above) Principles of Computer Networks and Communications Chapter 3 Overview [cont.] Signals (analog or digital) carry information (analog or digital) Leaving 4 possibilities: Analog information over analog signals Analog information over digital signals Digital information over analog signals Digital information over digital signals Principles of Computer Networks and Communications Chapter 3 Analog Signals Analog signals Are continuous Can take whatever shape or power needed to represent information Can assume an infinite number of values Cannot change shape instantaneously Principles of Computer Networks and Communications Chapter 3 Analog Signals Analog signals Are continuous Cannot change shape instantaneously ALL Signals are combinations of simple sine waves. Sine waves Principles of Computer Networks and Communications Chapter 3 Analog Signals—Sine Waves 3 characteristics of sine waves: Amplitude Frequency Phase Principles of Computer Networks and Communications Chapter 3 Analog Signals—Sine Wave Equation s(t) = Amplitude Wave location at time (t) Phase A sin (2 π f t + φ ) Wave frequency (cycles/second) Time Principles of Computer Networks and Communications Chapter 3 Analog Signals—Sine Waves Characteristics: amplitude, frequency, phase Two sine waves with same frequency same phase different amplitudes S2 amplitude > S1 amplitude Principles of Computer Networks and Communications Chapter 3 Analog Signals—Sine Waves Characteristics: amplitude, frequency, phase Two sine waves with different frequencies same phase same amplitudes S1 frequency > S2 frequency Principles of Computer Networks and Communications Chapter 3 Analog Signals—Sine Waves Characteristics: amplitude, frequency, phase Two sine waves with same frequencies different phases same amplitudes Same wave shifted to right Principles of Computer Networks and Communications Chapter 3 Analog Signals—Sine Waves Consolidated: amplitude, frequency, phase Principles of Computer Networks and Communications Chapter 3 Analog Signals as Digital Data Ways to represent digital data (1s or 0s) using analog signals: Vary peak amplitudes (A1 , A2) Vary frequencies (f1 , f2) Vary phases (φ1 , φ2) Principles of Computer Networks and Communications Chapter 3 Analog Signals—Advantages Faithful copy of original analog signal Conceptually represent real world events Can travel far without shape distortion from the medium Easy to create, handle Principles of Computer Networks and Communications Chapter 3 Analog Signals—Disadvantage Noise! For computer communications, susceptibility to damage from noise outweighs all other advantages for analog signals. Principles of Computer Networks and Communications Chapter 3 Analog Signals—Noise Adds to a signal Changes original signal shape Must be separated out to recover original signal Cannot be known accurately (because shapes/strengths are random) Reconstructing a noise-deformed analog signal exactly is an impossible task! Principles of Computer Networks and Communications Chapter 3 Digital Signals—Characteristics Digital signals are discrete Voltage is limited to small set of values Signal values change instantaneously No time elapses between amplitude changes Good approximations of real-world events Figure 3.3 – Some Digital Signal Shapes Principles of Computer Networks and Communications Chapter 3 Digital Signals—Advantages Can be restored to original shape even when corrupted by noise Natural and intuitive for representing computer information Principles of Computer Networks and Communications Chapter 3 Digital Signals—Advantages Can be restored to original shape even when corrupted by noise Natural and intuitive for representing computer information [1s or 0s] Fig 3.4 Principles of Computer Networks and Communications Chapter 3 Digital Signals—Disadvantages Digital signals never exactly represent real-world (analog) data Digital signals cannot travel as far through a medium without being distorted [Former Disadvantage] Technology to handle digital signals is more complex No longer an issue because costs have come down! Digital signals are standard in computer communications Principles of Computer Networks and Communications Chapter 3 Signal Amplification and Regeneration Why amplify and regenerate? Attenuation Form of distortion Signal energy is lost as signal travels through the medium (i.e., original signal shape is deformed) All signals suffer some attenuation as they travel Principles of Computer Networks and Communications Chapter 3 Signal Amplification and Regeneration Where to amplify and regenerate? Signals are intercepted at points where they are still accurately recognizable. Here, they are strengthened, and sent on. How many interception points? Depends on: Type of signal Media characteristics Distance Principles of Computer Networks and Communications Chapter 3 Signal Amplification and Regeneration Noise and other distortions change original signal shape and affect every signal Characteristics of an amplifier Signal enters and exits with the same shape Signal is increased in strength (including noise) and sent Characteristics of a regenerator Discerns original signal shape Re-creates original signal (without noise) and sends Principles of Computer Networks and Communications Chapter 3 Signal Amplification and Regeneration s(t) = Equation for a sine wave A sin (2 π f t + φ) s(t) = Equation for a sine wave after amplification by a factor of 10 A sin (2 π f t + φ) + 10 other distortions 10 + 10 noise Amplified components Principles of Computer Networks and Communications Chapter 3 Signal Analysis Signals that carry information must travel: Over thousands of miles of media Through a variety of equipment For a communications system to be useful: The media and equipment that interact with the signals must not change the signals beyond proper recognition Principles of Computer Networks and Communications Chapter 3 Signal Analysis Beam’s spectrum When a beam of light is separated into its component colors Signal spectrum When a signal (analog or digital) is separated into its elementary signals sine waves Principles of Computer Networks and Communications Chapter 3 Signal Analysis 2 Methods to determine the spectrum of a signal: Mathematical analysis—using Fourier’s technique to mathematically describe a signal Spectrum analyzer—graphic display of a “live” signal showing the sine waves that make up its spectrum Principles of Computer Networks and Communications Chapter 3 Historical Note—Newton and Sunlight Isaac Newton (1642–1727) observed: Streaming sunlight through a prism created a rainbow of colors Colors through a second prism could not be further decomposed; these were called primary colors. Colors could be recombined into white light by passing through an inverted prism Newton concluded that white light was actually composed of all the colors blended together Principles of Computer Networks and Communications Chapter 3 Historical Note—Fourier and Decomposition of Signals Jean Baptist Fourier (1768–1830) proved: Heat flows were a form of signal flows Any signal could be constructed by a combination of sinusoids Fourier series for periodic signals Fourier transform for aperiodic signals Principles of Computer Networks and Communications Chapter 3 Bandwidth To see how a signal evolves over time, use a 2-dimensional time domain view Fig 3.1 Principles of Computer Networks and Communications Chapter 3 Bandwidth To see sine wave components that create a signal’s spectrum, use a 2-dimensional frequency domain view Fig 3.8 Frequency components Principles of Computer Networks and Communications Chapter 3 Bandwidth What is bandwidth? For a signal, bandwidth is the significant range of frequencies in the spectrum For a system, bandwidth is the usable range of frequencies in the spectrum Principles of Computer Networks and Communications Chapter 3 Bandwidth Relationship between network (system) capability and signal requirement Bm bandwidth of signal to be carried If Bm ≤ BS BS bandwidth of network system Network can carry the signals If Bm > BS Network can not carry the signals Principles of Computer Networks and Communications Chapter 3 Bandwidth Significant range of frequencies in a signal’s spectrum: Bm = fh – fl Bandwidth of signal we need to carry Highest significant frequency in spectrum Lowest significant frequency in spectrum Principles of Computer Networks and Communications Chapter 3 Bandwidth Significant range of frequencies in a signal’s spectrum: Fig 3.9 Not significant Not significant significant Bandwidth tells us the range but does not tell us where the spectrum is Principles of Computer Networks and Communications Chapter 3 Bandwidth “For our signal to pass through a communications system successfully, all the frequencies in its spectrum must be able to pass successfully.” Fig 3.10 Initial power level for all signals Principles of Computer Networks and Communications Chapter 3 Bandwidth “For our signal to pass through a communications system successfully, all the frequencies in its spectrum must be able to pass successfully.” Fig 3.10 Actual power level for all signals after attenuation Principles of Computer Networks and Communications Chapter 3 Bandwidth “For our signal to pass through a communications system successfully, all the frequencies in its spectrum must be able to pass successfully.” Fig 3.10 Signals above half power level after attenuation Principles of Computer Networks and Communications Chapter 3 Bandwidth “For our signal to pass through a communications system successfully, all the frequencies in its spectrum must be able to pass successfully.” Fig 3.10 Signal bandwidth after attenuation 20 – 5 = 15 KHz Principles of Computer Networks and Communications Chapter 3 Bandwidth Attenuation Not uniform for all frequencies Frequencies at ends of spectrum attenuate more quickly than frequencies in the middle Higher frequencies attenuate more quickly than lower frequencies Attenuation for different frequencies is a characteristic of the wire Principles of Computer Networks and Communications Chapter 3 Bandwidth Wire bandwidth—“half power rule” “To be called usable, the power of the frequency received should be at least one half of the power sent.” Bandwidth calculation Difference between the highest and lowest frequencies received… Whose powers are at least half of that sent Bm = fh - fl Principles of Computer Networks and Communications Chapter 3: Signal Fundamentals Principles of Computer Networks and Communications M. Barry Dumas and Morris Schwartz Principles of Computer Networks and Communications Chapter 3 Objectives Differentiate between signals and information Describe characteristics, strengths, and weaknesses of analog and digital signals Understand the relationship between signals and sine waves Understand the role of noise in a system Differentiate between signal amplification and signal regeneration Describe the elements that are involved in and the methods for determining system bandwidth Principles of Computer Networks and Communications Chapter 3 Overview There are 2 basic forms of information: Analog Produced by real world events (e.g., voice, music) Can take on infinite values created from the event Digital Produced by computers Only two values: 1 or 0 There are also 2 basic forms of signals (see above) Principles of Computer Networks and Communications Chapter 3 Overview [cont.] Signals (analog or digital) carry information (analog or digital) Leaving 4 possibilities: Analog information over analog signals Analog information over digital signals Digital information over analog signals Digital information over digital signals Principles of Computer Networks and Communications Chapter 3 Analog Signals Analog signals Are continuous Can take whatever shape or power needed to represent information Can assume an infinite number of values Cannot change shape instantaneously Principles of Computer Networks and Communications Chapter 3 Analog Signals Analog signals Are continuous Cannot change shape instantaneously ALL Signals are combinations of simple sine waves. Sine waves Principles of Computer Networks and Communications Chapter 3 Analog Signals—Sine Waves 3 characteristics of sine waves: Amplitude Frequency Phase Principles of Computer Networks and Communications Chapter 3 Analog Signals—Sine Wave Equation s(t) = Amplitude Wave location at time (t) Phase A sin (2 π f t + φ ) Wave frequency (cycles/second) Time Principles of Computer Networks and Communications Chapter 3 Analog Signals—Sine Waves Characteristics: amplitude, frequency, phase Two sine waves with same frequency same phase different amplitudes S2 amplitude > S1 amplitude Principles of Computer Networks and Communications Chapter 3 Analog Signals—Sine Waves Characteristics: amplitude, frequency, phase Two sine waves with different frequencies same phase same amplitudes S1 frequency > S2 frequency Principles of Computer Networks and Communications Chapter 3 Analog Signals—Sine Waves Characteristics: amplitude, frequency, phase Two sine waves with same frequencies different phases same amplitudes Same wave shifted to right Principles of Computer Networks and Communications Chapter 3 Analog Signals—Sine Waves Consolidated: amplitude, frequency, phase Principles of Computer Networks and Communications Chapter 3 Analog Signals as Digital Data Ways to represent digital data (1s or 0s) using analog signals: Vary peak amplitudes (A1 , A2) Vary frequencies (f1 , f2) Vary phases (φ1 , φ2) Principles of Computer Networks and Communications Chapter 3 Analog Signals—Advantages Faithful copy of original analog signal Conceptually represent real world events Can travel far without shape distortion from the medium Easy to create, handle Principles of Computer Networks and Communications Chapter 3 Analog Signals—Disadvantage Noise! For computer communications, susceptibility to damage from noise outweighs all other advantages for analog signals. Principles of Computer Networks and Communications Chapter 3 Analog Signals—Noise Adds to a signal Changes original signal shape Must be separated out to recover original signal Cannot be known accurately (because shapes/strengths are random) Reconstructing a noise-deformed analog signal exactly is an impossible task! Principles of Computer Networks and Communications Chapter 3 Digital Signals—Characteristics Digital signals are discrete Voltage is limited to small set of values Signal values change instantaneously No time elapses between amplitude changes Good approximations of real-world events Figure 3.3 – Some Digital Signal Shapes Principles of Computer Networks and Communications Chapter 3 Digital Signals—Advantages Can be restored to original shape even when corrupted by noise Natural and intuitive for representing computer information Principles of Computer Networks and Communications Chapter 3 Digital Signals—Advantages Can be restored to original shape even when corrupted by noise Natural and intuitive for representing computer information [1s or 0s] Fig 3.4 Principles of Computer Networks and Communications Chapter 3 Digital Signals—Disadvantages Digital signals never exactly represent real-world (analog) data Digital signals cannot travel as far through a medium without being distorted [Former Disadvantage] Technology to handle digital signals is more complex No longer an issue because costs have come down! Digital signals are standard in computer communications Principles of Computer Networks and Communications Chapter 3 Signal Amplification and Regeneration Why amplify and regenerate? Attenuation Form of distortion Signal energy is lost as signal travels through the medium (i.e., original signal shape is deformed) All signals suffer some attenuation as they travel Principles of Computer Networks and Communications Chapter 3 Signal Amplification and Regeneration Where to amplify and regenerate? Signals are intercepted at points where they are still accurately recognizable. Here, they are strengthened, and sent on. How many interception points? Depends on: Type of signal Media characteristics Distance Principles of Computer Networks and Communications Chapter 3 Signal Amplification and Regeneration Noise and other distortions change original signal shape and affect every signal Characteristics of an amplifier Signal enters and exits with the same shape Signal is increased in strength (including noise) and sent Characteristics of a regenerator Discerns original signal shape Re-creates original signal (without noise) and sends Principles of Computer Networks and Communications Chapter 3 Signal Amplification and Regeneration s(t) = Equation for a sine wave A sin (2 π f t + φ) s(t) = Equation for a sine wave after amplification by a factor of 10 A sin (2 π f t + φ) + 10 other distortions 10 + 10 noise Amplified components Principles of Computer Networks and Communications Chapter 3 Signal Analysis Signals that carry information must travel: Over thousands of miles of media Through a variety of equipment For a communications system to be useful: The media and equipment that interact with the signals must not change the signals beyond proper recognition Principles of Computer Networks and Communications Chapter 3 Signal Analysis Beam’s spectrum When a beam of light is separated into its component colors Signal spectrum When a signal (analog or digital) is separated into its elementary signals sine waves Principles of Computer Networks and Communications Chapter 3 Signal Analysis 2 Methods to determine the spectrum of a signal: Mathematical analysis—using Fourier’s technique to mathematically describe a signal Spectrum analyzer—graphic display of a “live” signal showing the sine waves that make up its spectrum Principles of Computer Networks and Communications Chapter 3 Historical Note—Newton and Sunlight Isaac Newton (1642–1727) observed: Streaming sunlight through a prism created a rainbow of colors Colors through a second prism could not be further decomposed; these were called primary colors. Colors could be recombined into white light by passing through an inverted prism Newton concluded that white light was actually composed of all the colors blended together Principles of Computer Networks and Communications Chapter 3 Historical Note—Fourier and Decomposition of Signals Jean Baptist Fourier (1768–1830) proved: Heat flows were a form of signal flows Any signal could be constructed by a combination of sinusoids Fourier series for periodic signals Fourier transform for aperiodic signals Principles of Computer Networks and Communications Chapter 3 Bandwidth To see how a signal evolves over time, use a 2-dimensional time domain view Fig 3.1 Principles of Computer Networks and Communications Chapter 3 Bandwidth To see sine wave components that create a signal’s spectrum, use a 2-dimensional frequency domain view Fig 3.8 Frequency components Principles of Computer Networks and Communications Chapter 3 Bandwidth What is bandwidth? For a signal, bandwidth is the significant range of frequencies in the spectrum For a system, bandwidth is the usable range of frequencies in the spectrum Principles of Computer Networks and Communications Chapter 3 Bandwidth Relationship between network (system) capability and signal requirement Bm bandwidth of signal to be carried If Bm ≤ BS BS bandwidth of network system Network can carry the signals If Bm > BS Network can not carry the signals Principles of Computer Networks and Communications Chapter 3 Bandwidth Significant range of frequencies in a signal’s spectrum: Bm = fh – fl Bandwidth of signal we need to carry Highest significant frequency in spectrum Lowest significant frequency in spectrum Principles of Computer Networks and Communications Chapter 3 Bandwidth Significant range of frequencies in a signal’s spectrum: Fig 3.9 Not significant Not significant significant Bandwidth tells us the range but does not tell us where the spectrum is Principles of Computer Networks and Communications Chapter 3 Bandwidth “For our signal to pass through a communications system successfully, all the frequencies in its spectrum must be able to pass successfully.” Fig 3.10 Initial power level for all signals Principles of Computer Networks and Communications Chapter 3 Bandwidth “For our signal to pass through a communications system successfully, all the frequencies in its spectrum must be able to pass successfully.” Fig 3.10 Actual power level for all signals after attenuation Principles of Computer Networks and Communications Chapter 3 Bandwidth “For our signal to pass through a communications system successfully, all the frequencies in its spectrum must be able to pass successfully.” Fig 3.10 Signals above half power level after attenuation Principles of Computer Networks and Communications Chapter 3 Bandwidth “For our signal to pass through a communications system successfully, all the frequencies in its spectrum must be able to pass successfully.” Fig 3.10 Signal bandwidth after attenuation 20 – 5 = 15 KHz Principles of Computer Networks and Communications Chapter 3 Bandwidth Attenuation Not uniform for all frequencies Frequencies at ends of spectrum attenuate more quickly than frequencies in the middle Higher frequencies attenuate more quickly than lower frequencies Attenuation for different frequencies is a characteristic of the wire Principles of Computer Networks and Communications Chapter 3 Bandwidth Wire bandwidth—“half power rule” “To be called usable, the power of the frequency received should be at least one half of the power sent.” Bandwidth calculation Difference between the highest and lowest frequencies received… Whose powers are at least half of that sent Bm = fh - fl Principles of Computer Networks and Communications Chapter 3: Signal Fundamentals Principles of Computer Networks and Communications M. Barry Dumas and Morris Schwartz Principles of Computer Networks and Communications Chapter 3 Objectives Differentiate between signals and information Describe characteristics, strengths, and weaknesses of analog and digital signals Understand the relationship between signals and sine waves Understand the role of noise in a system Differentiate between signal amplification and signal regeneration Describe the elements that are involved in and the methods for determining system bandwidth Principles of Computer Networks and Communications Chapter 3 Overview There are 2 basic forms of information: Analog Produced by real world events (e.g., voice, music) Can take on infinite values created from the event Digital Produced by computers Only two values: 1 or 0 There are also 2 basic forms of signals (see above) Principles of Computer Networks and Communications Chapter 3 Overview [cont.] Signals (analog or digital) carry information (analog or digital) Leaving 4 possibilities: Analog information over analog signals Analog information over digital signals Digital information over analog signals Digital information over digital signals Principles of Computer Networks and Communications Chapter 3 Analog Signals Analog signals Are continuous Can take whatever shape or power needed to represent information Can assume an infinite number of values Cannot change shape instantaneously Principles of Computer Networks and Communications Chapter 3 Analog Signals Analog signals Are continuous Cannot change shape instantaneously ALL Signals are combinations of simple sine waves. Sine waves Principles of Computer Networks and Communications Chapter 3 Analog Signals—Sine Waves 3 characteristics of sine waves: Amplitude Frequency Phase Principles of Computer Networks and Communications Chapter 3 Analog Signals—Sine Wave Equation s(t) = Amplitude Wave location at time (t) Phase A sin (2 π f t + φ ) Wave frequency (cycles/second) Time Principles of Computer Networks and Communications Chapter 3 Analog Signals—Sine Waves Characteristics: amplitude, frequency, phase Two sine waves with same frequency same phase different amplitudes S2 amplitude > S1 amplitude Principles of Computer Networks and Communications Chapter 3 Analog Signals—Sine Waves Characteristics: amplitude, frequency, phase Two sine waves with different frequencies same phase same amplitudes S1 frequency > S2 frequency Principles of Computer Networks and Communications Chapter 3 Analog Signals—Sine Waves Characteristics: amplitude, frequency, phase Two sine waves with same frequencies different phases same amplitudes Same wave shifted to right Principles of Computer Networks and Communications Chapter 3 Analog Signals—Sine Waves Consolidated: amplitude, frequency, phase Principles of Computer Networks and Communications Chapter 3 Analog Signals as Digital Data Ways to represent digital data (1s or 0s) using analog signals: Vary peak amplitudes (A1 , A2) Vary frequencies (f1 , f2) Vary phases (φ1 , φ2) Principles of Computer Networks and Communications Chapter 3 Analog Signals—Advantages Faithful copy of original analog signal Conceptually represent real world events Can travel far without shape distortion from the medium Easy to create, handle Principles of Computer Networks and Communications Chapter 3 Analog Signals—Disadvantage Noise! For computer communications, susceptibility to damage from noise outweighs all other advantages for analog signals. Principles of Computer Networks and Communications Chapter 3 Analog Signals—Noise Adds to a signal Changes original signal shape Must be separated out to recover original signal Cannot be known accurately (because shapes/strengths are random) Reconstructing a noise-deformed analog signal exactly is an impossible task! Principles of Computer Networks and Communications Chapter 3 Digital Signals—Characteristics Digital signals are discrete Voltage is limited to small set of values Signal values change instantaneously No time elapses between amplitude changes Good approximations of real-world events Figure 3.3 – Some Digital Signal Shapes Principles of Computer Networks and Communications Chapter 3 Digital Signals—Advantages Can be restored to original shape even when corrupted by noise Natural and intuitive for representing computer information Principles of Computer Networks and Communications Chapter 3 Digital Signals—Advantages Can be restored to original shape even when corrupted by noise Natural and intuitive for representing computer information [1s or 0s] Fig 3.4 Principles of Computer Networks and Communications Chapter 3 Digital Signals—Disadvantages Digital signals never exactly represent real-world (analog) data Digital signals cannot travel as far through a medium without being distorted [Former Disadvantage] Technology to handle digital signals is more complex No longer an issue because costs have come down! Digital signals are standard in computer communications Principles of Computer Networks and Communications Chapter 3 Signal Amplification and Regeneration Why amplify and regenerate? Attenuation Form of distortion Signal energy is lost as signal travels through the medium (i.e., original signal shape is deformed) All signals suffer some attenuation as they travel Principles of Computer Networks and Communications Chapter 3 Signal Amplification and Regeneration Where to amplify and regenerate? Signals are intercepted at points where they are still accurately recognizable. Here, they are strengthened, and sent on. How many interception points? Depends on: Type of signal Media characteristics Distance Principles of Computer Networks and Communications Chapter 3 Signal Amplification and Regeneration Noise and other distortions change original signal shape and affect every signal Characteristics of an amplifier Signal enters and exits with the same shape Signal is increased in strength (including noise) and sent Characteristics of a regenerator Discerns original signal shape Re-creates original signal (without noise) and sends Principles of Computer Networks and Communications Chapter 3 Signal Amplification and Regeneration s(t) = Equation for a sine wave A sin (2 π f t + φ) s(t) = Equation for a sine wave after amplification by a factor of 10 A sin (2 π f t + φ) + 10 other distortions 10 + 10 noise Amplified components Principles of Computer Networks and Communications Chapter 3 Signal Analysis Signals that carry information must travel: Over thousands of miles of media Through a variety of equipment For a communications system to be useful: The media and equipment that interact with the signals must not change the signals beyond proper recognition Principles of Computer Networks and Communications Chapter 3 Signal Analysis Beam’s spectrum When a beam of light is separated into its component colors Signal spectrum When a signal (analog or digital) is separated into its elementary signals sine waves Principles of Computer Networks and Communications Chapter 3 Signal Analysis 2 Methods to determine the spectrum of a signal: Mathematical analysis—using Fourier’s technique to mathematically describe a signal Spectrum analyzer—graphic display of a “live” signal showing the sine waves that make up its spectrum Principles of Computer Networks and Communications Chapter 3 Historical Note—Newton and Sunlight Isaac Newton (1642–1727) observed: Streaming sunlight through a prism created a rainbow of colors Colors through a second prism could not be further decomposed; these were called primary colors. Colors could be recombined into white light by passing through an inverted prism Newton concluded that white light was actually composed of all the colors blended together Principles of Computer Networks and Communications Chapter 3 Historical Note—Fourier and Decomposition of Signals Jean Baptist Fourier (1768–1830) proved: Heat flows were a form of signal flows Any signal could be constructed by a combination of sinusoids Fourier series for periodic signals Fourier transform for aperiodic signals Principles of Computer Networks and Communications Chapter 3 Bandwidth To see how a signal evolves over time, use a 2-dimensional time domain view Fig 3.1 Principles of Computer Networks and Communications Chapter 3 Bandwidth To see sine wave components that create a signal’s spectrum, use a 2-dimensional frequency domain view Fig 3.8 Frequency components Principles of Computer Networks and Communications Chapter 3 Bandwidth What is bandwidth? For a signal, bandwidth is the significant range of frequencies in the spectrum For a system, bandwidth is the usable range of frequencies in the spectrum Principles of Computer Networks and Communications Chapter 3 Bandwidth Relationship between network (system) capability and signal requirement Bm bandwidth of signal to be carried If Bm ≤ BS BS bandwidth of network system Network can carry the signals If Bm > BS Network can not carry the signals Principles of Computer Networks and Communications Chapter 3 Bandwidth Significant range of frequencies in a signal’s spectrum: Bm = fh – fl Bandwidth of signal we need to carry Highest significant frequency in spectrum Lowest significant frequency in spectrum Principles of Computer Networks and Communications Chapter 3 Bandwidth Significant range of frequencies in a signal’s spectrum: Fig 3.9 Not significant Not significant significant Bandwidth tells us the range but does not tell us where the spectrum is Principles of Computer Networks and Communications Chapter 3 Bandwidth “For our signal to pass through a communications system successfully, all the frequencies in its spectrum must be able to pass successfully.” Fig 3.10 Initial power level for all signals Principles of Computer Networks and Communications Chapter 3 Bandwidth “For our signal to pass through a communications system successfully, all the frequencies in its spectrum must be able to pass successfully.” Fig 3.10 Actual power level for all signals after attenuation Principles of Computer Networks and Communications Chapter 3 Bandwidth “For our signal to pass through a communications system successfully, all the frequencies in its spectrum must be able to pass successfully.” Fig 3.10 Signals above half power level after attenuation Principles of Computer Networks and Communications Chapter 3 Bandwidth “For our signal to pass through a communications system successfully, all the frequencies in its spectrum must be able to pass successfully.” Fig 3.10 Signal bandwidth after attenuation 20 – 5 = 15 KHz Principles of Computer Networks and Communications Chapter 3 Bandwidth Attenuation Not uniform for all frequencies Frequencies at ends of spectrum attenuate more quickly than frequencies in the middle Higher frequencies attenuate more quickly than lower frequencies Attenuation for different frequencies is a characteristic of the wire Principles of Computer Networks and Communications Chapter 3 Bandwidth Wire bandwidth—“half power rule” “To be called usable, the power of the frequency received should be at least one half of the power sent.” Bandwidth calculation Difference between the highest and lowest frequencies received… Whose powers are at least half of that sent Bm = fh - fl Principles of Computer Networks and Communications