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1
Data Communications
Chapter 3
Data and Signals
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Terminology (1)
• Transmitter
• Receiver
• Medium– Guided medium
• e.g. twisted pair, optical fiber
– Unguided medium• e.g. air, water, vacuum
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Terminology (2)
• Direct link– No intermediate devices
• Point-to-point– Direct link – Only 2 devices share link
• Multi-point– More than two devices share the link
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Terminology (3)
• Simplex– One direction
• e.g. Television
• Half duplex– Either direction, but only one way at a time
• e.g. police radio
• Full duplex– Both directions at the same time
• e.g. telephone
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Frequency, Spectrum and Bandwidth
• Time domain concepts– Continuous signal
• Infinite number of points at any given time
– Discrete signal• Finite number of points at any given time; maintains a constant
level then changes to another constant level
– Periodic signal• Pattern repeated over time
– Aperiodic signal• Pattern not repeated over time
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Continuous & Discrete Signals
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Periodic Signals
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Sine Wave
• Peak Amplitude (A)– Maximum strength of signal– Typically volts
• Frequency (f)– Rate of change of signal– Hertz (Hz) or cycles per second– Period = time for one repetition (T)– T = 1/f
• Phase ()– Relative position in time
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Varying Sine Waves
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Frequency Domain Concepts
• Signal usually made up of many frequencies
• Components are sine waves
• Can be shown (Fourier analysis) that any signal is made up of component sine waves
• Can plot frequency domain functions
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Addition of Frequency
Components
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FrequencyDomain
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Spectrum & Bandwidth
• Spectrum– range of frequencies contained in signal
• Absolute bandwidth– width of spectrum
• Effective bandwidth– Often just bandwidth– Narrow band of frequencies containing most of the
energy
• DC component– Component of zero frequency
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Signal with DC Component
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Analog and Digital Data Transmission
• Data – Entities that convey meaning
• Signals– Electric or electromagnetic representations of
data
• Transmission– Communication of data by propagation and
processing of signals
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Data
• Analog– Continuous values within some interval– e.g. sound, video
• Digital– Discrete values– e.g. text, integers
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Signals
• Means by which data are propagated• Analog
– Continuously variable– Various media
• wire, fiber optic, space
– Speech bandwidth 100Hz to 7kHz– Telephone bandwidth 300Hz to 3400Hz– Video bandwidth 4MHz
• Digital– Uses two (or more) DC components
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Data and Signals
• Usually use digital signals for digital data and analog signals for analog data
• Can use analog signal to carry digital data– Modem
• Can use digital signal to carry analog data – Compact Disc audio
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Analog Signals Carrying Analog and Digital Data
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Digital Signals Carrying Analog and Digital Data
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Analog Transmission
• Analog signal transmitted without regard to content
• May be analog or digital data
• Attenuated over distance
• Use amplifiers to boost signal
• Also amplifies noise
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Digital Transmission
• Concerned with content• Integrity endangered by noise, attenuation etc.• Repeaters used• Repeater receives signal• Extracts bit pattern• Retransmits• Attenuation is overcome• Noise is not amplified
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Advantages of Digital Transmission
• Digital technology– Low cost LSI/VLSI technology
• Data integrity– Longer distances over lower quality lines
• Capacity utilization– High bandwidth links economical
– High degree of multiplexing easier with digital techniques
• Security & Privacy– Encryption
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Transmission Impairments
• Signal received may differ from signal transmitted
• Analog - degradation of signal quality• Digital - bit errors• Caused by
– Attenuation and attenuation distortion– Delay distortion– Noise
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Attenuation
• Signal strength falls off with distance• Depends on medium• Received signal strength:
– must be enough to be detected– must be sufficiently higher than noise to be
received without error
• Attenuation is an increasing function of frequency
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Attenuation
• Signal strength is measured in decibels (dB)• dB is a relative measure of loss (or gain)
• NdB = 10 log10 (P2 / P1)– P2 = ending power level in watts
– P1 = beginning power level in watts
• Example: P1 = 10 watts, P2 = 5 watts• Even easier – remember ½ rule• Losses and gains are additive
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Attenuation
• Signal to noise ratio shows the ratio of signal power to noise power
• S/NdB = 10 log10 (signal power/noise power)
• Example 1: Signal power = 1000 watts, noise power = 20 mw
• Example 2: Signal power = 100 w, noise power = 0.000002w
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Delay Distortion
• Only in guided media
• Propagation velocity varies with frequency
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Noise (1)
• Additional signals inserted between transmitter and receiver
• Thermal– Due to thermal agitation of electrons
– Uniformly distributed
– White noise
• Intermodulation– Signals that are the sum and difference of original
frequencies sharing a medium
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Noise (2)
• Crosstalk– A signal from one line is picked up by another
• Impulse– Irregular pulses or spikes– e.g. External electromagnetic interference– Short duration– High amplitude
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Data Rate and Bandwidth
• Any transmission system has a limited band of frequencies
• This limits the data rate that can be carried
• The faster the frequency, the faster the data rate
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Channel Capacity
• Data rate– In bits per second– Rate at which data can be communicated
• Bandwidth– In cycles per second of Hertz– Constrained by transmitter and medium
• Baud rate– Frequency with which the components change
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BPS vs. Baud
• Data rate rarely the same as baud rate
• Examples:
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Channel Capacity
• Nyquist– Maximum data rate of a noiseless channel =
– 2 H log2 V bps
– Where H = frequency– V = the number of discrete levels– Example: H = 4000 Hz, V = 2
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Channel Capacity
• Shannon (which includes noise)– Maximum data rate (in bps) = H log2 (1 + S/N)
– H = frequency
– S = signal power in watts
– N = noise power in watts
– Example: H = 3400 Hz, S = 0.2 w, N = 0.0002 w
– Max data rate = 3400 log2 (1+1000)
– = 3400 x 9.97
– = 33898 bps
36
Review Questions
• List the advantages and disadvantages of analog and digital
• What is the difference between data and signals?
• List the three components of a signal
• What is a composite signal?
• How do you calculate a dB?
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Review Questions
• How do you use the Nyquist formula?– 4000 Hz, 8 signal levels, data rate?– 50,000 bps data rate, 4000 Hz, how many
signal levels?
• How do you use the Shannon formula?– 8000 Hz, signal power = 20w, noise power =
0.002w, what is the data rate?– 5000 Hz, signal power = 50w, data rate =
20000bps, what is the possible noise power?