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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

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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?