Signals. Signals can be analog or digital. Analog signals can have an infinite number of values in a...
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Transcript of Signals. Signals can be analog or digital. Analog signals can have an infinite number of values in a...
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Signals
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Signals can be analog or digital. Analog signals can have an infinite number of values in a range; digital signals can have only a limited
number of values.
In data communication, we commonly use periodic analog signals and aperiodic digital signals.
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Frequency and period are inverses of each other.
Unit Equivalent Unit Equivalent
Seconds (s) 1 s hertz (Hz) 1 Hz
Milliseconds (ms) 10–3 s kilohertz (KHz) 103 Hz
Microseconds (ms) 10–6 s megahertz (MHz) 106 Hz
Nanoseconds (ns) 10–9 s gigahertz (GHz) 109 Hz
Picoseconds (ps) 10–12 s terahertz (THz) 1012 Hz
Frequency is the rate of change with respect to time. Change in a short span of time means high frequency. Change over a long span of time
means low frequency.
If a signal does not change at all, its
frequency is zero. If a signal changes
instantaneously, its frequency is infinite.
Phase describes the position of the waveform relative
to time zero.
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An analog signal is best represented in the frequency domain.
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A single-frequency sine wave is not useful in data communications; we need to change one or more of its characteristics to make it useful.
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According to Fourier analysis, any composite signal can be represented as a combination of simple sine waves with different frequencies,
phases, and amplitudes.
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The bandwidth is a property of a medium: It is the difference between the highest and the lowest frequencies that the medium can
satisfactorily pass.
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Example
If a periodic signal is decomposed into five sine waves with frequencies of 100, 300, 500, 700, and 900 Hz, what is the bandwidth? Draw the spectrum, assuming all components have a maximum amplitude of 10 V.
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B = fh - fl = 900 - 100 = 800 HzThe spectrum has only five spikes, at 100, 300, 500, 700, and 900
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A digital signal is a composite signal with an infinite bandwidth.
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• Transmission of digital signals– Baseband transmission
– Broadband transmission using modulation
Transmission of digital signals
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Transmission impairment• Attenuation
• Distortion
• Noise
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Performance
• Data rate limits– Shannon capacity BitRate ≤ bandwidth x log2(1+SNR)
• Throughput = transmitted bits per unit time• Latency = propagation time + transmission time
+ queueing time + processing delay– propagation time = distance / propagation speed– transmission time = message size / bandwidth’
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Bandwidth-Delay Product
• B-D product defines the number of bits that can fill the link (or the network)