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Signals

Signals. 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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Signals

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  1. Signals

  2. 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.

  3. Frequency and period are inverses of each other. 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. 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.

  4. An analog signal is best represented in the frequency domain.

  5. 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.

  6. According to Fourier analysis, any composite signal can be represented as a combination of simple sine waves with different frequencies, phases, and amplitudes.

  7. 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.

  8. 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.

  9. B = fh-fl = 900 - 100 = 800 Hz The spectrum has only five spikes, at 100, 300, 500, 700, and 900

  10. A digital signal is a composite signal with an infinite bandwidth.

  11. Transmission of digital signals • Transmission of digital signals • Baseband transmission • Broadband transmission using modulation

  12. Transmission impairment • Attenuation • Distortion • Noise

  13. 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’

  14. Bandwidth-Delay Product • B-D product defines the number of bits that can fill the link (or the network)

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