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The Physical Layer What kinds of signals can encode data? What kinds of media can carry these signals?

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Data vs. signals When talking broadly about networks, what we’re interested in is how data is sent. Data is the meaningful information generated by some device. A signal is the electronic encoding of data that is sent across some physical medium

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Types of data Digital data is data that comes in discreet values, such as 0 and 1. Analog data has continuous values

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Types of signals By extension, analog signals can have continuous values. Digital signals have a limited number of possible values.

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The Sine Wave The foundation of almost all analog signals is the sine wave. Any over-the-air signal is an analog signal.

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Describing a regular sine wave Amplitude - The height of the wave (volts) Frequency - How many cycles per second the wave goes through (Hertz) Phase - How far the wave is shifted from the zero point (degrees). Wavelength - The distance in space covered by one cycle of the wave (meters). A, F, and P completely describe any sine wave.

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Composite signals Simple signals cannot carry much information. Composite signals are generated by adding together a set of simple signals.

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More composite signals Periodic composite signals are generated by adding together other periodic signals (as in the example from the previous slide). Non-periodic composite signals are generated by the addition of an infinite number of sine waves, as in the human voice.

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Bandwidth An infinite number of frequencies is not the same thing as an infinite range of frequencies. The human voice can produce any frequency between 0 and 4 kHz. There are an infinite number of frequencies in this range, but the range is bounded. The bandwidth of a signal is the range of frequencies contained within that signal.

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Digital signals Analog signals are continuous in that they change in smooth, continuous ways. Digital signals are discreet. They change in abrupt ways. Simple digital signals have two values - on and off. More complicated signals can have multiple levels, where each level encodes multiple bits.

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Describing digital signals Almost all digital signals are non-periodic (the clock signal on your processor chip being the major exception). Thus, frequency has no meaning here. The bit rate of a digital signal is how many bits the signal can send in one second The bit length is the physical space occupied by one bit.

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Creating a digital signal At a very fundamental level, all signals are analog signals. How do we get a digital signal? A composite analog signal!

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CMP206 – Introduction to Data Communication & Networks Lecture 2 – Signals.

CMP206 – Introduction to Data Communication & Networks Lecture 2 – Signals.

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