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

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

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

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

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

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

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.

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

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.

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.

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.

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.

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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