Computer Networks: The Physical Layer

2.1 The Theoretical Basis for Data Communication

Highlights

2.1.1 Fourier Analysis

Without getting too technical, the point of this subsection is to show the mathematical basis for the way we send signals over wire today. We currently send bits over a wire using signals, which can be made if a wire can have more than one state. That is, if we can change the voltage of a signal on a wire, then we can send binary data over a wire. Many things can be changed besides the voltage, such as frequency. There are many ways to improve upon the simple change of voltage for sending data across a wire. With a good understanding of Fourier Analysis, people can devise ways to put large amounts of information on the wire.

2.1.2 Bandwidth-Limited Signals

This subsection discusses the physical limitations of the wire that is used for transmission. Sending data over a voice-grade wire, as is often done nowadays to reach the Internet, is limited in the available bandwidth. As we will see in the next section, the bandwidth determines in part the data rate on a line.

2.1.3 The Maximum Data Rate of a Channel

Nyquist's Theorem and Shannon's Result are the most important things to take away from this subsection. Nyquist's Theorem says that there is a limit to the amount of data that can be put on a line at a time. That limit is related to bandwidth (in Hz) and bits per signal change. Shannon carrier Nyquist's Theorem one step further by showing how noise on a line can reduce the number of bits per second (bps) that a line is capable of sustaining.

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