Lecture Notes on The Physical Layer

Lecture Notes on The Physical Layer - Chapter 2 The...

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Chapter 2 - The Physical Layer The physical layer is primarily concerned with physically transmitting data over some medium such as wire, optical fiber or electromagnetic. Usual concerns are: how far and how fast does the medium allow data transmission, how is data represented, and how efficiently is the medium utilized. 2.1 Theoretical Basis for Data Communication Computers operate on binary data. To be transmitted over some physical medium, binary data are generally converted into electrical or optical signals. Data - A binary 0 may be represented by a -12 volts and a binary 1 as +12 volts which the receiver of the data must interpret from voltage back to binary data. If the original signal transmitted becomes attenuated (e.g. lower voltage) or distorted (i.e. misshapen) by characteristics of the medium or external interference the electrical signal may be misinterpreted by the receiver. A few of the factors limiting the distance and data rate of a medium are discussed below. Capacitance - Wire and many other conductors act as capacitors, storing and releasing some of the energy over time rather than transmitting it instantaneously. The undesirable effect of capacitance on a signal is rounding and flattening, at high transmission rates the bits transmitted can run together by the time received. Capacitance generally increases as the length of the medium increases, placing an upper limit on the distance a signal can be transmitted. The figure at right shows cumulative capacitance, resistance and random noise effects on a square wave signal transmitted and received. Resistance - Electrical resistance is analogous to friction. Resistance consumes some portion of the power causing current to flow, converting the signal electrical energy into thermal energy or heat. Generally, the greater the length of the conductor, the greater the resistance and the more signal energy lost. This acts to limit communication distance as the power of the signal sent is dissipated and the signal attenuates. Amplifiers can compensate somewhat but amplify the signal and any noise. A repeater attempts to regenerate and transmit the original digital signal, in principal eliminating the noise.
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Noise - Noise distorts the original signal and can be caused by external sources such as motors, signals on nearby wires or thermal noise (i.e. random electron movement), etc. Amplitude - The height of a wave, usually represented on the y-axis. Represented by A. What is the maximum and minimum amplitude of the signal in the figure below? Waves - Data communication depends upon energy carried from the sender to the receiver. The amplitude of the energy level can be varied over time to represent 0's and 1's in digital communication, forming a wave. A single sine wave with amplitude between 1 and -1 is given in figure at right.
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