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commtech[8] - Instructor:YorisA.Au,Ph.D Part2 1 8. All data...

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1 IS 3413 – Introduction to  Telecommunications for Business Instructor: Yoris A. Au, Ph.D. Data Communications Basics Part 2
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8.2  The Three Main Sources of Transmission Errors     2 All data communications systems are susceptible to errors some of the problems are inherent in the physics of the universe and some result either from devices that fail or from equipment that does not meet the engineering standards Small errors that occur during transmission are more difficult to detect than complete failures We need ways/mechanisms to control and recover from such errors There are three main categories of transmission errors: Interference electromagnetic radiation emitted from devices such as electric motors and background cosmic radiation can cause noise that can disturb radio transmissions and signals traveling across wires
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8.2  The Three Main Sources of Transmission Errors     3 Distortion All physical systems distort signals As a pulse travels along an optical fiber, the pulse disperses Wires have properties of capacitance and inductance that block signals at some frequencies while admitting signals at other frequencies Placing a wire near a large metal object can change the set of frequencies that can pass through the wire Metal objects can block some frequencies of radio waves, while passing others Attenuation As a signal passes across a medium, the signal becomes weaker signals on wires or optical fibers become weaker over long distances, just as a radio signal becomes weaker with distance
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8.2  The Three Main Sources of Transmission Errors     4 Shannon's Theorem suggests one way to reduce errors: increase the signal-to-noise ratio (either by increasing the signal or lowering noise if possible) Mechanisms like shielded wiring can help lower noise Physical transmission system is always susceptible to errors it may not be possible to change the signal-to-noise ratio Noise/interference cannot be eliminated completely But many transmission errors can be detected In some cases, errors can be corrected automatically We will see that error detection adds overhead Error handling is a tradeoff in which a system designer must decide whether a given error is likely to occur, and if so, what the consequences will be
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8.3  Effect of Transmission Errors on Data     5 Figure 8.1 lists the three principal ways transmission errors affect data The figure points out that an underlying transmission error often manifests itself as a specific data error For example, extremely short duration interference, called a spike , is often the cause of a single bit error Longer duration interference or distortion can produce burst errors Sometimes a signal is neither clearly 1 nor clearly 0 , but falls in an ambiguous region, which is known as an erasure
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6
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8.3  Effect of  Transmission  Errors on 
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