Important Concepts - Local Area Network - Lecture Slides, Slides of Network security

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Important Concepts at the

Physical Layer

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Transmission Media

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Transmission Signal

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A Signal Is Made of Many Frequencies

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Frequency, Spectrum, Bandwidth

• The spectrum of a signal
  • The range of frequencies that it contains. (can be infinite.)
• The absolute bandwidth of a signal - The width of the spectrum
• The (effective) bandwidth of a signal - The band of frequencies where most of the energy of the signal is contained.

BW = 3f – f = 2f

3f

f

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 For a given data rate (bits/sec), if we are willing to use more bandwidth to send a signal, its quality at the receiver will be better, and the receiver can thus more correctly interpret the transmitted bits.

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Transmission Impairments Limit a Link’s Bandwidth

• Attenuation and attenuation distortion
  • Different frequency components are attenuated at different factors.

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• Delay distortion
  • Different frequency components are delayed at different factors.

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• Multipath interference (wireless broadcast)

 The same signal may be reflected and propagated along multiple different paths.  These signals may take different times to arrive at the receiver. (delay distortion)  When they arrive at the receiver, their signal strengths may vary a lot. (attenuation distortion)

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Channel Capacity

• Data rate, bandwidth, noise, and error rate are closely related.
• Nyquist bandwidth
  • C = B log 2 (M), M is the number of discrete signals or voltage levels.
• Shannon capacity
  • C = B log 2 ( 1 + SNR), SNR is signal to noise ratio
  • (SNR) (^) db = 10 log 10 (signal power/noise power) 2- 14

Data Encoding

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• Digital data -> digital signal
  • E.g. Ethernet (our focus)
• Digital data -> analog signal
  • E.g., Modem
• Analog data -> digital signal
  • Voice/Video over IP
• Analog data -> analog signal
  • E.g. AM/FM

The Four Different Applications Are

All Possible

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802.3 Ethernet

802.5 token ring

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No way to know when a string of bits has started or ended if a string of 0s is transmitted.

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