Physical Layer .pdf - CSE4344 – Computer Networks and Distributed Systems The Physical Layer Dr Padmaraj Nair Maximum Data Rate of a Channel •

Physical Layer .pdf - CSE4344 – Computer Networks and...

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CSE4344 – Computer Networks and Distributed Systems The Physical Layer Dr. Padmaraj Nair
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Maximum Data Rate of a Channel Nyquist’s theorem relates the data rate to the bandwidth (B) and number of signal levels (V): Shannon's theorem relates the data rate to the bandwidth (B) and signal strength (S) relative to the noise (N): Max. data rate = 2B log 2 V bits/sec Max. data rate = B log 2 (1 + S/N) bits/sec How fast signal can change How many levels can be seen
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Guided Transmission (Wires & Fiber) Media have different properties, hence performance Reality check Storage media » Wires: Twisted pairs » Coaxial cable » Power lines » Fiber cables »
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Wires – Twisted Pair Very common; used in LANs, telephone lines Twists reduce radiated signal (interference) Category 5 UTP cable with four twisted pairs
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Link Terminology Full-duplex link Used for transmission in both directions at once e.g., use different twisted pairs for each direction Half-duplex link Both directions, but not at the same time e.g., senders take turns on a wireless channel Simplex link Only one fixed direction at all times; not common
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Wires – Coaxial Cable (“Co-ax”) Also common. Better shielding and more bandwidth for longer distances and higher rates than twisted pair.
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Wires – Power Lines Household electrical wiring is another example of wires Convenient to use, but horrible for sending data
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Fiber Common for high rates and long distances Long distance ISP links, Fiber-to-the-Home Light carried in very long, thin strand of glass A miraculous technology without which the Internet couldn't exist in its present form Narinder Singh Kapany is known as the father of fiber optics Light source (LED, laser) Photodetector Light trapped by total internal reflection
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Fiber Fiber has enormous bandwidth (THz) and tiny signal loss – hence high rates over long distances
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Fiber Single-mode Core so narrow (10um) light can’t even bounce around Used with lasers for long distances, e.g., 100km Multi-mode Other main type of fiber Light can bounce (50um core) Used with LEDs for cheaper, shorter distance links Fibers in a cable
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Comparison of the properties of wires and fiber: Fiber Cables Property Wires Fiber Distance Short (100s of m) Long (tens of km) Bandwidth Moderate Very High Cost Inexpensive Less cheap Convenience Easy to use Less easy Security Easy to tap Hard to tap
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Wireless Transmission Electromagnetic Spectrum » Radio Transmission » Microwave Transmission » Light Transmission » Wireless vs. Wires/Fiber »
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Electromagnetic Spectrum Different bands have different uses: Radio: wide-area broadcast; Infrared/Light: line-of-sight Microwave: LANs and 3G/4G; Microwave Networking focus
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