media€¦ · types of media •mediapropagate signalsthat carry bits •we’ll look at some...
TRANSCRIPT
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Media
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Types of Media
•Media propagate signals that carry bits
•We’ll look at some common types:•Wires• Fiber (fiber optic cables)•Wireless
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Wires – Twisted Pair
•Very common; used in LANs and telephone lines• Twists reduce radiated signal
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Category 5 UTP cable with four twisted pairs
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Wires – Coaxial Cable
•Also common. Better shielding for better performance
•Other kinds of wires too: e.g., electrical power (§2.2.4)
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Fiber
• Long, thin, pure strands of glass• Enormous bandwidth (high speed) over long distances
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Light source(LED, laser)
Photo-detector
Light trapped bytotal internal reflection
Optical fiber
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Wireless
•Sender radiates signal over a region• In many directions, unlike a wire, to potentially many
receivers• Nearby signals (same freq.) interfere at a receiver; need to
coordinate use
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Wireless Interference
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WiFi
WiFi
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Wireless Bands
•Unlicensed (ISM) frequencies, e.g., WiFi, are widely used for computer networking
802.11b/g/n
802.11a/g/n
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Multipath
•Signals bounce off objects and take multiple paths• Some frequencies attenuated at receiver, varies with
location
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Many Other Types of Impact on Wireless
•Wireless propagation is complex, depends on environment
•Some key effects are highly frequency dependent, • E.g., multipath at microwave frequencies
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Fundamental Limits
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Topic
•How rapidly can we send information over a link? • Nyquist limit (~1924)• Shannon capacity (1948)
•Practical systems attempt to approach these limits
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Key Channel Properties
•The bandwidth (B), signal strength (S), and noise (N)• B (in hertz) limits the rate of transitions• S and N limit how many signal levels we can distinguish
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Bandwidth B Signal S,Noise N
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Nyquist Limit
•The maximum symbol rate is 2B
•Thus if there are V signal levels, ignoring noise, the maximum bit rate is:
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R = 2B log2V bits/sec
1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1
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Claude Shannon (1916-2001)
•Father of information theory• “A Mathematical Theory of
Communication”, 1948
•Fundamental contributions to digital computers, security, and communications
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Credit: Courtesy MIT Museum
Electromechanical mouse that “solves” mazes!
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Shannon Capacity
•How many levels we can distinguish depends on S/N• Or SNR, the Signal-to-Noise Ratio• Note noise is random, hence some errors
•SNR given on a log-scale in deciBels:• SNRdB = 10log10(S/N)
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0
1
2
3
N
S+N
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Shannon Capacity (2)
•Shannon limit is for capacity (C), the maximum information carrying rate of the channel:
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C = B log2(1 + S/N) bits/sec
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Shannon Capacity Takeaways
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C = B log2(1 + S/N) bits/sec
• There is some rate at which we can transmit data without loss over a random channel
• Assuming noise fixed, increasing the signal power yields diminishing returns : (
• Assuming signal is fixed, increasing bandwith increases capacity linearly!
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Wired/Wireless Perspective (2)
• Wires, and Fiber• Engineer link to have requisite SNR and B→Can fix data rate
• Wireless• Given B, but SNR varies greatly, e.g., up to 60 dB!→Can’t design for worst case, must adapt data rate
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Engineer SNR for data rate
Adapt data rate to SNR
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Putting it all together – DSL
•DSL (Digital Subscriber Line) is widely used for broadband; many variants offer 10s of Mbps• Reuses twisted pair telephone line to the home; it has up
to ~2 MHz of bandwidth but uses only the lowest ~4 kHz
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DSL (2)
•DSL uses passband modulation (called OFDM)• Separate bands for upstream and downstream (larger)
•Modulation varies both amplitude and phase (QAM)
• High SNR, up to 15 bits/symbol, low SNR only 1 bit/symbol
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Upstream Downstream
26 – 138kHz
0-4kHz 143 kHz to 1.1 MHz
Telephone
Freq.
Voice Up to 1 Mbps Up to 12 Mbps
ADSL2:
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Phy Layer Innovation Still Happening!
● Backscatter “zero power” wireless● mm wave 30GHz+ radio equipment● Free space optical (FSO)● Cooperative interference management● Massive MIMO and beamforming● Powerline Networking