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Transmission Media

Transmission Media. Transmission of Information. From physics Energy Electromagnetic wave propagation From mathematics Coding theory. Transmission Media. Copper wire Need two wires Possibilities Twisted pair Coaxial Cable Optical Fiber Flexible Light “stay in” Air/space

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Transmission Media

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  1. Transmission Media

  2. Transmission of Information • From physics • Energy • Electromagnetic wave propagation • From mathematics • Coding theory

  3. Transmission Media • Copper wire • Need two wires • Possibilities • Twisted pair • Coaxial Cable • Optical Fiber • Flexible • Light “stay in” • Air/space • Used for electromagnetic transmission

  4. Twisted pair wiring

  5. Coaxial Cable

  6. Type of Satellite • Geosynchronous Satellite • Low Earth Orbit • Low Earth Orbit Satellite Arrays

  7. Satellite

  8. Two important Physical Limits of a Transmission System • Propagation Delay • Time required for signal to travel across media • Example: electromagnetic radiation travels through space at the speed of light (3 * 108 meters per second) • Bandwidth • Maximum times per second the signal can change

  9. Transmission of Data • Network hardware encodes information for transmission • Two types of encoding • Analog (amount of energy proportional to the value of item sent) • Digital (two forms of energy to encode 0 and 1) • Computer networks use the latter

  10. Example Digital Encoding • Medium • Copper wire • Energy form • Electric current • Encoding • Negative voltage encodes 1 • Positive voltage encodes 0

  11. Illustration of Digital Encoding • Known as waveform diagram • X-axis corresponds to time • Y-axis corresponds to voltage

  12. Encoding Details • All details specified by a standard • ASCII standard • Several organizations produce networking standards • IEEE • Institute for Electrical and Electronic Engineers • ITU • International Telecommunications Union • EIA • Electronic Industries Association • Standard RS-232-C • Hardware that adheres to standard interoperable

  13. Standard RS-232-C • Example use • Connection to keyboard/mouse • Serial port on PC • Voltage +15 or -15 • Cable limit to < 50 feet • Uses asynchronous communication • When the transmitter has nothing to send, it leaves the wire with a negative voltage that corresponds to bit value 1

  14. Asynchronous Communication • Sender and receiver must agree on • Number of bits per character • Duration of each bit • Receiver • Does not know when a character will arrive • May wait forever • To ensure meaningful exchange • Start bit before character • One or more stop bits after character

  15. Illustration of RS-232 • Start bit • Same as 0 • Not part of data • Stop bit • Same as 1 • Follows data

  16. Duration of a bit in RS-232C • Determined by baud rate • Example baud rate: 9.6Kbaud, 28.8Kbaud, 33.6Kbaud • Duration of bit is 1/baud_rate • Sender and receiver must agree a priori • Received samples signal • Disagreement results in framing error

  17. Two Way Communication • Desirable in practice • Requires each side to have transmitter and receiver • Called full duplex • Distinguished from half duplex transmission (simplex transmission)

  18. Illustration of Full-Duplex Communication • Transmitter on one side connected to receiver on other side • Separate wires needed to carry current in each direction • Common ground wire • DB-9, DB-15, or DB-25 connector used • Pin 2 is transmit • Pin 3 is receive • A computer transmits on pin 2 and receives on pin 3 • A modem transmits on pin 3 and receives on pin 2

  19. Electrical Transmission • It’s is an ugly world • Electrical energy dissipates as it travels along • Wires have resistance, capacitance, and inductance which distort signals • Magnetic or electrical interference distort signals • Distortion can result in loss or misinterpretation

  20. Illustration of Distorted Signal For a Single Bit • In practice • Distortion can be much worse than illustrated

  21. Consequences • RS-232 hardware must handle minor distortions • Take multiple samples per bit • Tolerate less than full voltage • Cannot use electrical current for long-distance transmission

  22. Nyquist Intersymbol Interference Theorem • Nyquist’s theorem states that the maximum data rate in bits per second that can be achieved over a transmission system of bandwidth B is 2B • If the transmission system uses K possible values of voltage instead of two, D denotes the maximum data rate in bits per second, then: D = 2Blog2K

  23. Shannon’s Theorem • Noise: Background interference • C = Blog2(1+S/N) • C: effective limit on the channel capacity in bits per second • B: hardware bandwidth • S: average signal power • N: average noise power • Example: Voice telephone system • S/N ~ 1000, B = 3000Hz • C ~ 30,000bps

  24. The Bottom Line • Nyquist’s theorem means finding a way to encode more bits per cycle improves the data rate • Shannon’s theorem means that no amount of clever engineering can overcome the fundamental physical limits of a real transmission system

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