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Duplex

Duplex. Full-duplex transmission: both sides can transmit simultaneously Even if only one sends, still full-duplex line Even if neither is sending, still full-duplex line. A. B. A. B. A. B. Time 1 Both can send Both do. Time 1 Both can send Only A does. Time 1 Both can send

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Duplex

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  1. Duplex • Full-duplex transmission: both sides can transmit simultaneously • Even if only one sends, still full-duplex line • Even if neither is sending, still full-duplex line A B A B A B Time 1 Both can send Both do Time 1 Both can send Only A does Time 1 Both can send Neither does

  2. A B A B Duplex • Half-duplex transmission: only one can transmit at a time; must take turns • Still half duplex if neither transmits Time 1 Only one side Can send A does Time 2 Only one side Can send Neither does

  3. Duplex • Duplex is a Characteristic of the Transmission System, Not of Use at a Given Moment • In full duplex, both sides can transmit at once; in half duplex, only one side can transmit at a time • Still full duplex system if only one side or neither side actually is transmitting at a moment • Still half duplex if neither side actually is transmitting at a moment

  4. Radio Propagation • Broadcast signal • Not confined to a wire

  5. Radio Waves • When Electron Oscillates, Gives Off Radio Waves • Single electron gives a very weak signal • Many electrons in an antenna are forced to oscillate in unison to give a practical signal

  6. Radio Propagation Problems • Wires Propagation is Predictable • Signals go through a fixed path: the wire • Propagation problems can be easily anticipated • Problems can be addressed easily • Radio Propagation is Difficult • Signals begin propagating as a simple sphere • But they can be blocked • There are shadow zones Shadow Zone

  7. Radio Propagation Problems • Radio Propagation is Difficult • Signals are reflected • May arrive at a destination via multiple paths • Signals arriving by different paths can interfere with one another • This is called multipath interference

  8. Radio Propagation: Waves • Waves Frequency in hertz (Hz) Cycles per Second One Second 7 Cycles Wavelength (meters) Amplitude (strength) 1 Hz = 1 cycle per second

  9. Radio Propagation: Frequency Spectrum • Frequency Spectrum • Frequencies vary (like strings in a harp) • Frequencies measured in hertz (Hz) • Frequency spectrum: all possible frequencies from 0 Hz to infinity 0 Hz

  10. Frequencies • Metric system • kHz (1,000 Hz) kilohertz; note lower-case k • MHz (1,000 kHz) megahertz • GHz (1,000 MHz) gigahertz • THz (1,000 GHz) terahertz

  11. Radio Propagation: Service Bands • Service Bands • Divide spectrum into bands for services • A band is a contiguous range of frequencies • FM radio, cellular telephone service bands etc. Cellular Telephone Service Bands FM Radio AM Radio 0 Hz

  12. Radio Propagation: Channels and Bandwidth • Service Bands are Further Divided into Channels • Like television channels • Bandwidth of a channel is highest frequency minus lowest frequency Channel Bandwidth Channel 3 Service Band Channel 2 Channel 1 0 Hz

  13. Radio Propagation: Channels and Bandwidth • Example • Highest frequency of a radio channel is 43 kHz • Lowest frequency of the radio channel is 38 kHz • Bandwidth of radio channel is 5 kHz (43-38 kHz) Channel Bandwidth Channel 3 Service Band Channel 2 Channel 1 0 Hz

  14. Radio Propagation: Channels and Bandwidth • Shannon’s Equation • W is maximum possible (not actual) transmission speed in a channel • B is bandwidth of the channel: highest frequency minus lowest frequency • S/N is the signal-to-noise ratio W = B Log2 (1 + S/N)

  15. Radio Transmission: Broadband • Speed and Bandwidth • The wider the channel bandwidth (B), the faster the maximum possible transmission speed (W) • W = B Log2 (1+S/N) Maximum Possible Speed Bandwidth

  16. Telephony is Narrowband • Bandwidth in Telephone Channels is Narrow • Sounds below about 300 Hz cut off to reduce equipment hum within telephone system • Sounds above about 3,400 Hz cut off to reduce the bandwidth needed to send a telephone signal 3.1 kHz 300 Hz 20 kHz 3.4 kHz

  17. Error in Book Telephony is Narrowband • Bandwidth in Telephone Channels is Narrow • A radio channel would have to be from 0 to 3.4 kHz (3.4 kHz) • This would mean a maximum possible transmission speed of about 35 kbps Required Radio Channel 3.1 kHz 300 Hz 20 kHz 3.4 kHz

  18. Broadband • Two Uses of the Term “Broadband” • Technically, the signal is transmitted in a single channel AND the bandwidth of the channel is large • Therefore, maximum possible transmission speed is high • Popularly, if the signal is fast, the system is called “broadband” whether it uses channels at all

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