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 AB Time 1 Both can send Both do AB Time 1 Both can send Only A does AB Time 1 Both can send Neither does

2. Duplex Half-duplex transmission: only one can transmit at a time; must take turns –Still half duplex if neither transmits ABAB 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 Amplitude (strength) Wavelength (meters) Frequency in hertz (Hz) Cycles per Second One Second 7 Cycles 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. 0 Hz Cellular Telephone FM Radio AM Radio Service Bands

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 0 Hz Channel 3 Channel 2 Channel 1 Service Band Channel Bandwidth

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) 0 Hz Channel 3 Channel 2 Channel 1 Service Band Channel Bandwidth

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 Log 2 (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 Log 2 (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 20 kHz300 Hz 3.4 kHz 3.1 kHz

17. 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 20 kHz300 Hz 3.4 kHz 3.1 kHz Error in Book Required Radio Channel

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