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 Time 1 Time 1
Both can send Both can send Both can send
Both do Only A does Neither does
Duplex
• Half-duplex transmission: only one can
transmit at a time; must take turns
– Still half duplex if neither transmits
A B A B
Time 1 Time 2
Only one side Only one side
Can send Can send
A does Neither does
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
Radio Propagation
• Broadcast signal
– Not confined to a wire
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
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
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
Radio Propagation: Waves
• Waves Frequency in hertz (Hz)
Cycles per Second
Wavelength One Second
(meters) 7 Cycles
Amplitude
(strength)
1 Hz = 1 cycle per second
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
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
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
FM Radio
Bands
AM Radio
0 Hz
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
Channel 2 Band
Channel 1
0 Hz
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
Channel 2 Band
Channel 1
0 Hz
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)
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
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 3.4 kHz 20 kHz
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 3.4 kHz 20 kHz
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