Duplex

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
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

• Shannons 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 (1S/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
20 kHz
300 Hz
3.4 kHz
17
Telephony is Narrowband
Error in Book

• 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
20 kHz
300 Hz
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