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A little more complex Comm System

A little more complex Comm System


  • User of a PC wishes to send a message ‘m’
  • User activates electronic mail package e.g hotmail
  • Enters the message via input device (keyboard)
  • Character string is buffered in main memory as a sequence of bits ‘g’
  • PC is connected to some trans system such as a Telephone Network via an I/O Transmitter like Modem
  • Transmitter converts incoming stream ‘g’ into a signal ‘s’
RECEIVER SIDE
  • The transmitted signal ‘s’ is subject to a number of impairments depending upon the medium
  • Therefore, received signal ‘r’ may differ from ‘s’.
  • Receiver attempts to estimate original ‘s’ based on its knowledge of the medium and received signal ‘r’
  • Briefly buffered in the memory
  • Data is presented to the user via an output device like printer, screen etc.
  • The data viewed by user m’ will usually be an exact copy of the data sent ‘m’
  • Receiver produces a bit stream g’(t)
  • Briefly buffered in the memory
  • Data is presented to the user via an output device like printer, screenetc.
  • The data viewed by user m’ will usually be an exact copy of the data sent ‘m’
EXAMPLE-Telephone System
  • Input to the Telephone is a message ‘m’ in the form of sound waves
  • The sound waves are converted by telephone into electric signals of the same frequency
  • These signals are transmitted w/o any modification over the telephone line
  • Hence g(t) and s(t) are identical
  • S(t) will suffer some distortion so that r(t) will not be the same as s(t)
  • R(t) is converted back to sound waves with no attempt of correction or improvement of signal quality
  • Thus m’ is not an exact replica of m