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