- 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’
- 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
