Introduction to Data Communications
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The attenuation is due to the many pieces of electronic equipment and transmission media that the signal has to pass through, some can amplify the signal (make it a larger amplitude) and some may attenuate the signal (make it smaller).

There are maximum and minimum acceptable limits for Attenuation Distortion for phone lines. The Basic channel conditioning is:

Frequency RangeLoss (db)
500 - 2500-2 to +8
300 - 3000-3 to +12

The above Loss is a range of acceptable values for the frequency range. In the Basic Channelling Conditioning, it is acceptable to have a loss in signal in the frequency range of 500-2500 Hz of "8 db loss to -2 db loss" referenced to the amplitude at 1 kHz. Note that on the graph on the previous page that this is shown as -8db and +2 db.

+3 db attenuation is equal to -3 db in signal amplitude and +8 db attenuation equates to -8 db in signal amplitude.

25b. Propagation Delay

Signals transmitted down a phone line will take a finite time to reach the end of the line. The delay from the time the signal was transmitted to the time it was received is called Propagation Delay. If the propagation delay was the exact same across the frequency range, there would be no problem. This would imply that all frequencies from 300 to 3000 Hz have the same amount of delay in reaching their destination over the phone line. They would arrive at the destination at the same time but delayed by a small amount called the propagation delay.

This is heard as the delay when talking on long distance telephones. We have to wait a little longer before we speak to ensure that the other person hasn't already started to talk. All phone lines have propagation delay.

If the Propagation Delay is long enough, the modem or communications package may time-out and close the connection. It may think that the receive end has shut off!

Introduction to Data Communications
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