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!
Jitter: Amplitude and Phase
There are 2 types of Jitter:
Amplitude Jitter is the small constantly changing swings in the amplitude of a signal. It is principally caused by power supply noise (60 Hz) and ringing tone (20 Hz) on the signal.
Phase Jitter is the small constantly changing swings in the phase of a signal. It may result in the pulses moving into time slots allocated other data pulses when used with Time Domain Multiplexing.
Telephone company standards call for no more than 10 degrees between 20 and 300 Hz and no more than 15 degrees between 4 and 20 Hz.
Transients: Impulse Noise, Gain Hits, Dropouts & Phase Hits
Transients are irregular timed impairments. They appear randomly and are very difficult to troubleshoot. There are 4 basic types of Transients:
i. Impulse Noise
Impulse noise is sharp quick spikes on the signal caused from electromagnetic interference, lightning, sudden power switching, electromechanical switching, etc.. These appear on the telephone line as clicks and pops which are not a problem for voice communication but can appear as a loss of data or even as wrong data bits during data transfers. Impulse noise has a duration of less than 1 mSec and their effect is dissipated within 4 mSec.
ii. Gain Hits
Gain Hits are sudden increase in amplitude that last more than 4 mSec. Telephone company standards allow for no more than 8 gain hits in any 15 minute interval. A gain hit would be heard on a voice conversation as if the volume were turned up for just an instance. Amplitude modulated carriers are particularly sensitive to Gain Hits.
Dropouts are sudden loss of signal amplitude greater than 12 db that last longer than 4 mSec. They cause more errors than any other type of transients. Telephone company standards allow no more than 1 dropout for every 30 minute interval. Dropouts would be heard on a voice conversation similar to call waiting, where the line goes dead for a 1/2 second. This is a sufficient loss of signal for some digital transfer protocols such as SLIP, that the connection is lost and would have to be re-established.
iv. Phase Hits
Phase Hits are sudden large changes in the received signal phase (20 degrees) or frequency lasting longer than 4 mSec. Phase Hits generally occur when switching between Telcos, common carriers or transmitters. FSK and PSK are particularly sensitive to Phase Hits. The data may be incorrect until the out of phase condition is rectified. The telephone company standard allows no more than 8 phase hits in any 15 minutes.
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