Line Impairments are faults in the line that occur due to either improper line terminations or equipment out of specifications. These cannot be conditioned out, but can be measured to determine the amount of the impairment.
Crosstalk
Crosstalk is when one line induces a signal into another line. In voice communications, we often hear this as another conversation going on in the background. In digital communication, this can cause severe disruption of the data transfer. Cross talk can be caused by the overlapping of bands in a multiplexed system, or by poor shielding of cables running close to one another. There are no specific communications standards that are applied to the measurement of crosstalk.
26b. Echo or Signal Return
All media have a preferred termination condition for perfect transfer of signal power. The signal arriving at the end of a transmission line should be fully absorbed, otherwise it will be reflected back down the line to the sender (and appear as an Echo). Echo Suppressors are often fitted to transmission lines to reduce this effect.
Usually during data transmission, these suppressors must be disabled or they will prevent return communication in full duplex mode. Echo suppressors are disabled on the phone line if they hear carrier for 400ms or more. If the carrier is absent for 100 mSec, the echo suppressor is re-enabled.
Echo Cancellers are currently used in Modems to replicate the echo path response. These cancellers then combine the results to eliminate the echo (thus, no signal interruption is necessary).
Frequency Shift
Frequency shift is the difference between the transmitted frequency and the received frequency. This is caused by the lack of synchronization of the carrier oscillators.
Nonlinear Distortion
Nonlinear distortion changes the wave shape of the signal. If the signal was transmitted as a sine wave (and arrived as a square wave), it would be an example of severe nonlinear distortion. Amplitude modulated carriers would suffer drastically if the original wave shape was distorted.
Jitter: Amplitude and Phase
Here are the 2 types of Jitter:
Amplitude Jitter
Phase Jitter
Amplitude Jitter is the small constantly changing swing 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 swing in the phase of a signal. It may result in the pulses moving into time slots that are allocated to 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.
Impulse Noise
Gain Hits
Dropouts
Phase Hits
i. Impulse Noise
Impulse noise is a sharp and quick spike on the signal that can come from many sources: electromagnetic interference, lightning, sudden power switching, electromechanical switching, etc.. These appear on the telephone line as clicks and pops: they're 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 increases 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.
iii. Dropouts
Dropouts are sudden losses of signal amplitude that are greater than 12 db, and 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 can 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), where the connection is lost and would then have to be re-established.
iv. Phase Hits
Phase Hits are either a sudden--and large--change in the received signal phase (20 degrees), or a frequency that lasts 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 minute period.
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