Impulse noise is created by several mechanisms in which transients are induced into the loop by nearby sources. Such sources include dial pulse, electromechanical switches in the office, power line transients, current surges in heavy equipment such as elevators, and other devices in the home or office. Impulse noise tends to be isolated with short pulses of energy, in the order of microseconds. If the magnitude of the impulse is large enough, it can cause errors in data transmission. Impulse noise is a major impairment for ADSL systems, especially due to the heavy ADSL subscriber loop loss. One method of minimising the effects of impulse noise is by forward error corrective coding.
NEXT is brought about when signals transmitting toward the far end are coupled with the weak received signals originating from the far end.
FEXT is brought about when signals transmitting from the far end of one twisted pair is coupled with the weak received signals originating from the far end of another twisted pair. For ADSL systems, it turns out that FEXT is the predominant cross talk.
Figure 8. Far-end and Near-end.
The attenuation of twisted pairs increases in proportion to the length of the line and its operating frequency. The diameter of the wire (gauge) also affects the attenuation of the signal. Taking into consideration the channel impairments described, ADSL has a range of downstream speeds achievable depending on the distance and wire gauge:
