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Lines Impairment Factors

Twisted pairs have been in use long before the development of DSL, and were mainly designed for use as telephone lines (or POTS ). People back then didn't have in mind that it would someday be used for digital transmission. That's why many researchers and manufacturers nowadays are looking for solutions to some of the impairments caused to DSL lines, causing decrease in data rate and limiting the capabilities of modern DSL.

This section will describe some of the factors causing impairments on DSL lines, as well as possible solutions.

Bridge Taps

any portion of a loop that is not in the direct talking path between the central office and the consumer's
terminating equipment.
In the local loop they appear when the local phone company "taps" off an existing pair to provision a new
service to a new subscriber. Typically, they do not remove the unused cable segment and a bridged tap
is created. In the home, every unused phone jack represents a bridged tap.
A bridged tap creates an impedance mismatch within the transmission line, which creates signal
reflections. These reflections are generally not noticed in standard (POTS) but become significant with
high frequency such as xDSL, and might eventually result in signal loss and distortion. The distortion will be the maximum when the length of the bridge tap is equal to one quarter of the wavelength.

Possible solution : generally bridged taps are not welcome in a DSL environment and should be removed when possible. Otherwise, using an adaptive equalizer or A POTS splitter to isolates the house wiring and provides a direct path for the DSL signal to the modem should do the trick.

Impulse Noise

A short random burst of energy noise, that can cause data transmission errors. Examples of such impulse noise are ringing of the telephone, turning switch on and off, etc.
Possible solution : using Interleaving or special filters.

Line Attenuation

Reduction in the strength of a signal going through a transmission line, caused by the long distance of a loop. The longer the loop is - the line gets more attenuated. Also - higher frequencies are more vulnerable and are greatly attenuated.
Possible solution : basically the shorter we can keep the loop - the better. Employing DLCs (data loop carriers) might extend the loop and decrease attenuation.

RF Interference

RF interference from cordless phones, amatur radio signals, and other sources transmitting RF "over the air" which might share with the frequencies used by the xDSL method, can cause data errors and transmission delays.
Possible solution: using filters, or if possible choose a spectrum which has the least outside interferences.

Crosstalk

A telephone cable is combined from several twisted pairs. Crosstalk is signal interference between adjacent pairs on the same line or nearby cables. Usually caused by the electric and magnetic field generated by the current runinng through the pair of wires, the result is signal moving cross-side from one pair to another causing noise.
There are 2 types of crosstalk:

NEXT - Near End crossTalk, where a transmitter induces noise onto the nearby receiver on the same end of the cable, causing noise between signals traveling in opposite directions.
Possible solution : separated upstream and downstream channels, as DMT does.
FEXT - Far End crossTalk, where a transmitter induces noise onto the receiver on the other side of the cable, causing noise between signals traveling in the same direction.
Possible solution : reduce transmission power wisely, in regard to the length of the cable.

Figure 1. Crosstalk showing FEXT and NEXT

Next: xDSL types