ls from the far end of a twisted-pair couple with the weak received signals from the far end of another twisted pair. This is a dominant impairment in ADSL. Near-end crosstalk occurs when signals transmitted towards the far end couple with weak signals originating from the far end. A telecoms engineer from fifty years ago would be surprised and certainly impressed by what can now be squeezed along two pieces of copper wire. Many of the techniques used are, however, based on prior art used in legacy systems and close examination of the components used would reveal more similarities than differences. So, for example, the challenge today is to achieve higher data rates (capacity gain) by using higher frequencies in the twisted-pair local loop. The challenge in a legacy network has traditionally been to realise more voice capacity, for example on the coaxial circuits feeding the local loop. A 9.5-mm coaxial cable carrying 2700 voice channels would typically have a 6-dB per 2 kilometre propagation loss at 300 kHz and a 40-dB loss at 12.5 MHz. This required (still requires today in legacy networks) attenuation equalisation and phase equalisation. Attenuation equalisation is needed to avoid strong signals swamping weak signals. Phase equalisation is needed to mitigate the effects of group delay caused either by the propagation medium or by filters or repeater amplifiers. Amplifiers also of course introduce noise and create intermodulation. As with coaxial cable, the capacity-distance product for a twisted pair is a function of wire size and data rate. A DSL (digital subscriber line) example is shown in Table 2.8. The length and the quality of the local loop determine the upper limit of the frequencies that can be used, which in turn increases the capacity. Distance can be increased by using repeater amplifiers. Apparent capacity gain is also achieved by configuring the upstream/downstream to be asymmetric in the downstream direction, matching bandwidth availability to an assumed asymmetric traffic demand. Hence, the ter