Even though the penetration of the fibre optical cables into telecommunication access networks seems to be accelerated, most of the customers are connected to the central office or to the telecommunication nodes by the copper wire pairs of the old telephone network. Until the replacement of the lines, their needs for modern high speed data communications services can be fulfilled by these symmetrical wire pairs. Equipment manufacturers produce equipments of the latest technologies for these networks (e.g. VDSL2), however pre-qualification of wire pairs of access networks is also essential, as the estimation of the performance gives the possibility to optimize the offered ensemble of services for telecommunications service providers. Our previous performance evaluation method based on fuzzy rule bases at discrete values of the insertion loss at 6 characteristic frequencies is improved by wavelet analysis. We have also proved, that the fine structure of the insertion loss spectrum does not play role in the achievable data transfer rate of the lines.