onal bandwidth. The wider bandwidths will require higher sampling rates, which create DAC/ADC performance challenges that will be compounded by the additional clock cycles needed for the IFFT/FFT transform as the number of tones increase. But the real problem will be the crosstalk created as new high-bandwidth services are introduced that have to coshare with legacy services. The potential for crossinterference is highlighted in Table 2.10. There are effectively two constraint mechanisms - the fundamental propagation constraints of the medium and the unwanted crosstalk between services cosharing the medium. Note that the downstream data rates for VDSL derive from submultiples of SONET (syn- chronous optical network) and SDH (synchronous digital hierarchy) data rates that presumably would imply that 155.52 Mbps would be the next benchmark to aim for. VDSL is, however, presently configured as symmetric bandwidth. These higher data rate channels are susceptible to channel time dispersion (CDT) and frequency-dependent noise (FDN) and will degrade the performance of other legacy services sharing the medium. There are, however, various techniques that can be applied to mitigate these effects. These can be illustrated by tracing through the processing steps in the trans- mit/receive chain. In the transmitter an error-correction code such as Reed-Solomon is used to encode the high-speed data stream and then extracted into lower-speed data substreams that are then placed on each subcarrier using quadrature amplitude modulation. The number of carriers to be used is determined during bit loading and then transformed into a time domain signal using an inverse discrete Fourier transform (IDFT). A cyclic extension is added to mitigate channel dispersion effects. In the receiver the time-domain signal is equalised and a discrete Fourier transform brings the signal back into the frequency domain. Frequency equalisation is then performed on each carrier and then demodulated and passed on to the error decoder. Splitting the high d