4.1 The issues

DSL systems do not live in isolation from their neighbours. We have seen in Chapter 3 that in fact there is crosstalk between systems operating in the same part of the access network, and that this crosstalk can be categorised into two important types, near end crosstalk (NEXT) and far end crosstalk (FEXT). The distinction is important because these two types of crosstalk limit available transmission capacity in different ways. Duplex transmission suffers from the more severe NEXT but benefits from duplicate use of bandwidth for each direction of transmission while the less severe FEXT allows a higher throughput but only in one direction at a time or frequency.

It was shown that a critical frequency could be found below which it was favourable to transmit duplex against NEXT and above which it was favourable to transmit dual simplex against FEXT.

Crucially, though, this critical frequency was found to be loop-length dependent. The longer the loop the lower the critical frequency becomes. This is important because in most real access networks not all loops are the same length and indeed loops of different lengths often share the same cables and so crosstalk between them may well occur. This fact blurs the concept of the critical frequency to cover a band. The band stretches from the critical frequency for the longest loops in the cable, below which duplex transmission is beneficial for all, to the critical frequency for the...