3.1 Introduction

There are around 700 million copper pairs in telephony access networks that provide for extensive connectivity of the world's population. A combination of this existing copper infrastructure and new transmission technologies mean that a new era of universal broadband access can now begin at a fraction of the cost, and in a fraction of the time required for optical access networks.

However, there is a threat to this vision of the future it is relatively straight forward to design transmission systems that work well in computer simulations and in a few specific laboratory tests, but more difficult to deliver useful capacity when subjected to the hostile environment of the real network [1]. Also, the uncontrolled deployment of such advanced transmission systems in multi-pair cables may result in mutual interference (i.e. crosstalk). Such degradation can occur even when the systems are quite thoroughly specified, since they can be inadvertently operated so as to cause problems. Even a low rate of such instances would significantly pollute the copper network. It is therefore vital to understand the crosstalk environment and spectral compatibility issues for different DSLs [2,3] if the broadband potential of the existing copper access network is to be fully realised.

Most Telcos have a broadband access strategy that involves deployment of ISDN, ADSL, SDSL and VDSL transmission technologies for a new generation of services, and the use of HDSL/SDSL for reducing costs...