Asymmetric digital subscriber line (ADSL) systems-an introduction

The installed copper pair of the telephone access network can be a bottleneck for the provision of the digital “telephony+video+data services” of the future and for this reason there is currently much interest in finding high-bit-rate solutions. ADSL is seen as a cost effective method of providing entertainment quality uni-directional video on demand, over the twisted pairs of the existing telephone access network. Opinion differs as to the precise bit rate requirements, these will depend on the video coding techniques adopted. Using current MPEG standards a bit rate of 1.5 or 2 Mbit/s (using the MPEG-1 algorithm) is regarded as sufficient for domestic VCR quality but not for live broadcast quality where up to 6 Mbit/s (using the MPEG-2 algorithm) may be required. The principal features anticipated for ADSL systems are summarised. To achieve these features it is envisaged that the impairments in the network will have to be managed by adaptive digital equalisers, echo cancellation, and adaptive modulation techniques