DSL modem standards-removing the access bottleneck

Digital Subscriber Line (DSL) technology evolved in the early 1980s when advances in silicon chip design made it possible to begin to exploit the inherent information carrying capacity of the world's metallic or wireline access networks. These networks had been hitherto constrained to carrying analogue voice signals and low rate digital signals via voice-band modems. It is only relatively recently that advances in voice-band modem technology have pushed the access throughput up to speeds in excess of 28.8kbit/s. However, the voice-band developments are now slowing down because exploitation of the channel capacity is rapidly approaching the limits predicted by C.E. Shannon in the 1940s. Attention is now turning towards the use of frequencies above and beyond the voice-band limit of 3.4kHz where there is more freedom to achieve higher bit rates over the gently band-limited channels of twisted telephony wire-pairs. The region above the voice-band is the bastion of DSL modem technology and the paper is intended to acquaint the reader with developments in DSL modem standardisation. The paper identifies key published standards or technical reports relating to DSL modems and presents the author's interpretation of the current status of DSL related work in certain international and regional standards bodies and fora. Furthermore, it shows that advances in access technology are opening up exciting opportunities for both telephone companies (telcos) and their customers.