In January this year, several leading companies in the personal computer, telecommunications, and networking industries announced the formation of the `Universal’ Asymmetric Digital Subscriber Line (ADSL) Working Group. The aim of the group is to accelerate the availability of high-speed digital Internet access for the mass market. This will be achieved by the proposal of a simplified version of an existing ADSL standard which will enable high-speed communications over existing phone lines based on an open, interoperable International Telecommunication Union (ITU) standard. It will, however, be compatible with (and complementary to) current higher speed ADSL systems that are currently being deployed around the world.
By reducing the complexity of on-site installation and eliminating the need for new wiring at the user’s home, Universal ADSL will make it possible to increase bandwidth up to 25 times the speed of the current highest-speed analog modem technology. As a result, Web content developers will be able to enhance their sites with CD-quality audio and video, confident in the knowledge that the download time won’t be prohibitive.
The Universal ADSL Working Group (UAWG) itself is led by PC industry leaders Compaq, Intel and Microsoft, and telecommunications leaders Ameritech, Bell Atlantic, BellSouth, GTE, SBC Communications, Sprint and U S West.
Aside from its obvious speed benefits, the new ADSL should drop rapidly in price as it is implemented on silicon. Silicon vendors like AMD will be working on single chip devices to support the standard. In addition, software setup of Universal ADSL communications devices will be included in future versions of Microsoft Windows. This should ensure a simple plug and play access for consumers to high-speed communication services. PC companies are expected to begin packaging the new ADSL modems in consumer and small business PCs over the next two years.
The UAWG will build on the present ADSL T1.413 standard to allow the Universal ADSL standard to be quickly deployed. With the goal of assuring consumers that products and services will work together, the UAWG’s work will complement currently planned equipment deployment for full-rate ADSL and help to provide a migration path from today’s modems.
This latest development comes as service providers are already beginning to roll out ADSL services, which have been proven in extensive trials on four continents. In North America, ADSL services are already available in limited markets, and widespread commercial deployments are planned throughout 1998 and 1999.
Trials in europe
Here in the UK, BT will be mounting a major trial of ADSL services in West London later this year, working closely with suppliers Fujitsu and Alcatel. In Germany, Deutsche Telekom will be testing an ADSL-based system in a pilot project in the second quarter of 1998. To this end, 400 of Deutsche Telekom’s residential and business customers in Dusseldorf, Cologne and Bonn will be provided with ADSL lines. Before the end of this year, Deutsche Telekom will be offering ADSL lines to its customers in Berlin, Frankfurt, Hamburg, Munich and Stuttgart too.
The ADSL technology itself enables high speed data and video on a single line without affecting voice calls. In operation, an ADSL circuit connects an ADSL modem on each end of a twisted pair telephone line, creating three information channels – a high speed downstream channel, a medium speed duplex channel, and a POTS (Plain Old Telephone Service) channel. The POTS channel is split off from the digital modem by filters, thus guaranteeing uninterrupted POTS, even if ADSL fails. The high speed channel ranges from 1.5 to 6.1Mbps, while duplex rates range from 16 to 640kbps. Each channel can be submultiplexed to form multiple, lower rate channels.
The Universal ADSL approach is a version of ADSL that would eliminate the need for a splitter to be installed at the users premises. A splitter is a device that separates digital signals from voice signals in the phone line. The tradeoff for easier installation is speed: downstream speeds would be limited to 1.5 Mbps and upstream rates up to 256kbps.
The American National Standards Institute working Group T1E1.4 recently approved an ADSL standard at rates up to 6.1Mbps (ANSI Standard T1.413). The standard currently embodies a single terminal interface at the premise end. Future work will expand the standard to include a multiplexed interface at the premise end, as well as protocols for configuration and network management.
While the ADSL technology may solve some of the bandwidth problems faced by home PC users connected to the Internet via telephone lines over the next few decades, it does not come close to providing the sort of bandwidth that would be possible were those users connected via fibre optic cable. But Fibre To The Home (FTTH) is presently an unlikely near term solution due to its prohibitive cost. One alternative that’s likely to be more commercially practical, however, is a combination of fibre cables feeding Optical Network Units (ONUs) with the last leg to the home still fed by existing or new copper. This topology, which has been called Fibre To The Neighbourhood (FTTN) encompasses Fibre To The Curb (FTTC) with short drops to the home and Fibre to The Basement (FTTB) serving tall buildings.
One of the enabling technologies for FTTN is Very high rate Digital Subscriber Line, or VDSL. While no standards currently exist, the aim of VDSL is to allow the transmission of very high speed data over short reaches of twisted pair copper telephone lines, with speeds depending on line length. While ADSL employs advanced transmission techniques and forward error correction to achieve data rates from 1.5 to 9Mbps over twisted pair ranging to 18,000ft, VDSL employs the same techniques to realise faster data rates from 13 to 55Mbps over twisted pair ranging to 4500ft. While VDSL achieves data rates nearly ten times that of ADSL, ADSL is the more complex transmission technology, in part because ADSL must contend with larger dynamic ranges than VDSL.
The two techniques can be considered to be complementary, with VDSL providing a means of boosting bandwidth to the home once telephone companies have deployed ONUs in the field. For the present time, however, even the benefit that ADSL services will provide should provide PC users surfing the Internet at home and engineers who use the Internet at work with a significant performance enhancement.