ARTICLE SEPTEMBER 2003

COMMUNICATING FASTER AND MORE WIDELY

Broadband asynchronous digital subscriber line (ADSL) technology currently offers data transfer speeds of up to 8 Mbit/s downstream and 1 Mbit/s upstream. Such high transmission speeds are considered crucial to Internet applications such as interactive multimedia. While integrated service digital network (ISDN) lines should prevail for the next few years, so-called xDSL ? covering ADSL and higher speed DSL technologies ? is expected to become the most popular method for high-speed Internet access.

The main advantage of xDSL technologies is to allow reuse of existing copper telephone cables for high-speed digital connections, avoiding the need to install new cabling. This allows traditional local voice network operators to offer high-speed Internet connections, and compete with the alternative technologies such as cable modems and both wireless and satellite Internet access.

Other benefits of DSL include always-on connections, ease of use, reliability, good security as it offers point-to-point connections from the user to the exchange, and proven technology.

Making yourself at home

Broadband DSL has come a long way in a little over three years. Initial data rates and geographical reach of asynchronous DSL (ADSL) were relatively low. However, ratification of the new ADSL2 standard in 2002 and now ADSL2+ earlier this year will boost speeds or, more importantly, increase the distances possible between subscriber and exchange, encouraging more mass-market home use.

Wider geographical reach is crucial in improving coverage of rural areas. ADSL2 boosts reach from the exchange to 6 km, equivalent to a 20% increase in coverage. The main improvements include:

• Minimum 8 Mbit/s downstream transfer rate, scalable to a maximum of 15 Mbit/s and a minimum 800 kbit/s upstream rate, scalable to 1.5 Mbit/s;
• Loop diagnostic tools;
• Improved initialisation and faster start up;
• On-line reconfiguration and power management;
• Framing with up to four frame bearers and up to four latency paths;
• An optional all-digital model, making use of the voice bandwidth to boost the upstream data rate; and
• Higher layer adaptation, providing support for asynchronous transfer mode (ATM)-based ADSL and packet-based ADSL (for Ethernet).

ADSL2 is backwards compatible with the earlier ADSL standard and should also lead to improvements in interoperability between chipset suppliers. There is also a ?lite? version offering slightly lower performance but with simpler installation ? in-line filters in the DSL modem eliminate the need for splitters and offer easy plug-and-play consumer use. Finally, ADSL2+ doubles the downstream bandwidth, enabling downstream data rates as high as 25 Mbit/s on shorter phone lines.

Now the standards have been frozen technically, prior to full acceptance in October this year, equipment suppliers have been able to concentrate on building suitable systems. The DSL Forum has already organised three multi-vendor ?plugfests? in the USA open to all ADSL2 system vendors. This has made it possible to test interoperability of chipsets, digital subscriber line access multiplexers (DSLAMs), customer premises equipment (CPE) and test devices.

Higher speeds for business and pleasure

Ratification of the symmetric high bit rate DSL (G.SHDSL) standard aimed at users of DSL for voice, data and Internet access services by the International Telecommunications Union (ITU) in 2001 led to a revolution in the business market, particularly for small and medium-sized enterprises. G.HDSL offers data transfer rates of up to 2.35 Mbit/s ? allowing applications such as video conferencing ? and doubling the distance possible from the exchange compared with the earlier synchronous DSL (SDSL) standard.

• Such increases in performance could lead to a much wider range of applications in business as well as in the home. Major value-added services include:
• Peer-to-peer networking ? a key use that requires symmetrical data rates, continuous high volumes and would be charged on a volume basis;
• Video-on-demand ? demanding high downstream data rates, guaranteed for real-time transfer with content- and priority-related charging; and
• Voice-over-IP telephony ? necessitating guaranteed data rates, minimum latency and jitter with call-related charging.

Other uses could include online gaming, music services, teleworking and security.

Accessing the fibre optic backbone

Meanwhile very high speed DSL (VDSL) is set to offer much faster data rates for larger businesses, albeit over shorter distances. VDSL lines will offer data transfer rates from 13 to 55 Mb/s for up to 1500 m ? and the shorter the distance, the faster will be the connection rate. The intention is to connect consumers? premises through existing copper wiring to neighbourhood optical network units that are linked directly to the exchange main fibre optic backbone network. This would enable VDSL users to access the maximum bandwidth available. And it would allow service providers to combine high-quality digital video streams, high-speed Internet access and voice services ? the so-called ?triple play? ? using standard copper phone lines.

VDSL is already being trialled in countries around the world ? including Italy in Europe ? although standards questions remain to be resolved. While the European Telecommunications Standards Institute (ETSI), the American National Standards Institute (ANSI) and the ITU have approved basic transceiver technical standards, signal coding problems still remain, with different approaches being promoted by the two main interested groups:

• The VDSL alliance favours a multi-carrier system line-coding scheme based on discrete multitone (DMT) that is more compatible with existing ADSL technology.
• The VDSL coalition prefers a cheaper single-carrier line-coding scheme based on quadrature amplitude modulation (QAM) that also consumes less power.

Once these problems have been resolved, the sky effectively seems to be the limit!