Asymmetric Digital Subscriber Line (ADSL), a modem technology, converts existing twisted-pair telephone lines into access paths for multimedia and high-speed data communications. ADSL can transmit up to 6 Mbps to a subscriber, and as much as 832 kbps or more in both directions. Such rates expand existing access capacity by a factor of 50 or more without new cabling. ADSL is literally transforming the existing public information network from one limited to voice, text and low resolution graphics to a powerful, ubiquitous system capable of bringing multimedia, including full motion video, to everyone's home this century.

ADSL will play a crucial role over the next ten or more years as telephone companies, and other service providers, enter new markets for delivering information in video and multimedia formats. New broadband cabling will take decades to reach all prospective subscribers. But success of these new services will depend upon reaching as many subscribers as possible during the first few years. By bringing movies, television, video catalogs, remote CD-ROMs, corporate LANs, and the Internet into homes and small businesses, ADSL will make these markets viable, and profitable, for telephone companies and application suppliers alike.

Capabilities

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, depending on the implementation of the ADSL architecture, and a POTS (Plain Old Telephone Service) or an ISDN channel. The POTS/ISDN channel is split off from the digital modem by filters, thus guaranteeing uninterrupted POTS/ISDN, even if ADSL fails. The high speed channel ranges from 1.5 to 6.1 Mbps, while duplex rates range from 16 to 832 kbps. Each channel can be submultiplexed to form multiple, lower rate channels, depending on the system.

ADSL modems provide data rates consistent with North American and European digital hierarchies and can be purchased with various speed ranges and capabilities. The minimum configuration provides 1.5 or 2.0 Mbps downstream and a 16 kbps duplex channel; others provide rates of 6.1 Mbps and 64 kbps duplex. Products with downstream rates up to 8 Mbps and duplex rates up to 640 kbps are available today. ADSL modems will accommodate ATM transport with variable rates and compensation for ATM overhead, as well as IP protocols.

Downstream data rates depend on a number of factors, including the length of the copper line, its wire gauge, presence of bridged taps, and cross-coupled interference. Line attenuation increases with line length and frequency, and decreases as wire diameter increases. Ignoring bridged taps, ADSL will perform as follows:

While the measure varies from provider to provider, these capabilities can cover up to 95% of a loop plant depending on the desired data rate. Customers beyond these distances can be reached with fiber-based digital loop carrier systems. As these DLC systems become commercially available, telephone companies will offer virtually ubiquitous access in a relatively short time.

Many applications enabled by ADSL involve digital compressed video. As a real time signal, digital video cannot use link or network level error control procedures commonly found in data communications systems. ADSL modems therefore incorporate forward error correction that dramatically reduces errors caused by impulse noise. Error correction on a symbol-by-symbol basis also reduces errors caused by continuous noise coupled into a line.

ADSL depends upon advanced digital signal processing and creative algorithms to squeeze so much information through twisted-pair telephone lines. In addition, many advances have been required in transformers, analog filters, and A/D converters. Long telephone lines may attenuate signals at one megahertz (the outer edge of the band used by ADSL) by as much as 90 dB, forcing analog sections of ADSL modems to work very hard to realize large dynamic ranges, separate channels, and maintain low noise figures. On the outside, ADSL looks simple -- transparent synchronous data pipes at various data rates over ordinary telephone lines. On the inside, where all the transistors work, there is a miracle of modern technology.

To create multiple channels, ADSL modems divide the available bandwidth of a telephone line in one of two ways -- Frequency Division Multiplexing (FDM) or Echo Cancellation. FDM assigns one band for upstream data and another band for downstream data. The downstream path is then divided by time division multiplexing into one or more high speed channels and one or more low speed channels. The upstream path is also multiplexed into corresponding low speed channels. Echo Cancellation assigns the upstream band to over-lap the downstream, and separates the two by means of local echo cancellation, a technique well know in V.32 and V.34 modems. With either technique, ADSL splits off a 4 kHz region for POTS at the DC end of the band.

An ADSL modem organizes the aggregate data stream created by multiplexing downstream channels, duplex channels, and maintenance channels together into blocks, and attaches an error correction code to each block.

The receiver then corrects errors that occur during transmission up to the limits implied by the code and the block length. The unit may, at the users option, also create superblocks by interleaving data within subblocks; this allows the receiver to correct any combination of errors within a specific span of bits. This allows for effective transmission of both data and video signals alike.

The American National Standards Institute (ANSI), working group T1E1.4, approved the first ADSL in 1995. It supported data rates up to 6.1 Mbps (ANSI Standard T1.413). The European Telecommunications Standards Institute (ETSI) contributed an Annex to T1.413 to reflect European requirements. T1.413 (Issue I) was limited to a single terminal interface at the premise end. Issue II (T1.413i2), approved in 2001, expanded the standard to include a multiplexed interface at the premise end, protocols for configuration and network management, and other improvements.

Work towards an Issue III was ultimately submitted to the international standards body, the ITU-T, to develop the international standards for ADSL. The ITU-T standards for ADSL are most commonly referred to as G.lite (G.992.2) and G.dmt (G.992.1)–both of which are approved in June of 1999. Having an international standard has aided in moving towards vendor interoperability and service provider acceptance, further increasing deployment, and ultimately availability to the consumer.

The ATM Forum has recognized ADSL as a physical layer transmission protocol for unshielded twisted pair media.

The DSL Forum was formed in December of 1994 to promote the DSL concept and facilitate development of DSL system architectures, protocols, and interfaces for major DSL applications. The DSL Forum has expanded its efforts to address marketing issues surrounding awareness, and enabling high-speed applications via DSL. The DSL Forum has approximately 340 members representing service providers, equipment manufacturers, and content developers from throughout the world.

Market Status

ADSL is available in various speeds and pricing throughout the United States, Canada, Europe and Asia. There are emerging markets in the Caribbean and through out Latin America. North America has just hit its 1 (M) millionth customer. Worldwide the number is rapidly approaching 3 million in third quarter 2000.

There has been extensive work on interoperability, and the DSL Forum demonstrated any-to-any interoperability with 42 vendors at SUPERCOMM 2000. Deployment is broadening rapidly due to emerging solutions to reaching customers behind Digital Loop Carrier (DLC).

NOTE: The DSL Forum takes no position on particular implementations of ADSL, or specific vendor features, pricing, or performance. This monograph therefore omits any discussion of line code (the basic modulation system) or the various trade-offs between performance and costs. The Forum does maintain a roster of vendors who can address these areas. For further information send a message to info@dslforum.org.