One of the most promising technologies for breaking the bandwidth bottleneck is ADSL (Asymmetric Digital Subscriber Line, sometimes also called Asynchronous Digital Subscriber Line). ADSL is one of several similar technologies generically called xDSL. Another popular variant is HDSL (High bit-rate Digital Subscriber Loop).

All of the xDSL technologies involve the installation of electronic boxes on the ends of relatively standard telephone lines, which allows for transmission speeds of as much as 1.5 Mbps for HDSL and 6 Mbps for ADSL.

ADSL is really an extension of HDSL. It is the fastest of the xDSL technologies, and it is also designed to handle packet-switched transmission technologies, such as ATM. (Sending data in self-addressed packets-via routed rather than switched circuits-is a much cheaper way to send all forms of data than via voice circuits. This is how information is transmitted over the Internet, and this method of transmission will eventually become the standard for all transmissions.)

ADSL technology ADSL uses three transmission channels: 1. A high-speed (between 1.5 Mbps and 6.1 Mbps) downlink from the carrier to the customer. 2.A full-duplex data channel at 576 Kbps. 3.A plain old telephone service (POTS) channel. A key feature of ADSL is that regular telephone service (POTS) remains functional even if the other ADSL channels drop out.

ADSL is expected to support throughput (the amount of signal, measured in megabits, put through the circuit) as high as 6 Mbps in a single direction, with a much lower throughput of about 64 Kbps in the other direction.

An ADSL circuit connects ADSL modems on both ends of a twisted-pair telephone line. These modems create the three information channels: a high-speed downstream channel, a medium-speed duplex channel, and a POTS or 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 Mbps to 6.1 Mbps, and duplex rates range from 16 to 640 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 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 carry ATM (Asynchronous Transfer Mode) transmissions with variable rates.

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

- -Data- -Wire - -Distance- -Wire- -Distance - -Rate- -Gauge - -1.5 or- -24 AWG- -18,000 ft- -0.5mm- -5.5km - -2 Mbps - -1.5 or - -26 AWG- -15,000 ft- -0.4mm- -4.6mm - -2 Mbps - -6.1 Mbps- -24 AWG- -12,000 ft- -0.5mm - -3.7km - -6.1 Mbps- -26 AWG- -9000 ft- -0.4mm- -2.7km

Although distances vary from one telco (local telephone company) to another, ADSL, based upon the chart shown above, can cover up to 95% of a local loop, depending on the desired data rate. Customers beyond these distances can be reached with fiber-based DLC (digital loop carrier) systems. As these DLC systems become commercially available, telephone companies can offer access in a relatively short time. ADSL modems use a special type of error correction ("forward error correction") that dramatically reduces errors caused by impulse noise. This is necessary for the successful transmission of digital compressed video, which may be one of the more popular transmissions over ADSL circuits.

ADSL depends upon digital signal processing and algorithms (structured mathematical formulas) to get large quantities of information through twisted-pair telephone lines. Special transformers, analog filters, and A/D converters are also involved.

Long telephone lines may attenuate signals at 1MHz (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 support large dynamic ranges, separate channels, and maintain low noise figures.

Tech details To create multiple channels, ADSL modems divide the available bandwidth of a telephone line in two ways-Frequency Division Multiplexing (FDM) and 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 overlap the downstream, and separates the two by means of local echo cancellation, a technique well known 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, then attaches an error correction code to each block. The receiver corrects errors that occur during transmission up to the limits implied by the code and the block length. The unit may, at the user's option, also create "superblocks" by interweaving data within "sub-blocks." This allows the receiver to correct any combination of errors within a specific span of bits, allowing for effective transmission of both data and video signals alike.

The importance of ADSL

ADSL's transmission rates of 1.5 to 6 Mbps are only a fraction of what would be available with optical fiber to the home or office, which would be at least hundreds of gigabits, but compared to regular telephone circuits, it is phenomenal.

These transmission rates (or anything close to them), would expand existing bandwidth by a factor of 50 or more-without new cabling. ADSL could transform the existing 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, into millions of homes and offices.

ADSL could play a crucial role over the next ten or more years as telephone companies enter new markets for delivering information in video and multimedia formats. Success of these new services will depend upon reaching as many subscribers as possible. By bringing movies, television, video catalogs, remote CD-ROMs, corporate LANs, and the Internet into homes and small businesses, ADSL could make these markets viable for telephone companies and application suppliers alike.

Will this really happen? ADSL modems have been tested successfully in more than 100 telephone companies. Thousands of lines have been installed in various technology trials in North America, Europe, and Asia. Several telephone companies plan market trials using ADSL, principally for data access, but also including video applications for such applications as personal shopping, interactive games, and educational programming. Semiconductor companies have introduced transceiver chipsets already being used in market trials. These chipsets combine off-the-shelf components and programmable digital signal processors. Some industry "experts" expect the phone companies to act according to market conditions. That is, to provide the bandwidth desired by their customers to the home, but that may never happen on a large scale. Local telephone systems are not designed for high-speed circuits like ADSL; they were built to send each call through a switching system. That is how they charge for their services. ADSL, and all new data systems are designed for Internet-type routed systems. In other words, they do not run through switches. Instead, they are routed from one place to another, providing a much less expensive and far more versatile network. But, it leaves the phone companies with less to charge for.

Fundamentally, the phone companies are facing a choice. The can continue to charge $1500 per month for 1.5 Mbps T1 lines, or they can invest in a lot of equipment so they can sell 6 Mbps ADSL lines for $50 per month.

Obviously, it is not in the immediate interest of the phone companies to jump into ADSL. And because local telcos have government-enforced monopolies, there is no direct competition to spur them into action. A few years ago, ISDN was touted as the new high-speed digital circuit that would provide high-speed service to homes and businesses. It died on the vine, due essentially to the phone companies not really wanting it to succeed. There is a good chance the same thing could happen to ADSL. In that event, the technology will be developed, but not easily obtained or supported. Time will tell. (See November 1998 article, "Breaching the digital bottleneck.") Computer makers and software designers have prototypes sitting on their shelves that would give you incredible features, such as scanning a video log, and downloading your favorite NFL game, movie, or old TV show at a moment's notice, and watching it any time you like. But these products will stay on the shelf until there is enough bandwidth to make them usable. There is no point to downloading the 1986 Super Bowl game if it takes four hours and ties up your phone lines as well. But, if there is fiber to the home, you can download the whole game in the time it takes you to grab a Coke from the fridge.

The telephone company/government regulatory establishment has things closed off for the moment, and many wonder when they will see a need to change.