ADSL: The New Internet Link

This high-speed transmission technology could make downloading movies and video catalogs possible; but only if our telephone companies allow you to have it. One of the most promising technologies for breaking the bandwidth bottleneck is Asymmetric Digital Subscriber Line (ADSL, sometimes called Asynchronous Digital Subscriber Line). ADSL is one of several similar technologies generically called xDSL.

This high-speed transmission technology could make downloading movies and video catalogs possible; but only if our telephone companies allow you to have it.

One of the most promising technologies for breaking the bandwidth bottleneck is Asymmetric Digital Subscriber Line (ADSL, sometimes 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 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 megabits/sec for HDSL and 6 megabits/sec for ADSL.

ADSL is really an extension of HDSL. It's not only the fastest of the xDSL technologies, but it also handles packet-switched transmission technologies, such as ATM (Asynchronous Transfer Mode). Sending data in self-addressed packets, via routed rather than switched circuits, is a cheaper way to send all forms of data than via voice circuits. This is how information is transmitted over the Internet, and 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: POTS remains functional even if the other ADSL channels drop out.

In the future, ADSL should 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 line. These modems create three information channels: A high-speed downstream channel, medium speed duplex channel, and POTS or ISDN channel. The POTS/ISDN channel splits off from the digital modem by filters, guaranteeing uninterrupted POTS/ISDN, even if ADSL fails. The high-speed channel ranges from 1.5 Mbps to 6.1 Mbps, while duplex rates range from 16 kbps to 640 kbps. You can submultiplex each channel to form multiple, lower-rate channels, depending on the system.

ADSL modems provide data rates consistent with North American and European digital hierarchies. With various speed ranges and capabilities, the minimum configuration provides 1.5 Mbps 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 carry ATM 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-coupled interference. Line attenuation increases with line length and frequency, and decreases as wire diameter increases.

While 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. Reaching customers beyond these distances is possible with fiber-based DLC (digital loop carrier) systems. As these 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 the 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 factors.

Long telephone lines may attenuate signals at 1 MHz (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. Here are some technical details for added interest: 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 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, separating 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.

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 the code implies and the block length. The unit may, at the user's option, also create "superblocks" by interleaving 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 data and video signals.

The importance of ADSL. ADSL's transmission rates of 1.5 Mbps 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). Compared to regular telephone circuits, it's phenomenal.

These rates would expand existing bandwidth by a factor of 50 or more. 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 10 or more years as telephone companies enter new markets for delivering information in video and multimedia formats. Success of these new services depends on 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? This is the big question. ADSL modems test successfully in more than 100 telephone companies, telecom operators, and thousands of lines 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 for such applications as personal shopping, interactive games, and educational programming.

Semiconductor companies introduce transceiver chipsets already 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 to the home desired by their customer. However, this 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's how they charge for their services. ADSL, and all new data systems, are designed for Internet-type routed systems. That is, they do not run through switches. Instead, installers route them from one place to another. This provides a less expensive, more versatile network. However, it leaves the phone companies with less to charge for.

Fundamentally, the choices facing phone companies are: A. Continue to charge $1,500.00 per month for 1.5 Mb/s T1 lines, or B. Invest in equipment to sell 6 Mbps ADSL lines for $50.00 per month.

You can see why it's not in the immediate interest of the phone companies to jump into ADSL. And since local telcos have government-enforced monopolies, there's no direct competition to spur them into action.

A few years ago, some touted ISDN as the new high-speed digital circuit that would provide high-speed service to homes and businesses. Since telcos didn't want it to succeed, it died on the vine. The same could happen to ADSL. And as the technology develops, support and obtainability won't be easy.

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. But these products will stay on the shelf until there is enough bandwidth to make them usable. There is no point to download the 1986 Super Bowl game if it takes four hours to do it, and it 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 observers wonder when they will see a need to change. Time will tell.

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