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High-Tech Times Article 037

Digital Subscriber Line

Welcome back to the High-Tech Times. This month’s article comes from our frustrated Hawaii OnLine publisher, who asks, “what is an easy way to explain what DSL is and how it works?”

Well, Jon, there is some good news and bad news in your question. Digital Subscriber Line (DSL) takes advantage of standard copper telephone lines to provide high?speed Internet access. Unlike traditional dial?up connections such as analog modems, DSL delivers continuous “always-on” access.
A major advantage of this high?speed, dedicated, point?to?point technology is that it uses the existing copper telephone wires to your office and home. With 800 million phone lines deployed throughout the world, there is very little need for new wiring. If you have a standard RJ?11 phone jack, you are probably already wired. All you need is a DSL modem that's compatible with your service provider's central office equipment (the DSLAM or Digital Subscriber Line Access Multiplexer) and downloads will flash before your eyes.
The bad news is that DSL is designed for the “local-loop” copper from the telephone company's central office to the end user's business or home ? a range of only 18,000 feet or 3.4 miles - with only a few exceptions. Assuming you are within this radius of service, here are a few more facts and figures about DSL.
DSL comes in several flavors: Asymmetric DSL (ADSL), ADSL Lite (also known as G.Lite), Symmetrical DSL (SDSL), and Integrated Services Digital Network (ISDN) over DSL (IDSL). If all of this alphabet soup frustrates you, let me take a few minutes to explain.
ADSL provides more bandwidth downstream for faster downloads to your computer where it's needed, than for uploads to somewhere else. ADSL supports downstream speeds up to 8 Mbps, and upstream rates up to 1.1 Mbps. To appreciate how fast an ADSL download really is, that's up to 278 times faster than a 28.8 Kbps modem, and up to 143 times faster than a 56 Kbps modem. How can ADSL get so much more performance out of the same copper wires than, say, a 56K modem? ADSL modems leverage signal-processing techniques that insert and extract more digital data onto analog lines beyond the frequencies of normal voice service. Because the high-frequency carrier signal can be modified, a larger digital data “payload” can be carried over greater distances using standard phone lines.
An additional device installed at the customer premise, commonly known as a POTS (Plain Old Telephone Service) splitter, separates the regular voice telephone signal from the high?speed digital data-signal modulated above it. This keeps the voice line free for incoming voice or FAX calls, as opposed to ISDN or 56K which borrows bandwidth from the voice frequencies. For the user, this means they can access the Internet and make a phone call at the same time without slowing data access.
ADSL Lite provides slower services - at lower cost and bandwidth - that dispenses with the need for the phone company to install and maintain an end-user?based POTS splitter. Essentially, by reducing the data rate, line noise interference is manageable, and thus a POTS splitter is not required. By eliminating a phone company visit as well as the additional splitter equipment, a significant cost reduction can be passed on to the consumer.
Customers will be able to take the G.LITE modems they buy at their local computer retail store and connect them directly to their phone jacks. It's plug-and-play. By simplifying installation and reducing cost, ADSL Lite will be more attractive to the larger consumer market. As it will support both data and voice, G.Lite provides an evolution to full?rate ADSL if more bandwidth is required.
SDSL and IDSL are mentioned here mostly because you’ll read about them in the news. The key ways in which these two technologies differ from ADSL is that in each case the download and upload speeds are the same, making them ideal for LAN?to?LAN traffic and high-bandwidth applications such as full-motion video-conferencing, Web-hosting and, collaborative computing. Neither service supports voice, so POTS splitters are not needed. And the services are currently nonstandard so customers need to know which modem will work with their service provider's central office equipment.
DSL services were first conceived as the phone company's answer to cable data services. DSL was initially designed to provide video?on?demand and interactive-TV applications over twisted?pair wires. When fiber?based broadband loops proved too costly for widespread deployment, interest in developing DSL local-loop services intensified. 
Another boost to DSL came with the passage of the Telecommunications Reform Act of 1996, allowing local phone companies, ILECs (Incumbent Local Exchange Carriers), IXCs (Interexchange Carriers), ISPs (Internet Service Providers), CLECs (Competitive Local Exchange Carriers), satellite, and cable companies and radio/television broadcasters in the U.S. to compete in one another's markets. The race for affordable broadband bandwidth was on! So far, ADSL and ADSL Lite have emerged as the DSL services most in demand for consumers. For businesses, SDSL is the preferred service. They all support applications like integrated Internet access, intranet access, remote LAN access, video-on-demand, and more. 
Cable companies like Oceanic’s RoadRunner have done a good job marketing their medium as an infrastructure that can provide broadband Internet access as well as television. And it certainly does that here Hawaii where the service is available. At this point, cable has better residential?market penetration than ADSL. However, cable companies are still working to resolve a number of issues that prevent it from becoming a professional business tool, such as availability at commercial sites and customer service -  issues that telephone companies have addressed and continue to improve. 
Today, cable modems are mostly targeted at consumers for residential use ? and for good reason. Theoretically, cable modems offer downstream speeds up to 30 Mbps and an upstream connection up to 10 Mbps back to the cable head-end. However, unlike DSL and ISDN, cable modems are a shared ? not dedicated ? access technology. That means that total available bandwidth is shared among users in a neighborhood, just as if they were on a local area network. Therefore, not everyone on the network will get the top speeds of 10 to 30 Mbps.
So during off?peak hours your cable broadband connection speed can be pretty good. However, if you share your cable connection with the local high school, for example, and 400 students jump on the Internet at 3:00 PM, your cable bandwidth would now be split many more times over. Nearby users could suddenly experience a dramatic slowdown through their cable connection. DSL guarantees your speed over a line dedicated to your use, and offers valued?added applications such as increased (dynamic) bandwidth when you need it ? something cable cannot yet do.
The following is a table that summarizes the four types of DSL that I have discussed:

ADSL 32K-8 Mbps Downloads 18,000 feet

ADSL 32K-1.1 Mbps Uploads 18,000 feet
ADSL Lite 64K-1.5 Mbps Downloads 18,000 feet
ADSL Lite 32K - 512 Kbps Uploads 18,000 feet
SDSL 1.544 Mbps Symmetric 22,000 feet
SDSL 2.048 Mbps Symmetric 22,000 feet
IDSL 144 Kbps Symmetric 18,000 feet
IDSL 144 Kbps Symmetric 36,000 feet with repeaters

You may still be confused about Digital Subscriber Line services, but now you’re confused in an educated way! See you next month.