The Low-down on DSL


Article reprinted from ZDTV DSL Help Guide


If you've been paying attention to the press lately, you know that DSL (Digital Subscriber Line) is the hottest trend in high-speed Internet access. But if you've been wondering what exactly this new technology is, or how you may benefit from it, this article's for you.

First, we'll sort through the alphabet soup of acronyms surrounding DSL technology. Then we'll explain how this technology works, and take a look at where it's currently available. We'll also go through all the steps involved in setting up your system, and provide plenty of online DSL resources.

DSL is definitely on the rise, and is expected to replace ISDN and make a serious challenge to the cable modem market in the next few years. TeleChoice, Inc. forecasts over one million DSL lines installed by the end of 2000. So let's get to know this rising star. Digital Subscriber Line service is a high-speed data service that works over copper telephone lines and is typically offered by telephone companies. The real beauty of DSL technology is that it works on existing POTS lines -- Plain Old Telephone Service -- which allows the phone companies to provide this service without costly installation of higher-grade cable.

DSL uses a different part of the frequency spectrum then analog voice signals, so it can work in conjunction with your standard analog POTS service, sharing the same pair of wires.

This may seem counter-intuitive, but that is one of the real strengths of this technology -- it can piggy-back right on top of your existing phone line, without even disturbing that service. You can even use your analog portion of the phone line as a modem or fax line, while simultaneously using the data portion for your DSL access. Not surprisingly, there's a slew of terms and acronyms that get used when discussing DSL technology. Starting at the beginning, DSL refers to a digital subscriber line that a telephone company central office provides to an end user.

There are a host of versions and flavors of DSL, which has led to the common designation of "xDSL" when referring to this type of technology in general.

The most common service, and the one you'll be looking at if you're considering home DSL Internet access, is ADSL, for Asymmetric Digital Subscriber Line. Being the most common form, the "A" is often dropped, and when someone is just talking DSL, it's probably ADSL.

ADSL can support downstream bandwidths of up to 8 Mbps and upstream bandwidths of 1.5 Mbps. For comparison, a T-1 connection also provides 1.5 Mbps.

An important variation of ADSL is called G.Lite, DSL-Lite, or UADSL (Universal ADSL), and is a notched-down version aimed at the immediate consumer market. Going by many names, this service provides speeds up to 1.5 Mbps downstream and 384 kbps upstream. Another similar offering is CDSL (Consumer DSL), which is a smaller-bandwidth Rockwell variation.

Some of the other variations include HDSL (High-bit-rate DSL), SDSL (Symmetric DSL) and VDSL (Very-high-bit-rate DSL).

HDSL was the original form of DSL technology, developed in the early 90's, as an improved way to provide T1/E1 (1.5/2.0 Mbps) services by the telephone companies. It uses 4 copper wires (2 pairs) and offers a wider coverage area than previous methods.

SDSL, also sometimes called HDSL-2, is an enhanced version of HDSL that allows it to work with only one pair of wires. It accomplishes this with only a slight (.2 km) decrease in loop length.

Both HDSL and SDSL are symmetric forms of DSL technology, which means they have the same bandwidth capability in both directions.

VDSL, also sometimes called BDSL, is targeted at high-access demanding companies and can support speeds of 52 Mbps downstream and 13 Mbps upstream.

Believe it or not this isn't even all of the xDSLs. For a great online reference of terms and abbreviations that you may come across in the DSL world take a look at Aware's xDSL Glossary. ADSL (Asymmetrical) is the type of DSL being offered for high-speed Internet access. It is asymmetrical because it provides different bandwidths in the upstream and downstream directions, giving the user a much bigger "pipe" in the downstream direction.

This scheme works well for the typical Internet user -- where upstream communication is usually small (link requests) compared to downstream communication (Web pages with graphics, downloads).

An ADSL circuit works by connecting an ADSL modem on each end of a twisted-pair telephone line, creating three information channels -- a high speed downstream channel (to your home), a medium speed upstream channel (from your home), and a POTS voice channel.

The ability to provide separate voice and data "channels" on the same line is one of the aspects of DSL technology that makes it so attractive to the telephone companies. Standard ADSL service requires the use of a "splitter" on both ends, to separate the voice channel from the data channels.

One advantage of being able to split the data and voice like this is that the phone companies can keep them on separate networks. The Internet data calls can stop clogging up the PSTN (Public Switched Telephone System) and be sent directly to the packet-switched network.

With the advent of G.Lite DSL, the service can now be deployed to homes without even the need for a telephone company installed splitter -- hence in addition to its other acronyms this service is often referred to as "splitterless-DSL."

The exciting promise of DSL lies in this ability to implement the service easily with existing phone wiring. By being able to offer a high-speed data service that will work on your existing phone line -- and can be turned on without any installation visit -- the potential audience and rate of deployment far surpasses other high-speed options. ADSL works well for two types of applications, interactive video and high-speed data communications. High-speed data services break out into two main areas, Internet access and remote LAN access, the realm of telecommuters. In this article we focus on using ADSL for high-speed Internet access.

Besides higher bandwidth, some of the advantages of ADSL access from telephone companies are that there are no per-minute charges, and you get an "always-on" connection for your monthly fee.

G.Lite ADSL was developed as a cheaper, lower bandwidth version of ADSL service, that could be turned on without a visit from a telephone technician. Companies like Microsoft, Compaq and Intel have been involved in the G.Lite effort, all hoping to establish a high-speed data service that is as easy for consumers to install as today's analog modems.

In late 1998, G.992.2 was adopted by the ITU as the standard that began as the G.Lite. Formal ratification of the new G.992.2 standard is official as of June 1999. At 1.5 Mbps downstream and 386 Kbps upstream, G.Lite DSL is still 8 to 10 times faster than the ISDN services offered today for Internet access, and more than 25 times faster than 56k modems.

Most providers are not yet offering G.Lite ADSL service, but you can expect this to change in the near future. If you order an ADSL service today, you will most likely still need a technician to visit and install an ADSL splitter. There will be a dramatic change in the number of people can get DSL services. Up until recently, you had to be in a select area, usually involved in a carrier's field test. Now, however, the rate of deployment is picking up.

DSL providers have been announcing aggressive implementation plans this year. In January, SBC Communications (parent of Pacific Bell, Southwestern Bell, Nevada Bell and SNET) announced plans to have DSL service available to over 8 million residential customers by the end of 1999. Bell Atlantic met the challenge and promised to make their DSL offering available to another 7.5 million customers by year's end.

AOL is working prominently with both companies, and is offering high-speed access to their service to these customers.

One consideration is that with today's DSL you have to be within 18,000 feet of the telephone company central office, and sometimes less. Estimates have placed just over half of US residences within DSL range of their central office.

The easiest way to find out if you have DSL access as an option is to call your local phone company, or visit their Web site. All the carriers offering DSL have it prominently advertised on their site, and many offer specific telephone number look-up services, to determine if you are in a qualifying area. Ordering the service from a telephone company is done just as ordering any other telephone line.

For an extensive and up-to-date list of DSL trials and offerings visit the ADSL Forum's ADSL Deployments section. Most modern computers can be easily equipped to connect to a DSL service. This is accomplished by connecting an ADSL modem, which is a significantly different beast that your traditional analog modem, to an Ethernet network (NIC) card in your PC.

Today, ADSL modems are external devices that accept the data line from the telephone company, and provide a 10-baseT Ethernet interface to connect to your computer. (Expect to see internal ADSL modems very soon.) Many computers today come with built-in Ethernet capability, but don't worry if yours doesn't. The cost and installation of the network card is generally included by the DSL provider.

The basic requirements for a system to work with today's ADSL modems is either a PC with at least a 66 MHz 486 processor or a Macintosh with at least a 68030 processor, and 16 Mb of memory. Of course performance will improve with faster processors and more RAM on either platform.

Getting started with ADSL Internet access is quite a bit different than with a regular dial-up ISP. You will either be dealing with your local telephone company -- the provider of the ADSL service -- or an ADSL-equipped ISP who will coordinate with the phone company.

In many cases the telephone companies offering the ADSL services are also getting into the ISP business, and they may handle both aspects of the service for you. This can simplify billing and service considerations.

With traditional ADSL services, a technician will install a splitter at your telephone line point-of-presence. This device will split out the standard analog voice line that gets wired to your home jacks, and the data line that gets connected to an ADSL modem.

If you are not dealing with your local telephone company, you will most likely need two separate installation visits to get it all going. First the ISP will arrange for the telephone company to turn on the DSL line and install the splitter at your home, then a technician will come and install your ADSL modem on this line, and possibly the network card in your computer. With G.Lite ADSL, there will be no need for an installation visit. You will simply order the service, install the ADSL modem, and plug it into your regular telephone line.

The ADSL modem is generally provided by the DSL provider. They may charge you up to $200 for this device, discount it heavily or even throw it in for signing up. There is often a monthly rental option as well. At this time you must use the brand and model of ADSL modem specified by your provider.

Costs for DSL services vary more than cable modem services, though this area has recently been getting more competitive. There has been a strong effort to get monthly costs down below the $50 mark, which conventional wisdom says is necessary for widespread acceptance.

DSL providers typically offer several different pricing/bandwidth options. Today, you may pay anywhere from $39 to $80 a month for a basic ADSL service that provides 384 kbps downstream and 128 kbps upstream. For higher bandwidth options the price obviously climbs. A typical 1.5 Mbps downstream / 384 kbps upstream connection will be in the $100-200 range.

Of course there are installation and set-up fees as well. These also vary greatly, and are often waived or reduced for one-year commitments. Typically, installation fees range from $200-$400.

Whatever the specific arrangements your ADSL provider makes with you, rest assured they will be much more involved in setting up your connection than a standard dial-up Internet service provider. While the speeds and costs associated with DSL access seem almost too good to be true, like any technology there are potential downsides.

There are fairly strict distance limitations that DSL circuits can operate within. To receive G.Lite ADSL a customer typically has to be within 18,000 feet of the central office -- not always an option. DSL services that provide greater that 1.5 Mbps require even shorter distances to the central office, usually 10,000 to 12,000 feet. Compare this to a cable modem that can be located up to 30 miles away from the service provider.

The quality of the wiring is an issue as well. Even if you live within the distance requirement of a central office equipped for DSL, if your neighborhood or building has deteriorating telephone cable it still may not work. In these cases the local phone company may be able to provide a "cleaned" or "conditioned" line for you, but you will pay dearly for this.

In some instances DSL circuits can suffer from interference from telephone handsets, or poorly functioning telephones. It may be necessary to install by-pass filters at offending telephone jacks, or replace some telephone instruments. High growth is expected in this industry from all corners, especially in the next several years. TeleChoice, Inc. estimates that installed domestic DSL lines will reach 500,000 this year, and one million before 2001.

With less than 25,000 installed lines by the end of 1998, this is some pretty serious projected growth. There is no disputing that DSL's primary competition in the residential high-speed Internet access race, cable operators, have a strong lead.

For comparison, Kinetic Strategies, Inc. estimates that there were 500,000 cable modems in use in the U.S. by the end of 1998, and they are expected to reach the million mark during the third quarter of 1999. Forrester Research predicts cable modems will have 80 percent of the broadband market by 2002.

The bottom line is that while it may be difficult to get an ADSL line today, there is every reason to expect very rapid deployment of these services in the near future. The greatest strength of this technology is its ability to reuse existing copper phone lines, which gives DSL a real advantage in ease of deployment, compared to other high-speed options. Here are the two best sources of DSL information on the Web:

Home