How It Works: DSL

How It's Sold: DSL

Each DSL type supports an upstream speed--from you to your ISP--and a downstream speed--from your ISP to you; these speeds may be different from each other. While you can get such exotic varieties as IDSL (ISDN over DSL, 144 kbps downstream and upstream) and VDSL (Very-high-speed DSL, with bit rates of up to 52 mbps downstream and 1.5 mbps upstream), you'll most often see Symmetric DSL and Asymmetric DSL. To add to the mess, the latter comes in two varieties: full-rate ADSL and ADSL-Lite, also called G.Lite.

As its name implies, Symmetric DSL provides the same speed downstream and upstream, which theoretically maxes out at about 1.5 mbps, the equivalent of a T1 line. SDSL is relatively slow, and it's not cheap. In a recent survey of ISPs, we found prices for business SDSL service starting at about $120 per month for 144 kbps. Given this, a symmetric connection is best suited to businesses that need to host Web sites and do videoconferencing.

Of all the DSL varieties, full-rate ADSL is the most common type in use. While it's theoretically capable of downstream speeds of 8 mbps (1 mbps upstream), 256 kbps to 1.5 mbps is more typical. PC World found during testing that ADSL accounts promising speeds of up to 1.5 mbps downstream and 384 kbps upstream actually delivered an average of 375 kbps downstream and 284 kbps upstream. (Check out our March feature "Bandwidth on Demand," for details.)

ADSL lets you simultaneously use one phone line for both voice and data (SDSL does not). But the phone company needs to install a splitter on your phone, and these installations can be costly. Some vendors are beginning to offer microfilters instead, which you can install at each phone jack yourself.

Historically, the country's various phone companies have deployed different forms of full-rate ADSL, all incompatible with one another. But those companies are slowly standardizing on a form called G.DMT, which could bring prices down.

Because of its cost (about $200 per month for 1.5 mbps downstream), full-rate ADSL is primarily aimed at small businesses. Many ISPs are now offering pared-down ADSL service--$49 per month for 384 kbps downstream, 128 kbps up--to home users. Still in development, G.Lite is aimed at the consumer market. By limiting downstream speeds to 1.5 mbps and upstream speeds to about 384 kbps, G.Lite avoids requiring a splitter to be installed in order for voice and data to exist on the same line. (Some phone companies will install splitters anyway to limit possible interference between the divided frequencies.)

To get any form of DSL installed, you first need to find out if it's available in your area. Contact your local phone company or use Telco Exchange's DSL Pricing and Availability Web page. Second, you need Internet service, and most phone companies double as ISPs. Independent local and national ISPs also offer DSL, but your phone company still needs to provide some necessary services. For a list of some ISPs that support DSL, check out xDSL Resource's ISP Directory or PC World Online's ISP Finder.

You may also need to purchase a DSL modem. Because of the various DSL types out there, you'll probably have to get the modem through your phone company or ISP. The current lack of a single standard means you can't buy just any modem and expect it to work with your phone company's DSLAM. Standards such as G.DMT and G.Lite could fix this, making it possible to buy a DSL modem off the shelf, the same way you buy a standard modem today.

Many ISPs offer promotions in which you buy the modem for $200 to $300 and get free installation. External modems require a computer equipped with Ethernet or a network interface card. And remember that for full-rate ADSL, the phone company will need to install a splitter, or you will need to install microfilters at your phone jacks. Your ISP should have information on service charges as well as which components you need.

For additional information, take a look at the sites for the ADSL Forum and xDSL Resource.

Michael Gowan is a staff editor for PC World Online.