Q I’ve been connecting to the Internet with my shack PC at 28.8 kbps. Just this week I received word that DSL will soon be available in my area and that it would greatly accelerate my Internet access. What can you tell me about DSL?
A DSL stands for stands for digital subscriber line, and it takes advantage of the fact that the total capacity of the telephone line coming into your home is vastly under-utilized. Simple voice communication uses only about 1% of the available bandwidth, leaving huge amounts of unused capacity that can support digital transmissions.
DSL uses your telephone line by skipping analog conversion. That is, there is no digital-to-analog conversation process like you see (and hear) in conventional modems. Instead, DSL keys digital data directly on the line. The result is mind-boggling speed—theoretically as high as 8 mbps (megabits, or millions of bits, per second), which is more than 250 times the rate you get at 28.8 kbps. And because the analog voice signal and digital DSL signals use different frequencies, they can be transmitted simultaneously. So not only do you get much higher transmission rates, you can leave your computer connected to the Internet 24 hours a day and still receive telephone calls—all over a single copper phone line.
Of course, there is a catch. (Isn’t there always?) DSL signals attenuate rapidly with distance. For most DSL technologies, the signal is only viable for 18,000 feet. In other words, if you don’t live within about 31/2 miles of your phone company’s central office, you can forget about DSL. That excludes something in excess of one-third of all homes in the United States.
DSL actually comes in a number of flavors. One of the most common is ADSL (asymmetric digital subscriber line). Most home and small office computer users download a lot of data off the Web, but send relatively little data in the other direction.
ADSL makes use of that by reserving more bandwidth for downstream data flow—from the Web to your computer—than upstream. With ADSL, downstream speeds of up to 6 mbps are possible (although 1.5 mbps is more typical); upstream tops out at 640 kbps. ADSL requires the installation of a voice/data splitter at your home. A slightly slower version, called DSL Lite, does the splitting at the phone company. Other varieties include HDSL (high bit-rate DSL) which carries equal amounts of data in both directions and has a maximum rate that is lower than ADSL; RADSL (rate-adaptive DSL), which analyzes the capacity of a customer’s phone line and adjusts the rate accordingly; and VDSL (very high data rate DSL), which can send data at an astonishing rate of 55 mbps, but only for about 1,000 feet.
From QST June 2000
