Digital Subscriber Line

The copper-wire-based digital subscriber line (DSL) service is one of the two main competing forms of broadband Internet connectivity, the other being cable. (Wireless and satellite broadband services are not commercially available, although both show some promise.) DSL differs significantly from cable in its technology, which is based on copper twisted-pair wires, not fiber-based coaxial cables. It also differs in its data speeds, which are slower than cable's, at least theoretically. DSL differs greatly from cable in its point of origin as well—usually telephone companies rather than cable-TV system centers.

But despite cable's apparent advantages, DSL is emerging as a serious challenger to cable broadband, especially among small businesses and home offices, largely because DSL's one-to-one connection between customer and provider is more secure; cable services are shared in a network among users, meaning that the more customers there are online at once, the slower the overall cable Internet service flows. Also, cables' shared customer network has prompted fears that cable-service hackers could break into other cable customers' computers. Although it offers slower peak data speeds than cable, DSL service nonetheless is up to 50 times faster than 28.8 kbps (kilobits per second) dialup modem Internet connections.

DSL was first developed in 1989, primarily for use with video. At the time, video-on-demand was considered a high priority among telephone company executives, who saw it as a way to compete against cable TV providers. But the video-on-demand market never flourished, and DSL foundered. Then, in the mid- 1990s, as online music and video, large documents and images, and other multimedia content were gaining in popularity, the technology reemerged as a perfect way to provide broadband Internet service to businesses and residences. DSL was also a way for telephone companies to get involved in the broadband Internet business, after it became clear that the costs of upgrading the entire phone network with fiber-optic cable would be prohibitively high.

DSL service requires no special telephone wires to provide high-speed Internet access at potential speeds of up to 1.1 mbps (megabytes per second); standard phone lines already in place are sufficient. Also, because DSL services utilize the unused high-frequency end of the spectrum on the same wires that deliver voice telephone service to homes and businesses, it is possible for both voice and data to run simultaneously over the phone line without having to install new phone lines or maintain separate Internet services. In other words, unlike dial-up connections, the phone will still ring if a DSL customer is surfing the Web, and the customer can take the call while remaining online. In fact, DSL service is “always on,” whether the user is currently on the Internet or not.

However, DSL has some serious drawbacks. It is often very difficult for the user to set up and install the service without the help of visiting technicians. Also, DSL is not available at all in many areas. Customers hoping to gain DSL access, for instance, must be located within certain short distances from a telephone company's central offices; the most common consumer form of DSL service, ADSL, requires that customers be located within 12,000 feet of the telephone company hub.

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