Digital subscriber line (DSL) technology is proving to be an important broadband access method for information technology professionals. DSL lines provide a high-speed and affordable method of linking employees and providing remote connectivity to corporate servers, LANs, intranets, and the Internet. Data Local Exchange Carriers (DLECs) and the Baby Bells are moving aggressively to introduce DSL service to new markets, and DSL could soon support such emerging technologies as Virtual Private Networks (VPNs) and packetized voice.
The advent of Digital Subscriber Line (DSL) technology is beginning to change the way companies connect their remote employees to corporate servers, intranets and the Internet. DSL also is providing a new, efficient method for connecting satellite offices to corporate headquarters.

Just a few years ago, when Pentium II processors were first appearing, most observers never would have believed megabits of bandwidth would soon be purchased for a fraction of the cost of a T-1 line. But that was before DSL. Utilizing plain twisted-pair copper wiring, DSL proved that data transmission speeds could exceed 1.5 megabits per second (Mbps) on regular telephone lines.

Today, DSL is delivering on its promise of providing high-speed, affordable bandwidth to the masses. Thanks in part to the evolution of data local exchange carriers (DLECs) and such organizations as the Universal ADSL Working Group ( and the ADSL Forum (, the availability of DSL service is rapidly increasing.

Heavy hitters 3Com, Compaq, Microsoft, and Intel are among those that have thrown support behind these organizations in attempts to bring standardization to this emerging technology. At the same time, DSL is battling cable to be the broadband access method of choice.

The flavors of DSL
One factor that complicates life for any company looking at DSL is the many different varieties of DSL technology. Here are the main “flavors” of DSL:

  • ADSL Asymmetric DSL: ADSL was the first DSL technology to gain any kind of industry or consumer acceptance. One of its limitations is that traffic headed downstream (from the carrier’s central office) travels faster than upstream (toward the central office).
  • SDSL Symmetric DSL: The advantage of SDSL over ADSL is that the upstream and downstream data speeds are the same.
  • IDSL: This is a hybrid of DSL and ISDN technology and usually features data rates of 128 Kbps to 144 Kbps.
  • HDSL High-speed DSL: This version of DSL features faster data transmission speeds.
  • VDSL Very High-speed DSL: As the name implies, VDSL is even faster than HDSL, not to mention ADSL: 13 Mbps to 50 Mbps.

How it works
Building a DSL network takes time and significant capital. The existing local loop (the famous “last mile” in telephony parlance) from a telephone company’s CO to its subscriber’s facility generally requires no change. Indeed, one of the great attractions of all DSL technologies is that they bypass the “last mile problem.”

Instead, DSL access multiplexers (DSLAMS) are installed in the COs and used to route data traffic around conventional switches. A high-speed DS-3 or ATM line then gathers the aggregated traffic for transport. DSL circuits are completed at the customer’s location via a DSL modem, which connects to a subscriber’s computer or server via an Ethernet card and category 5 cable.

Subscriber installations are complicated by telephone company “truck roll” service calls necessary to ensure proper configuration. The adoption of a G.Lite standard, actively pursued by the Universal ADSL Working Group and others, should eliminate the need for the installation of splitters for ADSL, thereby reducing costs. Splitters separate voice traffic from the data stream, permitting simultaneous voice and data traffic. G.Lite, scheduled for ratification in June, also would standardize requirements for ADSL modems.
Before you start researching your data services provider, make sure you check how far your location is from the CO. In addition to requiring different equipment, DSL varieties include distance limitations that dictate how far an office or residence can be located from the provider’s CO. Generally, ADSL customers must be located within 12,000 feet to 18,000 feet, while IDSL subscribers can be located up to 18,000 feet away. HDSL subscribers must be within 12,000 feet. VDSL requires that subscribers be within 1,000 feet to 4,500 feet of the provider’s DSL equipment.
Instead of replacing voice lines, many DLECs view DSL as supplementing customers’ existing voice services. DLECs aim to provide enterprises with connections that are always on, meaning subscribers never encounter busy signals when connecting to a network. Unlike traditional T-1 circuits, DSL lines are distance transparent. This means subscribers pay the same whether they are contacting a server across town or across the world.

DLECs such as Covad Communications Co. ( and NorthPoint Communications Inc. ( are betting that businesses needing to connect their workers will best be served by SDSL service. Their networks are being designed accordingly.

Some of the players
Both companies have aggressive expansion plans, and each aim to have service in more than 20 markets by year-end. In fact, NorthPoint announced in late 1998 that its deployment would exceed that of all the Baby Bells combined by the second half of 1999.

Although DLECs’ pricing is higher than the Baby Bells’, DLECs’ services are tailored to closely fit the needs of businesses. For example, DLECs offer the ability to service multiple locations nationwide via one provider, and others are announcing initiatives aimed at businesses.

Microsoft unveiled a relationship with Rhythms NetConnections Inc. ( in March. The two companies are joining to provide value-added services to business subscribers. Rhythms’ DSL business customers will have access to a co-branded MSN portal; Hotmail; a search engine; and other enterprise applications including multimedia support.

DLECs are also exploring supporting VPNs and packetized voice over DSL, both of which could prove useful in the enterprise.

If experts’ forecasts are any indication, DSL is going to increase substantially. According to Dataquest, some 510,000 DSL lines were estimated to be in place by the end of 1998, and that number was expected to grow to more than 5.8 million by year-end 1999. Other figures, including those provided by International Data Corporation, show ADSL lines alone reaching 2.5 million by 2001.

Whatever the predictions, one thing is certain. DSL is providing tremendous bandwidth at affordable prices. For example, BellAtlantic’s InfoSpeed service offers up to 7.1 Mbps downstream for just $109.95 a month. Non-recurring installation costs and a modem purchase total $300 to $550. Industry giant America Online ( is among those implementing agreements with BellAtlantic ( for DSL service. BellSouth Corporation (, Pacific Bell (, US West Inc. (, and others offer similar service plans.

Pricing from DLECs, meanwhile, can range from $100 to $600 a month or more, depending on several factors. Installation costs remain about the same as the Bells or slightly higher if a router is required. The market, or region of the country, and the amount of bandwidth selected are the two most significant determinants of price.

Erik Eckel is a Web editor for