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Telecommunicatons expert Annabel Dodd provides an overview of DSL: who sells it, how it works, and how residential and commercial DSL services differ.

Telecommunicatons expert Annabel Dodd provides an overview of DSL: who sells it, how it works, and how residential and commercial DSL services differ.

The growth of the Internet and the proliferation of powerful computers have created a demand for lower-cost, high-speed remote access to corporations and the Internet. High-speed services such as T1 are affordable for large and midsize organizations. But until the advent of DSL, teleworkers, small businesses, and branch offices of large companies were largely left out of the loop.

Small and medium-sized companies as well as residential consumers buy DSL for Internet access. Businesses purchase it as a lower-cost alternative to T1 service. DSL works over the same copper already installed by telephone companies for voice traffic. Consumers want it because it is "always on" and is faster than dial-up services such as those sold by AOL. DSL is an alternative for residential customers to cable TV modems.

What Is DSL Service?

Digital Subscriber Line (DSL) service is a high-speed digital service that works on the same copper cabling used for voice. Telephone companies, Internet service providers such as EarthLink, and competitive local exchange carriers (CLECs) sell it. There are many flavors of DSL, including asymmetric DSL (ADSL) and symmetric DSL(SDSL), which are described in this article.

What type of DSL service do incumbent local telephone companies sell?

Local telcos such as Verizon (formerly Bell Atlantic) and SBC sell asymmetric DSL. They each have a different brand name for their offerings. Infospeed DSL is the name of the Verizon offering; PACbel and SBC simply call their service DSL, and BellSouth offers MegaBit Select and MegaBit Deluxe.

Who buys DSL from incumbent telephone companies?

Currently, mostly residential customers who live close enough to telephone company offices to qualify buy DSL. Most business customers do not purchase DSL from incumbent telephone companies because they want symmetric, not asymmetric, service. Business customers need high-speed service for both sending and receiving files from the Internet.

Who else sells DSL service?

In addition to incumbent telephone companies such as SBC, BellSouth, and US West, DSL is sold by Internet service providers, long-distance companies, and competitive local exchange carriers. Service providers such as Concentric Networks, EarthLink, WorldCom, Cyber Access, Flashcom, ShoreNet, and Verio offer DSL. For the most part, they resell DSL from wholesalers such as NorthPoint, Covad, incumbent telephone companies, and Rhythms.

Some companies such as HarvardNet, Vittsm, and Digital Broadband sell DSL directly to end users without using wholesalers for the DSL modem and telephone company coordination. These service providers also install the DSL modem or modem/router if the end user needs Internet access for multiple computers. The router directs email and Web traffic to the appropriate computer.

In addition to Internet access, all of these providers sell email, security software, and Web page hosting.

What is a wholesaler's role?

Wholesalers such as Covad, Rhythms, and NorthPoint buy copper local loops that run from telephone companies to customers' premises. They also rent space in telephone company central offices, where they place their DSL equipment. Specialized DSL equipment in the telephone company offices aggregate traffic from multiple DSL users and send it in one high-speed stream to the Internet service provider or competitive local exchange carrier.

In addition, the incumbent telephone companies such as Verizon sell their DSL service wholesale to Internet service providers, who resell it to residential consumers.

Wholesalers install the DSL modems at customer premises for service providers that sell directly to end users. See figure 1 for a sample DSL configuration.

Figure 1

A sample DSL configuration.

What type of DSL do most Internet service providers sell?

The DSL service offered by incumbent telephone company competitors is usually symmetric DSL (SDSL). SDSL has the same speed for downloading and sending to the Internet. LAN connectivity is offered to other corporate sites, Internet access, and Web hosting for the customer's Internet domain name, as well as email service. Because the telephone company ADSL service is asymmetric, slower speeds are available for sending to the Internet than when receiving information from the Internet.

Service providers claim that businesses send large files and thus need equal speeds for sending as well as downloading from the Internet. Residential consumers send mainly email messages that do not require high speeds. However, businesspeople tend to send large files such as PowerPoint and Excell attachments in addition to simple text emails.

What is ADSL Lite?

ADSL Lite has asymmetric, lower speeds from the user to the Internet than from the Internet to the user. The "Lite" refers to an ADSL standard that does not require a splitter. (A splitter separates voice signals from data signals on cable that supports voice and data.) Not having a splitter makes ADSL easier to install. Telephone companies hope to install DSL service without dispatching a technician. Telephone companies sell a mix of ADSL and ADSL Lite.

What about home pages for DSL provided by incumbent telephone companies?

If customers have an Internet home page, the address for that home page will be something like www.telcousername.com rather than www.username.com.

How is DSL different from ISDN?

Unlike ISDN, DSL is "always on." No dialing is required, and there are no usage charges. DSL provides faster Internet and LAN downloads.

What are sample ADSL rates and speeds for telephone company DSL?

The following table gives some sample ADSL rates and speeds:

Monthly Cost

Download from the Internet

Upstream to the Internet or LAN










*Thousand bits per second **Million bits per second

What are sample speeds and costs for residential consumers?

Residential fees for ADSL are generally $40 to $50 per month. Speeds are about 90 thousand bits per second from customers' computers to the Internet, and 640 thousand bits per second from the Internet to users' computers. EarthLink and other Internet service providers resell this type of incumbent telephone company asymmetric DSL to residential consumers.

What are sample speeds and costs for business customers?

Speeds range from 144, 160, and 784 thousand bits per second to 1.54 million bits per second. Higher-speed DSL is more costly than lower-speed service, and the number of IP and mailbox addresses and hosting requirements also impacts costs. Monthly service ranges from $150 for the lowest-speed service to $800.

Where is DSL available?

Service is most readily available in large metropolitan areas and their immediate suburbs.

Service is available only at sites less than 2 miles from a telco central office. This includes about 85 percent of customers.

The condition of the outside telephone company cabling impacts availability. DSL does not work on cabling that has bridge taps. (Bridge taps allow multiple customers to share a length of cabling.) According to Verizon, about 20 percent of metropolitan and 60 percent of suburban locations have bridge taps in the Verizon territory. In addition, customers must not be more than a certain number of feet from the nearest telephone company central office. About 85 percent of customers are close enough to central offices to get DSL service.

How are repairs handled?

If a customer has a repair problem, he or she calls the service provider, who determines whether the trouble is in the DSL hardware or the telephone line. If it is in the telephone line, the provider calls the wholesaler, such as Covad or NorthPoint. The wholesaler reports the trouble to the incumbent telephone company, such as SBC.

What equipment is needed for residential DSL service?

A DSL modem and an Ethernet card are required. The service works with both Mac- and Windows-based computers. Some new computers come with DSL Lite modems preinstalled.

Presently most telco-provided DSL service works with only one computer per site. LAN connections at the originating site are not supported. However, a telecommuter with DSL can connect to a remote LAN. In addition, customers can hook up their own routers to access the same DSL line from multiple computers. However, the telephone company will not help customers with the configuration.

Improvements in Availability of DSL

The key enhancement that most customers want is a higher availability of DSL. High-speed Internet access is not obtainable uniformly throughout the United States. Rural areas have the least availability because of the prevalence of long distances between customers and the telephone company central offices. Even in metropolitan and suburban locations, availability is uneven.

New technologies are now available that are capable of extending the reach of DSL. However, they require investments by both incumbent and competitive local exchange carriers.

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