The Scoop on DSL

Digital Subscriber Line (DSL) is a common form of broadband access to the Internet. With varying speeds of connection, DSL is much more convenient and easy to use than traditional dial-up modem service. This article provides the fundamentals of what DSL is, how it operates, why it's popular, and why you may or may not be able to order the service where you work and live.

What is DSL?

DSL is a constant connection to the Internet that works over your existing telephone line. DSL doesn't require a separate line or use a dial-up modem. DSL allows users to use the Internet and make phone calls at the same time. What really sets it apart, however, is the speed. While most modems today connect at 56 kbps, DSL is capable of much greater connection speeds. These speeds can vary, but even the slower DSL offerings have much greater speeds than dial-up modem connections.

Who offers the service?

DSL is offered either by the incumbent local telephone company or a competitor telephone company, who in many cases leases the existing loops (wires going from the telephone company's central office to your house or business) from the incumbent phone company.

How does DSL work?

Think of the telephone line to your house. That line—called a local loop—connects
your telephone to the telephone company's local switch. When you make a call,
the voice signal is carried over the line, switched in the central office to
the party you are calling, and your connection is completed. Because a dial-up
connection also uses the telephone line, it also uses this loop. The signal
must be switched by the telephone company to the ISP service, and go through
a modem bank to complete your Internet connection. The result: a delayed connection
to the Internet. To compensate for their phones lines being tied up, many people
put in a second phone line just for the modem. However, modem connection speed
is relatively slow—probably 56k at best—likely much slower.

DSL changes the way you connect. By attaching a DSL modem to your computer
and using a Digital Subscriber Line Access Multiplexor (DSLAM) provided by the
service provider (at the telephone company's switching office), your online
activities bypass the local switch. This eliminates switching connections, the
dial-up modem at your house, and the modem bank at the ISP, resulting in a much
quicker connection speed. Because you aren't using the local switch, you're
always connected to the Internet.

Why don't I need a second line anymore? And why don't my online activities
interfere with my voice communication?

Again think of that local loop going from your house to the telephone switch.
What DSL does, essentially, is to separate the data traffic (your online activities)
from voice communication. Data traffic travels on a high frequency going from
your computer over the local loop. But instead of getting switched, the DSLAM
directs the high frequency traffic directly to the Internet. Voice communication,
on the other hand, gets routed over the low frequency portion of the local loop
and goes through the telephone company switch. It's almost like having two lines
in one—high frequency for data traffic and low frequency for voice traffic—thus
eliminating the need for a second line.

How fast is DSL?

The speed depends on the service offered by the service provider, the equipment
used, and the offering you choose. Some common forms of DSL are—

• Asymmetric DSL (ADSL), the most common offering, has a higher download speed than upload speed;


• Integrated DSL (IDSL), with a bit speed of 128kbps;


• Symmetric DSL (SDSL) and High Bit Rate DSL (HDSL), with a bit speed of 1.5mbps; and


• Very High Bit Rate DSL (VDSL), with very high download speeds, in the 20mbps range.

Prices vary with each offering.

This sounds too good; there must be some drawbacks.

There are drawbacks. Because the high frequency data speeds are fast, local
loops need to be reliable and data needs to flow uninterrupted. Technical limitations
dictate that customers must be within a certain distance from the telephone
switch to order DSL service. The most common limitation used by most regional
Bell companies is 18,000 feet (about three miles) from the local switch. These
limitations will change as technology improves.

So, if you're within three miles of a switch and the telephone company or competitive provider offers DSL service from that central office, you can probably order the service.

Some technical problems can also complicate the deployment of DSL services.

1. DSL services function on existing copper wires. Sometimes devices are added
   to the wires to boost voice signals over long loops. If too many of these
   devices are already on the lines, high-speed DSL service will not work well
   with large data transmissions.
2. If too many customers are "tapped" into an existing deployed copper wire
   line, DSL functionality simply will not work.

There are also some situations where DSL will not work because the local loop path is not entirely copper.

Do all telephone companies offer the service?

No. Providing DSL services likely involves additional investment by the phone
company. To upgrade to DSL the incumbent phone company or a competitive local
exchange carrier will need to install DSLAMs in all central offices where they
plan to offer the service. In addition, the outside plant may require expensive
upgrades for the service to work properly. As a result, the incumbent phone
company or a competitive local exchange carrier may offer DSL, but not necessarily
in all service locations. And, not surprisingly, DSL is often not offered in
the most rural areas.

If my telephone company is not offering the service where I want it, what
can I do?

You should first get others to request the service, too. If enough people want
DSL the phone company may start offering it, because they can make a profit
selling the service. A competitive local exchange carrier that doesn't have
major investments in infrastructure may also be able to offer services—provided
they can make a profit on selling their services and using existing infrastructure.

You should also ask the local phone company when they are scheduled to offer
DSL in your area. They may tell you; they may not. Some companies consider this
information to be proprietary and are not willing to share it. If that happens
you can contact your public service commission to see if they can obtain the
information.

Asking questions may give you an idea about when to expect DSL services to be deployed, but it won't convince the phone company to deploy services faster. Keep in mind: the more people who ask for the service, the sooner it will become available.

Are other issues affecting the availability of this technology?

Yes. Federal Communications Commission regulations involving competition rules are affecting the willingness of local phone companies to deploy new technology that will enhance high-speed data communication. If an incumbent local phone company deploys new technologies on infrastructure in its network, competitive local service carriers can sometimes be allowed to come into the market and under cut the local phone company's rates of service. The industry is working to resolve these problems.