Hardware Reference
In-Depth Information
DSL Availability
Just as distance to a telephone company's central switch (CS) is an important considera-
tion for people purchasing an ISDN connection, distance also affects who can use DSL in
the markets offering it. For example, most DSL service types require that you be within
about 18,000 feet (about 3 miles) wire distance to a telephone company (telco) offering
DSL; some won't offer it if you're beyond 15,000 feet wire distance because the speed
drops significantly at longer distances. Repeaters or a local loop that has been extended
by the telco with fiber-optic line might provide longer distances. The speed of your DSL
connection varies with distance: The closer you are to the telco, the faster your DSL ac-
cess is. Many telcos that offer some type of DSL service provide websites that help you
determine whether, and what type of, DSL is available to you.
If you want to locate DSL service providers in your area, compare rates, and see reviews
from users of the hundreds of ISPs now providing DSL service, set your browser to
www.dslreports.com
.
The site provides a verdict on many of the ISPs reviewed, summar-
izing users' experiences and ranking each ISP in five categories.
Major Types of DSL
Although the term
DSL
is used in advertising and popular discussions to refer to any form
ofDSL,many,manyvariationsofDSLareusedindifferentmarketsandfordifferentsitu-
ations. This section discusses the most common forms of DSL and provides a table that
compares the various types of DSL service. Although many types of DSL service exist,
the two most popular types are ADSL and SDSL:
•
ADSL (Asymmetrical DSL)
—The type of DSL used most often, especially in res-
idential installations. Asymmetrical means that downstream (download) speeds are
muchfasterthanupstream(upload)speeds.Formostusers,thisisnoproblembecause
downloads of web pages, graphics, and files are the major use of Internet connec-
tions. Maximum downstream speeds can be as high as 24Mbps, with up to 3.5Mbps
upstream. Most vendors who offer ADSL provide varying levels of service at lower
speeds and prices, as well. Voice calls are routed over the same wire using a small
amountofbandwidth,makingasingle-lineservicethatcarriesvoiceanddatapossible.
•
SDSL (Symmetrical DSL)
—This type of DSL service provides the same speed for
upstream as for downstream service. Generally, SDSL is offered to business rather
thanresidentialcustomersbecauseitrequiresnewcabling(ratherthanreusingexisting
phone lines). A long-term contract frequently is required.
•
VDSL(Very High-Data-Rate DSL)
—This type of DSL service provides much faster
service than traditional ADSL or SDSL services, but requires a connection within
5,000 wire feet of the telco. VDSL takes advantage of telco's gradual replacement of
