Hardware Reference
In-Depth Information
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)
—Most often used, especially in residential installations.
Asymmetrical means that downstream (download) speeds are much faster than upstream
(upload) speeds. For most users, this is no problem because downloads of web pages, graphics,
and files are the major use of Internet connections. 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 amount of bandwidth, making a single-line service that carries voice and
data possible.
•
SDSL (Symmetrical DSL)
—Provides the same speed for upstream as for downstream service.
Generally, SDSL is offered to business rather than residential customers because it requires
new cabling (rather than reusing existing phone lines). A long-term contract frequently is
required.
•
VDSL (Very High-Data-Rate DSL)
—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 traditional copper wiring with fiber-optic lines to
carry signals to the neighborhood, curb, or directly to your home. To make the final leg of the
connection, VDSL can be implemented over copper wires, coaxial cable, or Ethernet cable.
AT&T sells VDSL as AT&T U-verse. Verizon FiOS provides a VDSL-like service that
supports digital cable TV, phone, and Internet via fiber-optic service to the home.
Note
Fiber to the home (FTTH), Fiber to the premises (FTTP), Fiber to the curb (FTTC), and Fiber
to the neighborhood (FTTN) describes the replacement of conventional copper telephone or
coaxial cable TV connections with fiber-optic cables. Fiber-optic cables carry many more
signals much faster than conventional wiring.
Rolling out fiber-optic cables for at least part of the DSL or cable infrastructure is essential to
achieving higher speeds and greater numbers of services. FTTx rollouts are taking place in
larger cities and metropolitan areas first, and will take longer to happen in smaller cities and
towns.
Just as with a cable modem, you should connect a DSL modem to computers through a router or
gateway, which is then connected to one or more computers. Where a DSL modem differs from a
cable modem is how the Internet connection is made. DSL modems are connected to the Internet via a
standard telephone cable attached between the DSL modem and the RJ-11 port that has been set up
for DSL service. To prevent DSL signals from interfering with standard voice telephone operation,
splitters and microfilters must be installed on a DSL line where any phones are connected.
Note
Many DSL modems now include wired and wireless routers or gateways.
