Digital subscriber line (DSL)
is the result of demand for cheaper and higher bandwidth services
over the already existing copper phone-line network. As with ISDN, there was, and is, a great
deal of installed and widely available sub-Category 3 cable that, with a new encoding method,
could provide high-bandwidth services.
Within this chapter the terms
DSL
and
xDSL
are used. By convention, both
mean the same thing, although
xDSL
is a generic term that means all DSL technologies,
including ADSL and HDSL. These variants of DSL are described later
in this chapter.
DSL
is typically used to describe the base technology.
However, this existing cable currently supports analog voice services, so the new technology,
again like ISDN, needs to support legacy voice services in addition to providing the new data
service. So DSL is a voice and data service that supports multi-megabit data rates over the same
cable that previously supported only voice.
The
digital subscriber line access multiplexer (DSLAM)
provides the cornerstone of the
DSL infrastructure. This device provides two important functions in the DSL network: First,
it separates voice and data traffic from each line, and, second, it terminates each connection
to the residence or business. Figure 27.1 illustrates a typical DSL residential installation with
an access terminal (DSLAM) extending the link from the central office. Note that a remote
access terminal is not required and that a one-mile copper connection could extend directly
from a central-office-located DSLAM.
As an overview, DSL provides the following benefits:
Voice and data services over the same copper pair
Significantly greater bandwidth than ISDN or analog services over comparable physical media
Unfortunately, DSL also has some negatives, including these:
Significant distance limitations at higher data rates
Low tolerance for low-quality copper wiring
Complex, labor-consuming installation procedures for some versions
An inability to work with legacy line-conditioning equipment, including load coils
This chapter covers the flavors of DSL that are available to the administrator for remote
access solutions, in addition to covering configuration and troubleshooting of this technology.