ZyXEL Communications Prestige 661H Series, Руководство пользователя, Страница: 40 / 547

Prestige 661H/HW Series User’s Guide
Introduction to DSL 40
Introduction to DSL
DSL (Digital Subscriber Line) technology enhances the data capacity of the existing twisted-
pair wire that runs between the local telephone company switching offices and most homes
and offices. While the wire itself can handle higher frequencies, the telephone switching
equipment is designed to cut off signals above 4,000 Hz to filter noise off the voice line, but
now everybody is searching for ways to get more bandwidth to improve access to the Web -
hence DSL technologies.
There are actually seven types of DSL service, ranging in speeds from 16 Kbits/sec to 52
Mbits/sec. The services are either symmetrical (traffic flows at the same speed in both
directions), or asymmetrical (the downstream capacity is higher than the upstream capacity).
Asymmetrical services (ADSL) are suitable for Internet users because more information is
usually downloaded than uploaded. For example, a simple button click in a web browser can
start an extended download that includes graphics and text.
As data rates increase, the carrying distance decreases. That means that users who are beyond
a certain distance from the telephone company’s central office may not be able to obtain the
higher speeds.
A DSL connection is a point-to-point dedicated circuit, meaning that the link is always up and
there is no dialing required.
Introduction to ADSL
It is an asymmetrical technology, meaning that the downstream data rate is much higher than
the upstream data rate. As mentioned, this works well for a typical Internet session in which
more information is downloaded, for example, from Web servers, than is uploaded. ADSL
operates in a frequency range that is above the frequency range of voice services, so the two
systems can operate over the same cable.