ZyXEL Communications Prestige 623ME-T, User Manual

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Prestige 623ME-T User’s Guide
Introduction to DSL xxvii
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.