ZyXEL Communications Prestige 642M series, User Manual

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Prestige 642M Series ADSL Bridge
What is DSL? xv
What is DSL?
DSL (Digital Subscriber Line) enhances the data capacity of the existing twisted-pair wire that runs
between the local telephone company switching offices, 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. Now, however, 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. In other words, if your computer is beyond a certain
distance, from the central office of a telephone company, then you 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 that
there is no dialing required.
What is 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, e.g., 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.