ZyXEL Communications Prestige 652 User Manual page 25

Prestige 652 ADSL Security Router
What is 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.
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, 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.
What is DSL? xxv