What Is Dsl - ZyXEL Communications Prestige 861H-G1 User Manual

Prestige 861H User's Guide

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 VDSL?
VDSL is the next generation of DSL technology that offers a much higher bandwidth than most
DSL technologies. VDSL is the only feasible solution for bandwidth-demanding and video-rich
applications such as video-on-demand, high definition television, tele-medicine, surveillance
systems and other switched video services. VDSL supports both symmetric and asymmetric
applications using existing copper wire (telephone wire) and therefore saving the cost of using
traditional T1/E1 service for small/medium-sized business and residential users.
What is DSL? xxiii