ZyXEL Communications P-861H-G1, User Manual, Page: 35 / 341

Prestige 861H Series User’s Guide
Introduction to DSL 34
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 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.