ZyXEL ADSL2+ User Manual download pdf (Page 30)

P-660R-T Series User’s Guide

Introduction to DSL 30

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.

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ZyXEL ADSL2+ User Manual Page 30