ZyXEL Communications Prestige 642R Series - Manual (Page 23)

Prestige 642R Series ADSL Router
What is DSL? xxiii
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 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, 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. What are the
advantages of ADSL from the point of view of the Network Service Provider (NSP) and the end user?
Advantages to the Network Service Provider (NSP)
1. ADSL enables telephone companies (telcos) to use the world's nearly 750 million existing copper wires
to deliver affordable high-speed remote access to the Internet, corporate networks and on-line services
over ordinary phone lines.
2. ADSL enables new applications that require real-time, interactive multimedia and broadcast-quality
video. Such applications include collaborative computing, video conferencing, distance learning and
video-on-demand.
3. The industry is rapidly converging on standards that will enable interoperability and ultimately make a
mass market possible.
4. ADSL empowers service providers to provide either a guaranteed sustained or adaptive rate, or best
effort service similar to analog modems.
¾
Nearly 300 times faster than 24.4 Kbps modems
¾ Over 100 times faster than 56 Kbps modems
¾ 70 times faster than 128 Kbps ISDN
5. Both residential and business properties around the world are already running out of spare lines on
existing installed telephone cables. ADSL provides service providers with the capability to use one line
to provide new data services while maintaining the telephone service on the same line, thus leveraging
the existing infrastructure.