ZyXEL Communications P-660HW-T - V2, User Manual

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P-660HW-T v2 User’s Guide
Appendix B About ADSL 261
A
P P E N D I X
B
About ADSL
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.
ADSL Overview
Asynchronous Digital Subscriber Line (ADSL) technology provides high-speed data access
across regular telephone or ISDN lines by making use of previously unused high-frequency
bandwidth. ADSL is asymmetric in the sense that it provides a higher downstream data rate
transfer (up to 8Mbps), than in the upstream transfer (up to 832 Kbps). Asymmetric operation
is ideal for typical home and small office use where files and information are downloaded
more frequently than uploaded.
Advantages of ADSL
1 ADSL provides a private (unlike cable telephone and modem services where the line is
shared), dedicated and secure channel of communications between you and your service
provider.