ZyXEL P-861H-G2 User Guide - Page 35

Prestige 861H Series User's Guide Introduction to DSL DSL (Digital Subscriber Line) technology enhances the data capacity of the existing twistedpair 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. Introduction to DSL 34