#12001/3/13 14:05:53
谁要有时间可以翻译一下这篇文章，我不是特别有空，也不太在行翻译东西。
Cable Modem Basics
Traditional dial-up modems provide online access through the public telephone network at up to 56 kbps (equal to 56,000 bits per second). A cable modem, on the other hand, gives users high-speed Internet access through a cable TV network at more than 1 mbps (1 million bits per second), or about 20 times faster.
When a cable modem unit is installed next to your computer, a splitter also is placed on the side of your household. It separates the coaxial cable line serving the cable modem from the line that serves your TV sets. A separate coaxial cable line is then run from the splitter to the cable modem. See diagram
Cable modems typically connect to computers through a standard 10Base-T Ethernet interface. A wire, called "Category 5 cabling," is run from the cable modem to an Ethernet card in the computer. Data is transmitted between the cable modem and computer at 10 mbps. Tomorrow's cable modem technology also will support Universal Serial Bus (USB)and home phone-line networking connections to computers.
Understanding Networks
A cable television system typically has 60 or more channels, and most of them are used for programming services like CNN, ESPN and HBO. These channels also can be used to offer high-speed Internet access. Each channel offers 27 mbps of downstream capacity and 10 mbps of upstream capacity, which is shared by a small cluster of homes. Because data traffic is bursty, several hundred cable modem users can surf at the same time without any loss of performance. If speeds begin to fall off due to heavy traffic, the cable operator can allocate more channel space to preserve high performance levels.
A device called a cable modem termination system (CMTS), located at the local cable operator's network hub, controls access to cable modems on the network. Traffic is routed from the CMTS to the backbone of a cable Internet service provider (ISP), such as @Home or Road Runner, which, in turn, connects to the Internet.
With newer cable modem systems, all traffic from the CMTS to the cable modem is encrypted to ensure privacy and security for users.
Like many other ISPs, @Home and Road Runner use proxy and caching servers to store copies of popular Web sites closer to their subscribers. The upshot: A customer with a cable modem connection isn't forced to travel across the increasingly congested Internet, which can be a slow process, to access popular Web sites.
Cable Modem Standards
Almost 3 million cable modems have been installed in the U.S. and Canada. But all of the hardware and software supporting those connections isn't interoperable, or able to work together. If, for example, a cable company uses Motorola network equipment, only a Motorola modem can be plugged into it.
To try to promote cable modem rollouts, as well as relieve technological confusion, CableLabs, an industry trade organization, drafted a standard for cable modem products in 1996 called DOCSIS, which stands for Data Over Cable Service Interface Specification. The standard was developed to ensure that cable modem equipment built by a variety of manufacturers is compatible, as dial-up modems are.
Today, CableLabs continues to manage a rigorous testing process for DOCSIS cable modems, stamping the products that pass the test "CableLabs Certified." Through June, 2000, DOCSIS modems from 12 manufacturers had been approved. Among the companies marketing DOCSIS cable modems: 3Com, Cisco Systems, Com21, Motorola, Nortel Networks, Philips, Thomson, Toshiba, Samsung and Sony.
Buying Cable Modems
Cable modems haven't been widely available to consumers who'd like to buy their own units. Because most products in this space are proprietary, cable companies usually lease modems to subscribers so that they won't have to deal with obsolescence issues as the technology evolves.
However, as cable companies deploy more DOCSIS modems, several manufacturers are beginning to offer the units at retail on a trial basis. But because widespread availability isn't expected for another 18 months or so, we suggest that you lease a modem from your cable company for now, and buy a new DOCSIS unit down the road.
For More Information ...
Want to learn more about about cable modems? Visit our Product Information area. Many common cable modem issues are also addressed in our Questions and Answers section. What's more, you can surf to our Cable Modem Message Board to discuss your questions directly with other prospective and active cable modem users.
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