The weakness of a dial-up connection comes from its use of analog phone equipment, not the wires used for phone lines. A DSL, or Digital Subscriber Line, transmits data digitally over normal phone lines and thus allows much higher data transfer speeds.
Unlike a dial-up connection, which is available to anyone with a phone line, DSL requires that your phone company install special equipment. A phone line is a pair of wires leading from a home or office to a building known as a central office, or CO, and each pair from the CO to a phone is known as a local loop.
You have probably seen these windowless one- or two-story CO buildings, either with a phone company's name on the outside or no markings whatsoever. The DSL equipment must be installed at a CO to provide DSL service to all the lines that connect to that CO so the line never passes through analog switches.
The performance of DSL is heavily dependent on the length of the local loop. The longer the wire, the more the signal degrades, and if the speed isn't lowered, too many errors occur for the connection to be useful. Therefore, the closer the home or business is to a CO, the better the DSL will perform.
Unlike a dial-up connection, because DSL uses a higher range of frequencies than is used for voice communication, it doesn't tie up the phone line. Someone can talk on the phone while the DSL is in use without any interference between the two.
DSL comes in different bandwidth arrangements. DSL for home use is usually ADSL, where A stands for "asynchronous," which has a much higher downstream bandwidth than upstream bandwidth. This arrangement makes sense for home users because the most common home use of the Internet is the Web, where lots of data in documents is sent down to the user's computer but little data is sent back up.
A Typical ADSL service offers a 1.5-megabits-per-second downstream, and a 256-kilobits-per-second upstream, which, although slower, is still about eight times faster than a dial-up connection's upstream. DSL services intended for business use offer even higher downstream rates, such as 3 megabits per second or more. At these speeds, they are a good alternative to a T1 line.
Another popular way to get a high-speed connection to the Internet is through the "cable company," the same company that provides a home with cable television. Access is through a box known as a cable modem, which, despite the name, is not a modem at all, because there is no digital-to-analog conversion. Instead, a cable modem is a gateway, converting between two different LAN protocols so a computer can communicate over the cable line.
The cable modem has two connection ports, one for the cable itself and one for the connection to the user's computer. The computer and the cable modem usually communicate through Ethernet - it's a miniature LAN. Some cable modems connect through a USB port on the computer.
The cable modem communicates with the cable company with a different protocol; like Ethernet, though, the connection is shared with other users. A device called a CMTS, or Cable Modem Termination System, is installed by the cable company for a set of subscribers in an area. The CMTS is another gateway, pulling data from the cable line and translating it into the Internet's protocols, or vice versa.
If you've ever used a cable modem, you may have noticed an "activity" light that comes on even when you are not sending to, or receiving from, the Internet. This light indicates some activity between your CMTS and all the cable modems it serves, not just activity involving your cable modem.
It is this sharing of the local cable that limits transmission speed. The cable company actually builds transfer limits into the system. If one user could use 100 percent of the cable, the speed would greatly outclass that of DSL, but because it must be shared, the speeds are "throttled" to something similar to DSL, in a range of about 700 kilobits to 2 megabits per second or more. Because the "throttle" is built into the system, the cable company can offer tiers of service. Thus, users can pay for just the bandwidth they need.
Both DSL and cable modem connections to the Internet are often referred to as "broadband," but this term is more advertisers' talk than anything that has technical meaning. In networking, "broadband" refers to a signal that uses a wide range (a broad band) of frequencies. In the vernacular, though, broadband simply means a connection that's a lot faster than a dial-up connection.
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02132011
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