【導(dǎo)讀】apoint–to–piontorstore–and–forwardsub.

　　

【正文】 on its end of the line, you could have the fastest modem in the world, and only be able to connect at Kbps. Before you invest in a Kbps or a 56 Kbps modem, make sure your ISP supports them. Speed It Up There are faster ways to transmit data by using an ISDN or leased line. In many parts of the ., phone panies are offering home ISDN at less than $30 a month. ISDN requires a socalled ISDN adapter instead of a modem, and a phone line with a special connection that allows it to send and receive digital signals. You have to arrange with your phone pany to have this equipment installed. For more about ISDN, visit Dan Kegel39。s ISDN Page. An ISDN line has a data transfer rate of between 57,600 bits per second and 128,000 bits per second, which is at least double the rate of a Kbps modem. Leased lines e in two configurations: T1 and T3. A T1 line offers a data transfer rate of million bits per second. Unlike ISDN, a T1 line is a dedicated connection, meaning that it is permanently connected to the Inter. This is useful for web servers or other puters that need to be connected to the Inter all the time. It is possible to lease only a portion of a T1 line using one of two systems: fractional T1 or Frame Relay. You can lease them in blocks ranging from 128 Kbps to Mbps. The differences are not worth going into in detail, but fractional T1 will be more expensive at the slower available speeds and Frame Relay will be slightly more expensive as you approach the full T1 speed of Mbps. A T3 line is significantly faster, at 45 million bits per second. The backbone of the Inter consists of T3 lines. Leased lines are very expensive and are generally only used by panies whose business is built around the Inter or need to transfer massive amounts of data. ISDN, on the other hand, is available in some cities for a very reasonable price. Not all phone panies offer residential ISDN service. Check with your local phone pany for availability in your area. Cable Modems A relatively new development is a device that provides highspeed Inter access via a cable TV work. With speeds of up to 36 Mbps, cable modems can download data in seconds that might take fifty times longer with a dialup connection. Because it works with your TV cable, it doesn39。t tie up a telephone line. Best of all, it39。s always on, so there is no need to connectno more busy signals! This service is now available in some cities in the United States and Europe. The download times in the table above are relative and are meant to give you a general idea of how long it would take to download different sized files at different connection speeds, under the best of circumstances. Many things can interfere with the speed of your file transfer. These can range from excessive line noise on your telephone line and the speed of the web server from which you are downloading files, to the number of other people who are simultaneously trying to access the same file or other files in the same directory. DSL DSL (Digital Subscriber Line) is another highspeed technology that is being increasingly popular. DSL lines are always connected to the Inter, so you don39。t need to dialup. Typically, data can be transferred at rates up to Mbps downstream and about 128 Kbps upstream over ordinary telephone lines. Since a DSL line carries both voice and data, you don39。t have to install another phone line. You can use your existing line to establish DSL service, provided service is available in your area and you are within the specified distance from the telephone pany39。s central switching office. DSL service requires a special modem. Prices for equipment, DSL installation and monthly service can vary considerably, so check with your local phone pany and Inter service provider. The good news is that prices are ing down as petition heats up. The NetWorks Birth of the Net The Inter has had a relatively brief, but explosive history so far. It grew out of an experiment begun in the 196039。s by the . Department of Defense. The DoD wanted to create a puter work that would continue to function in the event of a disaster, such as a nuclear war. If part of the work were damaged or destroyed, the rest of the system still had to work. That work was ARPANET, which linked . scientific and academic researchers. It was the forerunner of today39。s Inter. In 1985, the National Science Foundation (NSF) created NSFNET, a series of works for research and education munication. Based on ARPANET protocols, the NSFNET created a national backbone service, provided free to any . research and educational institution. At the same time, regional works were created to link individual institutions with the national backbone service. NSFNET grew rapidly as people discovered its potential, and as new software applications were created to make access easier. Corporations such as Sprint and MCI began to build their own works, which they linked to NSFNET. As mercial firms and other regional work providers have taken over the operation of the major Inter arteries, NSF has withdrawn from the backbone business. NSF also coordinated a service called InterNIC, which registered all addresses on the Inter so that data could be routed to the right system. This service has now been taken over by Network Solutions, Inc., in cooperation with NSF. How the Web Works The World Wide Web, the graphical portion of the Inter, is the most popular part of the Inter by far. Once you spend time on the Web,you will begin to feel like there is no limit to what you can discover. The Web allows rich and diverse munication by displaying text, graphics, animation, photos, sound and video. So just what is this miraculous creation? The Web physically consists of your personal puter, web browser