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Mirror Server
Sometimes a Web server will receive more traffic than it can handle. When this happens, the server's administrator may add extra servers--containing identical data--to accommodate the flow. These duplicates are called mirror servers. By adding mirror servers and telling users how to access them, an administrator can keep users from receiving error messages or unacceptably slow response times when they try to access a site. Mirror servers also act as backups if the primary site goes down.
Mirror Site
Because the Internet population has exploded in recent years, a lot of archive servers can't cope with the load. One solution is to create an exact copy of a server--a process called mirroring. Mirror sites divert some of the traffic from the original site. It's not unusual to find a dozen or more mirrors of busy ftp sites.
Modem
A modem is an external box or internal circuitry that converts computer data into sound that can be transmitted over phone lines. First used to send telegrams, early modems alternated between two different tones. This is called modulation, and the process of modulating (and demodulating at the receiving end) gave the modem its name. These days modems transmit data with lots of different tones, signals, and complex mathematical processing, so modem is a bit of a misnomer.
Modulation
Modulation refers to the process of encoding digital data into analog signals for transmission. When transferring data over phone lines, for example, a modem modulates the data into audible tones "carried" on frequencies between 0 Hz and 4 KHz. Once the data reaches its intended destination, another modem demodulates the signal back into digital data. Cable TV networks also use modulation techniques to transfer data. But instead of audible tones, cable has sophisticated digital modulation schemes to greatly increase the amount of data that can be sent.