Explanation:Modem, an acronym for modulator/demodulator, is a device that allows one computer to "talk" with another one over a standard telephone line. Modems act as a kind of interpreter between a computer and the telephone line. Computers transmit digital data, expressed as electrical impulses, whereas telephones transmit voice frequencies as analog signals. To transmit digital data, the sending modem must first modulate, or encode, a computer's digital signal into an analog signal that can travel over the phone line. The receiving modem must then demodulate, or decode, the analog signal back into a digital signal recognizable to a computer. A modem transmits data in bits per second (bps), with the fastest modems transmitting at 56K (kilobits per second). An internal modem is housed within the computer itself, while an external modern is a separate device that is connected to the computer via a cable.
A variety of different rules, called protocols, govern the conversion of data to and from digital and analog. These protocols also govern error correction and data compression. Error correction is necessary to detect and correct data that may have become lost or garbled as the result of a poor telephone connection. Data compression speeds the data transfer by eliminating any redundant data sent between two modems, which the receiving modem then restores to its original form. Individual modems vary in the types of protocols they support, depending on such factors as manufacturer and age.
Communications software enables a modem to perform the many tasks necessary to complete a session of sending and receiving data. To initiate a modem session, the user issues the command appropriate to the software being used, and then the software takes over and begins the complicated process of opening the session, transferring the data, and closing the session.
To open the session, the software dials the receiving modem and waits for an answering signal from it. Once the two modems have established a connection, they engage in a process called "handshaking," wherein they exchange information about the types of protocols each uses, ultimately agreeing to use a set common to both. For example, if one modem supports a more recent set of protocols then does the other, the first modem will agree to use the earlier set so that each is sending data at the same rate, with error correction and data compression appropriate to those protocols. The handshaking process itself is governed by its own protocol.
In addition to transmitting and receiving data, the communications software may also automate other tasks for the user, such as dialing, answering, redialing, and logging onto an online service.
ALTERNATIVES TO THE TRADITIONAL MODEM
The functionality provided by a traditional dialup modem—the ability to send and receive information electronically—is also offered in other technologies that offer faster transmission speeds, although each is not without its disadvantages. Integrated Services Digital Network (ISDN), Asymmetric Digital Subscriber Lines (ADSL), and Digital Subscriber Lines (DSL) all use more capacity of the existing phone to provide services.
At 128K, ISDN is more than twice as fast as a dialup modem, but not nearly as fast as ADSL or DSL. ADSL can deliver data at 8mbps, but is available only in selected urban areas. DSL transmits at a high rate of speed, but to ensure reliable service, the user must be located near the phone company's central office. In addition, a DSL connection is always "on," and so makes a computer more vulnerable to attacks from hackers. To secure a DSL connection, a user should install either a software package called a firewall or a piece of hardware called a router. With either of these in place, the DSL connection cannot be detected by outsiders.
Cable modems do not use phone lines. Instead, they utilize the same line that provides cable TV services to consumers. Offered by cable television companies, cable modems are about 50 times faster than a dialup modem, but transmission speed is dependent on the number of subscribers using the service at the same time. Because the service uses a shared connection, its speed decreases as the number of users increases. Satellite, or wireless, services are faster than a 56K modem, but slower than a DSL. In addition, the initial satellite installation is expensive. However, for users in rural areas who do not have access to other services, wireless service may be a viable option.
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