Modem
Modem, device that enables computers, facsimile machines, and other equipment to communicate with each other across telephone lines or over cable television network cables. In the strictest sense, a modem is a device that converts between analog signals, such as sound waves, and digital signals, which are used by computers. However, the term has also come to include devices that permit the transmission of entirely digital signals.
Modems transmit data at different speeds, measured by the number of bits of data they send per second (bps). A 28.8 Kbps modem sends data at 28,800 bits per second. A 56 Kbps modem is twice as fast, sending and receiving data at a rate of 56,000 bits per second.
An analog modem converts the digital signals of the sending computer to analog signals that can be transmitted through telephone lines. When the signal reaches its destination, another modem reconstructs the original digital signal, which is processed by the receiving computer. A standard analog modem has a maximum speed of 33.6 Kbps.
The word modem is an acronym formed from the two basic functions of an analog modem: modulation and demodulation. To convert a digital signal to an analog one, the modem generates a carrier wave and modulates, or adjusts, it according to the digital signal. The kind of modulation used depends on the application and the speed of operation for which the modem is designed. For example, many high-speed modems use a combination of amplitude modulation, in which the amplitude (see Wave Motion) of the carrier wave is changed to encode the digital information, and phase modulation, in which the phase of the carrier wave is changed to encode the digital information. The process of receiving the analog signal and converting it back to a digital signal is called demodulation.
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| INTEGRATED SERVICES DIGITAL NETWORK |
Instead of converting between analog telephone lines and digital applications, Integrated Services Digital Network (ISDN) carries digital signals throughout the transmission process. Because an ISDN modem does not convert between digital and analog signals, it does not perform the modulation and demodulation functions from which modems derived their name. An ISDN modem simply processes the digital signal between the computer and the ISDN lines.
ISDN transmission lines are ordinary two-wire telephone lines that carry digital signals on three separate channels. The telephone company uses one channel for tracking and control purposes; the remaining two channels can be used to transmit voice, data, or both. Digital data bypasses the analog voice network, enabling it to travel much faster. ISDN has a maximum speed of 128 Kbps.
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| ASYMMETRICAL DIGITAL SUBSCRIBER LINE |
Like ISDN, Asymmetrical Digital Subscriber Line (ADSL) permits the transmission of digital data over ordinary telephone lines. It is called asymmetrical because it transmits data in one direction (from the network) faster than it does in the other direction (to the network). ADSL carries signals to the network at speeds of up to 640 Kbps, and it can deliver data from the network at speeds of up to 8.1 million bits per second (Mbps).
An ADSL modem splits an ordinary telephone line into three separate data channels, each with different capacities and speeds. The lowest-capacity channel transmits analog voice data; a second, medium-capacity channel transmits data to the network; and the highest-capacity channel transmits data from the network.
A number of other forms of DSL are also available, depending on the speed of data transmission and the distance of the customer from the central office. These include High-Data-Rate Digital Subscriber Line (HDSL), Very-High-Data Rate Digital Subscriber Line (VDSL), and Symmetric Digital Subscriber Line (SDSL).
Cable modems permit the transmission of data over community antenna television (CATV) networks—that is, the network of cables used to distribute cable television. A cable modem transmits data from the network at about 3 Mbps and transmits data to the network at between 500 Kbps and 2.5 Mbps.
Like a standard analog modem, a cable modem converts between a digital signal and an analog signal. Cable modems are much more complex than standard analog modems. They also incorporate a tuner that separates the digital data from the rest of the broadcast television signal. Because users in multiple locations share the same cable, the modem also includes hardware that permits multiple connections and an encryption/decryption device that prevents data from being intercepted by another user or being sent to the wrong place.
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