20. Modem. 1239

The word modem is an acronym formed from the two basic functions of an analog modem: modulation and demodulation.

Modem is a device that enables computers, facsimile machines, and other equipment to communicate with each other across telephone lines or over cable television network cables. It converts analog signals into digital signals, which are used by computers.

Modems transmit data at different speeds, measured by the number of bits of data they send per second (bps). A standard analog modem has a maximum speed of 33.6 Kbps.

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.

To convert a digital signal to an analog one, the modem generates a carrier wave and modulates, or adjust, it according to the digital signal.

Many high-speed modems use a combination of amplitude modulation and phase modulation, in which the phase of the carrier wave is changed to encode the digital information.

The process of receiving an analog signal and converting it back to a digital signal is called demodulation.

21. Voice Over Internet Protocol. 1134

IP telephony (Internet Protocol telephony) is a general term for the technologies that use the Internet Protocol's packet-switched connections to exchange voice, fax, and other forms of information that have traditionally been carried over the dedicated circuit-switched connections of the public switched telephone network (PSTN).

Using the Internet, calls travel as packets of data on shared lines, avoiding the tolls of the PSTN.

IP telephony service providers include local telephone companies, long distance providers, cable TV companies, Internet service providers (ISPs) and fixed service wireless operators.

IP telephony is an important part of the convergence of computers, telephones, and television into a single integrated information environment.

Most personal computers communicate with each other and with larger networks, such as the Internet, by using the ordinary telephone network. Since the telephone network functions by converting sound into electronic signals, the computer must first convert its digital data into sound. Computers do this with a device called a modem, which is short for modulator/demodulator.

22. A Computer System. 1173

A computer system is a collection of components that work together to process data. We use a computer to solve our problems. A functioning computer system is a combination of hardware and software elements.

The hardware elements are the mechanical devices. Circuits, displays, power supplies, cables, keyboards, printers, scanners, modems. and mice are all hardware.

Software refers to parts of the computer which do not have a material form. The software elements are the programs written for the system and applications (programs for specific tasks).

Usually, a computer system consists of the central processor unit, which performs all data processing, a terminal device, which helps users to communicate with their computer system and a memory storing programs and data.

To function hardware and software, a computer needs an operation system program. Some operation systems require users to type in commands to tell the computer what to do.

Many computers use a graphical interface or point-and-click interface such as Windows. Some interfaces allow plug-and-play, the possibility of connecting new hardware. The interface program recognizes the hardware automatically.