The osi model provides a standard for layered

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Unformatted text preview: ysical channels (communication media) through which data can be transmitted from one point to another. Some of the most common data transmission medium are wire pairs, coaxial cable, microwave system, communications satellite and optical fibers. -*'** . ' 7. The technique by which a digital signal is converted to its analog form is known as modulation. The reverse process, that is the conversion of analog signal to its digital form, at a destination device, is called demodulation. 8. The three forms of modulation are amplitude modulation, frequency modulation and phase modulation. 9. The process of modulation and demodulation, that is, the conversion of digital data to analog form and vice-versa, is carried out by a special device called a modem (modulator/demodulator). 10. Data transmission service providers are popularly known as common carriers. The various types of services offered by the common carriers are dial-up line, leased line, Integrated Services Digital Network (ISDN), and Value Added Network (VAN). 11. The method of dividing a physical channel into many logical channels, so that a number of independent signals may be simultaneously transmitted on it, is known as multiplexing. The electronic device that performs this task is known as a multiplexer. 12. The two basic methods of multiplexing channels are frequency-division multiplexing and time-division multiplexing. 13. The two modes of data transmission on a communication line are asynchronous and synchronous. In asynchronous transmission, the sender can send data at any convenient time and the receiver will accept it. On the other hand, in synchronous transmission, the sender and receiver must synchronize with each other to get ready for the data transmission before it takes place. 14. The three different methods of establishing communication links between the sender and receiver in a communication network are circuit switching, message switching and packet switching. 15. The selection of the actual path to be used to transmit a packet in a WAN is determined by the...
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