Explain briefly the principle of working of modem. Or Explain the main transmit and main receive functional blocks of a modem. Explain the function of each unit.

Explain briefly the principle of working of modem. Or Explain the main transmit and main receive functional blocks of a modem. Explain the function of each unit.

Ans. The device which performs modulation is called a modulator and the

device which recovers the information signal from the modulated carrier is called

a demodulator. In data transmission, we usually come across devices which

perform both modulation as well as demodulation functions and these devices are

called modems. When data is to be transmitted over long distances, modems are

needed. In a modem, the input signal modulates a carrier which is transmitted to

the distant end. At the distant end, another modem demodulators the received

carrier to obtain the digital signal. Thus, a pair of modems is always required

The term ‘modem’ is derived from the words, Modulator and

DEmodulator. A modem contains a modulator as well as a demodulator. nearly

all the modems are designed for utilizing the analog voice band service provided

by the telecommunication network. Thus, the modulated carrier produced by

a modem fits into the 300-400 Hz bandwidth of the speech channel.

Fig. 2.23 shows a typical data connection set up using modems. 1 he digital

terminal devices exchange digital signals and are known as Data Terminal

Equipment (DTE). Two modems are always needed, one at each end. At the

transmitting end, the modem converts the digital signal from the DTE into an

analog signal by modulating a carrier. At the receiving end, the modem demodulates

the carrier and hands over the demodulated digital signal to the DTE.

implementing a data circuit using modem

A dedicated leased circuit or a switched telephone circuit can be used as

transmission medium between the two modems. In the latter case, moderns

are connected to the local telephone exchanges. Whenever data transmission

is needed, connection between the modems is established through the telephone

exchanges. Modems are also needed within a building to connect terminals

which are located at distances, more than 15 metres from the host.

A block diagram of a modem is shown in fig. 2.24, which comprises a

transmitter, a receiver and two interfaces. The digital signal to be sent is given

to the transmitter. The modulated carrier that is received from the distant end

is given to the receiver. TI1e digital interface connects the modem to the DTE

which produces and receives the digital signals. In order to transmit it and receive

the modulated signals, the line interface connects the modem to the transmission

channel. Modems connected to telephone exchanges have additional provision

for connecting a telephone instrument, which enables establishment of the

telephone connection.

block diagram of a modem

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