Ans:

Modem

It is device which is used to convert digital signals generated by the computer into an analog signal to be carried by a public access telephone line. It is also the device that converts the analog signal received over a phone. line into digital signal usable by the computer. A Modem derives its meaning from a modulation and demodulation is a composite word that refers to two functional units that make up device. A signal modulator and a signal demodulator. A modulator converts digital signal in to an analog signal. A demodulator converts analog signal into digital signal.

Modem can be classified into many categories to include the mode of transmission and their techniques as well as by the application features they contain and the type of lies they are built to service.

Figure 3: Signal conversion by Modems i.e. Modulation and demodulation

Speed: Modem speed range from 300 bps to 56 kbps. It normally transmit about 10 bits/character (each character has 8 bits); maximum rate of characters for a high speed modem is 2880 character/sec. For example a compressed image of 20 KB (equivalent to 20,000 characters) will take nearly 6 seconds to load on the fastest modem. The tasks which modem can perform are:

1. Automatically dial another modem using either touch-tone or pulse dialing.
2. Auto answer i.e., automatically answer another modem for making connection.
3. Disconnect a telephone connection when data transfer has completed or it an error occurs.
4. Automatic speed negotiation between two modems
5. Convert bits into the form suitable for the line (Modulator)
6. Transfer data reliably with the correct type of handshaking
7. Convert received signals back into bits (demodulator)