A modulator-demodulator, popularly known as modem, is a device that helps in modulating analog carrier signal for the process of encoding digital information and decoding the information that has been transmitted. The main aim of this device is to create a signal that can be easily transmitted as well as decoded for the reproduction of original digital data. Modems can be used in each and every thing that needs to undertake the process of transmitting analog signals from LED (light emitting diodes) to radio. A basic model type is the one that turns the computer’s digital data into modulated electrical signal for the purpose of transmission over telephone lines and then demodulated by another modem right at the side of the receiver in order to recover digital data.
These devices are usually distinguished by the data amount that they can send in a particular time unit, generally expressed in bps (bits per second) (symbol bit/s), or even bytes per second (the symbol of which is B/s). Modems can also be distinguished on the basis of their symbol rate, which is measured in baud. Symbols per second, or number of times modem sends a new signal every second, are denoted by baud unit. For an instance, ITU V.21 standard used the shift keying of audio frequency with two possible frequencies, which corresponded to two different symbols (or even one bit per symbol), for the purpose of carrying 300 bits every second using 300 baud. Contrastingly, the original standard of ITU V.22, which could be used for transmitting and receiving four distinct symbols (that is two bits per symbol), used phase shift keying to transmit 1,200 bits with the help of sending 600 symbols every second (600 baud).