Modem

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  • A modem (modulator-demodulator) is a device that modulates an analog carrier signal to encode digital information, and also demodulates such a carrier signal to decode the transmitted information.

  • The goal is to produce a signal that can be transmitted easily and decoded to reproduce the original digital data.

  • Modems can be used over any means of transmitting analog signals, from light emitting diodes to radio.

  • The most familiar example is a voice band modem that turns the digital data of a personal computer into modulated electrical signals in the voice frequency range of a telephone channel.

  • These signals can be transmitted over telephone lines and demodulated by another modem at the receiver side to recover the digital data modems are generally classified by the amount of data they can send in a given unit of time, usually expressed in bits per second (bit/s, or bps).

  •  Modems can alternatively be classified by their symbol rate, measured in baud.

  •  The baud unit denotes symbols per second, or the number of times per second the modem sends a new signal.

  • Modulation is the process of changing the form of the signal carrying the information.

  • The demodulation process does the task of extracting information from the signals that are modulated. Analog signals can be transmitted by using devices such as a radio or a diode.

  •  Modems are classified on the basis of two criteria, data sent per unit time, change in the state of the signal per unit time computer sends information in the form of digital signals.

  • But the information over the telephone lines needs to be transmitted in the form of analog signals to solve this problem, the functionality of a modem comes into play converts the digital signals into analog signals.

  • These analog signals are carried over the telephone line these signals reach another computer, the analog signals are converted back to the digital form by its modem.

  • In addition to converting digital signals into analog signals, the modems carry out many other tasks. modems minimize the errors that occur while the transmission of signals.

  • They also have the functionality of compressing the data sent via signals modems also do the task of regulating the information sent over a network.

  • Error Correction: In this process the modem checks if the information they receive is undamaged.

  • The modems involved in error correction divide the information into packets called frames.

  • Before sending this information, the modems tag each of the frames with checksums. checksum is a method of checking redundancy in the data present on the computer.

  • The modems that receive the information, verify if the information matches with checksums, sent by the error-correcting modem.

  • If it fails to match with the checksum, the information is sent back.

  • Compressing the Data: For compressing the data, it is sent together in many bits the bits are grouped together by the modem, in order to compress them.

  • Flow Control: Different modems vary in their speed of sending signals thus, it creates problems in receiving the signals if either one of the modems is slow.

  • In the flow control mechanism, the slower modem signals the faster one to pause, by sending a 'character'. when it is ready to catch up with the faster modem, a different character is sent, which in turn resumes the flow of signals.

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