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 encode the digital information at the time of modulation and decode it back during demodulation to transmit the data, 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
A 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 modem converts the digital signals into analog signals. These analog signals are carried over the telephone lines. When these signals reach another computer, the analog signals are converted back to the digital form.
In addition to converting digital signals into analog signals, the modems carry out many other tasks. They minimize the errors that occur while the transmission of signals. They also have the functionality of compressing the data sent via signals. They also do the task of regulating the information sent over a network.
- Error Correction: In this process the device checks if the information they receive is undamaged. It divides the information into packets called frames. Before sending this information, it tags each of the frames with checksums. Checksum is a method of checking redundancy in the data present on the computer. The device that receives the information, verifies 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 device in order to compress them.
- Flow Control: Different devices vary in their speed of sending signals. Thus, it creates problems in receiving the signals if either one of them is slow. In the flow control mechanism, the slower one signals the faster one to pause, by sending a 'character'. When it is ready to catch up with the faster one, a different character is sent, which in turn resumes the flow of signals.
Modems are classified on the basis of different criteria such as the place where they are installed, the manner in which they accept information, and the way they transmit signals. Based on these criteria, they are divided into the following types.
- Internal and External: The former is in the form of circuit boards, fitted in expansion slots of the motherboard. The latter is enclosed in a separate case, and not inside the system units. It is connected to the computer and the telephone line by means of two separate cables.
- Intelligent and Standard: The former accepts instructions entered from the keyboard of a microcomputer. The latter responds to commands and transmits information simultaneously. It is done with the help of microprocessor chips.
- Wireless and Short-Haul: The former transfers data by means of cables. Since they don't need an external power source, they are also called 'modern eliminators'. The latter does not require any kind of cable to transmit the data signals.