A modem is an external or internal device thats function is to transmit digital data over communication lines. Traditional modems once used frequently with dial-up networks convert analog data from telephone lines into computer digital data. Broadband modems transfer digital data between two digital devices.
Modems use two different forms of data transmission: synchronous and asynchronous. Synchronous transmission uses timing signals, and asynchronous uses error-correcting formulas in its transmission. Modems can use one form of transmission or the other, or they can use both.
The functions of modems have changed over the years; first they were used for telegrams and to transmit data for American air defense in the 1950s. Modems were used with computers for the first time in 1977 to transmit data between computers. In the beginning uses of modems and computers, data only circulated among a small amount of computers in the same vicinity while being controlled by a host computer. As modems improved and were able to transmit information faster, data was able to be transmitted between more hosts, and the Internet network slowly spread.
The four types of modems are the fax modem, which solely transfers data between fax machines, the traditional ISDN modem, the Digital Subscriber Line and the cable modem