Modem is an acronym for MODulator DEModulator. A modem is a device that converts data from digital computer signals to analog signals that can be sent over a phone line. This is called modulation. The analog signals are then converted back into digital data by the receiving modem. This is called demodulation.
"Analog" refers to information being presented continuously, while "digital" refers to data defined in individual steps. Analog information's advantage is its ability to fully represent a continuous stream of information. Digital data, on the other hand, is less affected by unwanted interference (noise). In computers, data is stored in individual bits, which have a value of either 1 (on) or 0 (off). If graphed, analog signals are shaped as sinusoids, while digital signals are shaped like ladders. Sound is analog, as it is always changing. Thus, in order to send information over a phone line, a modem must take the digital data given it by the computer and convert it into sound, an analog signal. The receiving modem must convert these analog signals back into the original digital data.
Error correction is the method by which modems verify that the information sent to them has been undamaged during the transfer. Error-correcting modems break up information into small packets, called frames. The sending modem attaches a checksum (a symbolic representation of the data sent, i.e. a number) to each of these frames. The receiving modem checks whether the checksum matches the information sent. If not, the entire frame is resent. Though error correction may slow down data transfer on noisy lines, it does provide greater reliability.
Often, one modem in a connection is capable of sending data much faster than the other can receive. Flow control allows the receiving modem to tell the other side to pause while it catches up. Flow control exists as either software (XON/XOFF) flow control, or hardware (RTS/CTS) flow control. With software flow control, when a modem needs to tell the other to pause, it sends a certain character, usually Control-S. When it is ready to resume, it sends a different character, such as Control-Q. Software flow control's only advantage is that it can use a serial cable with only three wires. Since software flow control regulates transmissions by sending certain characters, line noise could generate the character commanding a pause, thus hanging the transfer until the proper character (such as Control-Q) is sent. Also, binary files must never be sent using software flow control, as binary files can contain the control characters. Hardware (RTS/CTS) flow control uses wires in the modem cable or, in the case of internal modems, hardware in the modem. This is faster and much more reliable than software flow control. [an error occurred while processing this directive]