A modem converts the computer's digital information to and from sound signals sent across phone lines. The term "modem," comes from a contraction of "MOdulate and DEModulate." The modem converts the binary ones and zeroes, sent by the computer, into different frequencies of sound. These tones can be sent over standard telephone lines. A modem on the receiving end takes these sounds and transforms them back into binary code.
A modem can be internal or external. An internal modem is connected through an |expansion slot| on the |motherboard|. Phone lines connect to the modem through expansion apertures at the back of the system. The external modem is a separate unit which uses an RS-232 cable to connect to the computer's serial port. The term serial is used because it transmits data in a bit-by-bit fashion. The phone lines connect at the external unit.
Modems are generally categorized by the speed they transmit data. The two units of measurement used are the baud rate, and the bits per second (bps). Though these two terms are often used interchangeably, they are not the same. Baud refers to the number of packets of data sent each second. If each packet of data contains a single bit, then the baud and bps are the same. Newer modems can include several data bits in each packet. This means the number of bits per second is a multiple of the baud rate. For example, a 2400 baud modem, transmitting 4 bits per packet, transmits data at 9,600 bits per second. Commonly used speeds range from 300 baud (now considered quite slow) to 14,400 bps, though faster speeds are available.
