A modem is a device that is capable of encoding digital information by varying one or more properties of the carrier signal, sending this signal through a telephone line, and decoding the information once it has been transmitted by demodulating the signal. The modem on one side of the communication line modulates and transmits the data, while the modem on the other side receives and demodulates it.
Before a modem can exchange information with another modem, it first needs to initiate communication by opening the line, dialing the number and awaiting confirmation. The final step, commonly known as “handshaking,” is where two devices align their speed of communication. The transfer speed of older modem variants, such as the V.90 dial-up modems, is limited to 56 kilobits per seconds.
Modern modem variants, such as ADSL, work by dividing the available frequencies in the telephone line. The “A” in “ADSL” stands for “asymmetric,” which reflects the higher speed of downstream in relation to upstream bandwidth. Although ADSL modems provide higher speeds in comparison to dial-up modems, they are affected by the distance between the user and the central office of the Internet service provider. This is the result of incompatibility between voice signal amplifiers in the line and ADSL signals.
Cable modems send and receive data through a coaxial cable that normally carries television signals. The downstream data takes a single 6-megahertz slot, whereas the upstream requires only 2-megahertz worth of bandwidth. For the communication to be functional, the end with the ISP needs to have a cable modem termination system.