3.2 Modems

The telephone network is often used for long-distance connections. The telephone is used for audio communication. If we want to use phone lines for computer communication, then the data information has to be modulated at the source party and demodulated at the destination party. The communication happens both ways, so both parties need a modulator/demodulator, i.e., a modem.

A modem is an appliance that connects to a PC or a router using a modem cable (i.e., in the case of a PC, by the V.24 interface on the COM port). The second outlet of the modem is used to connect to the phone line. The modem might be built into the computer or in the form of a PCMCIA card inserted into a notebook or laptop. In such cases, there is no need for a modem cable. Today general users are recommend to use modems with USB ports. USB modems are connected to a USB port on the computer’s side.

We can build the line between modems ourselves (for example, between two buildings). In such cases, it...