As it was explained earlier, communications channels can operate in either the Analog or digital mode. However, a given circuit can operate only in one mode not both of them at the same time. Thus, there is need for translating from one to the other of the two modes. All voice communication occurs in the analog mode; yet, when we use voice-grade channels to transmit data we need to communicate with the receiving or sending computer in the digital mode. Modems are devices which function as "translators", changing the digital signals coming out of the terminal to analog so that it can travel along a voice-grade channel. At the receiving end, there is another modem, retranslating the analog signal into the digital signal the computer can accept Modem stands, for modulation-demodulation. Some modems are built into the terminal.