carrier detect (Modem) (CD)

The term carrier detect( CD) is used in interface and modem technology, furthermore in local area networks and in radio networks.

As far as interface and modem technology is concerned, the carrier detect signal is sent from the modem or peripheral device to the personal computer( PC) to indicate to it that the modem/peripheral device is ready for operation. The modem obtains the CD signal from the carrier signal of the telephone network. If the carrier signal is present, the modem activates the CD signal. The computer monitors the activated line and starts the connection setup.

In local networks, carrier detection prevents two or more stations from accessing the transmission medium at the same time. For this purpose, the stations willing to transmit monitor the level on the transmission medium. Only when there is no level can a station access the transmission medium.

In radio LANs, collisions occur due to interference from carrier signals. To avoid collisions, a station willing to transmitchecks the radio channels to see whether they are occupied or free and selects an unoccupied radio channel for radio transmission.