The term carrier detect (CD) is used in interface and modem technology, and also in local area networks and 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 the PC 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
. Forlocal networks, the carrier detection prevents two or more stations from accessing
at thesame 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 wireless LANs, collisions occur due to interference from carrier signals.
To avoid collisions, a station willing to transmit checks the radio channels whether they are occupied or free and selects an unoccupied radio channel for radio transmission.