Which signaling method is used to indicate that a call has been connected by a modem?

The signaling method that indicates a call has been connected by a modem is Carrier Detect (CD). This signal is part of the modem's operation, showing that a successful connection has been made and that the modem is ready to transmit or receive data. When a modem establishes a link, it sends a signal back to the device indicating that the connection is active, which is essential for ensuring reliable communication.

In contrast, signal strength measurement pertains to assessing the quality of the received signal but does not directly indicate a call connection status. Data encryption codes are used for securing data during transmission and are not involved in signaling connection status. Similarly, an IP transmission signal relates to packet-switched networks and is not applicable in the context of modem connections, which typically rely on analog or circuit-switched signaling.