What is the role of a CD (carrier detect) signal?

The role of a CD (carrier detect) signal is to indicate whether an interface is active. This signal is used in telecommunications to determine if a communication line is currently in use. When the CD signal is active, it means that the carrier signal is present, indicating that the device is ready to send or receive data. This status is vital for devices in a network, as it helps in managing the flow of data and ensuring proper communication between devices.

When an interface is deemed active due to the presence of a carrier detect signal, it allows the network devices to synchronize and effectively manage their communications. This function is crucial for preventing data collisions and ensuring that transmissions occur smoothly. The other options do not accurately describe the role of the CD signal, as they pertain to different functionalities unrelated to the indication of interface activity.