Depending on the type of carrier detection used, packet signals are not always present when carrier is detected. It is possible to use an external means of carrier detect. The receive signal is still connected as described above, to provide reception of packet data. Energy Carrier Detect. Many TNCs use energy carrier detect as a default.
Energy is any signal the receive circuits detect.
This includes packet signals, voice signals, and any type of noise. If you are listening to the frequency, anything you can hear is considered energy. Data Carrier Detect. Data carrier detect circuits are built in several different ways. On the other hand, if a signal is being received that does not contain real data, the TNC will transmit.