This status light is directly coupled with what the modem is sending the DCD line.
So long as the DCD signal is high, the computer can assume that any data coming from the modem was sent from the remote side.
A null modem typically connects the DTR output of each computer to both the DCD and DSR inputs of the other.
When used in this scenario, DCD is used to simply detect the presence and/or readiness of the other side to start a session.
Other equipment (such as the console port of a router) may expect characters to be transmitted,[clarification needed] but the DCD signal high is still a prerequisite for every communication.
Typically, the time-code output of the time source is transmitted to the computer over the same serial line.
Although these both refer to the same physical port, one important distinction between the way Linux treats these two device names has to do with the DCD line.
When cua0 is used - as it is when placing telephone calls - Linux assumes that the software needs to access the port while DCD is low for the purpose of dialing the number, so this blocking behavior doesn't exist.