Hi Dave,
Well it is not like a keepalive signal.
Presence of DCD signal indicates the the Link connected to the serial interface is UP and ready for use.
On Router serial interface, basically you will be using a V.35 interface interface cable to connect to a Lease line modem, which inturn connects to the telco network using copper wires ( local circuit)
Between the Serial interface of the router and the LL modem, V.35 standard is used for data communication. DCD is an indication signal provided by the modem to the router, stating that the leased link is up. If leased line is down, then the modem will not send the DCD signal to the router's serial interface, hence the router will declare that the line is down.
HTH
-VJ
