I observe in serail interface have common line i.e. DCD=up DSR=up DTR=up RTS=up CTS=up?? what is mean this and use.
These are control signals (dedicated wires in the cable) used in serial interfaces that describe the state of two interconnected devices.
- DCD - Data Carrier Detect. Commonly used with analog modems, this signal is set by a modem if a successful connection has been established with the opposite modem (a carrier wave has been detected). I am not sure if there is any sensible meaning for this signal when using serial interconnections directly between two routers.
- DSR - Data Set Ready, DTR - Data Terminal Ready. Each of this signals is used by one end of a serial connection to announce its readiness (whether the device is activated). One end of a serial link sets the DSR and verifies the DTR, the other end does this in the opposite manner. Usually, a router is called Data Terminal while a modem is called Data Set.
- RTS - Request To Send, CTS - Clear To Send. These two signals are used for hardware flow control to enable both devices on a serial link to announce their readiness to accept data.
You may find detailed information if you look for details on RS-232.