If the line is down, a problem may exist with the cabling, equipment somewhere in the circuit may be powered off or malfunctioning, or one end may be administratively down.

• 假如连线是关闭的，此问题可能是因为配线、在电路中某处的设备可能是关闭电源或功能不正常、或者一个端点可能被管理地关闭。