The cable port's link state. The physicaldown(1) state indicates
that the CLT is not powered on. The linkdown(2) state indicates
that the CLT is powered on, but registers fail. The linkup(3)
state indicates that the CLT is powered on, and registers
successful. The loopback(4) state indecates that the CLT is
powered on and registers successful, but loopback has detected.