Abstract: In one embodiment, physical layer error signatures of a communications link are determined by ascertaining characteristics of erred and correct modulation symbols received by a physical receiver of a communications device, such as, but not limited to, by a receiver of a packet switching device in a communications network normally used to receive data traffic. In one embodiment, a physical receiver of a physical communications device receives a particular modulation symbol and determines whether the received particular modulation symbol is an expected modulation symbol in a predetermined sequence of modulation symbols. One or more storage devices are maintained with one or more error signature measurements based on determining whether the particular modulation symbol is the expected modulation symbol in the predetermined sequence of modulation symbols, with the error signature measurements including performing a comparison to a threshold value or a current maximum value.