Common mode signals are voltages that appear on both conductors of a two wire twisted pair network. Differential signals are the useful voltages that normally convey the information. However all linear systems e.g. amplifier front ends in communication transceivers can only copy with a certain amount of common mode voltages. Eventually at a certain level the front end receiver cannot reject the common signal and this manifests itself into a small but significant output signal which is the difference between the common input from the amplifier. Any amplifier is never perfect and there is always an output even if the signals on each conductor are identical and this is expressed as the common mode rejection ratio for a differential amplifier. The CMRR usually depends on frequency of the noise
Depending on the frequency of the common mode input signal the resultant difference might seem as if it is real communication signal thus occupying the available bandwidth but not conveying the intended data. A neuron see would see this typically as a corrupt packet with a bad CRC.
Both the Smart Transceiver and to a lesser extent the FTT-10A transceiver have the ability to work in the most harsh common mode noise environments. Specifically the EN 61000-4-6 level 3 Conducted CM Immunity test puts a 51Vpp (10Vrms) signal in Common Mode onto the network, and for an FT smart Transceiver at 100Khz we will see about 3Vpp inboard of the transformer into the FT receiver that we are able to reject and communicate without errors.