Presented by: Richard Mellitz It has long been thought that common mode impairments may be easily controlled with common mode specifications. Differential simulation may indeed have some common mode included within the signal and noise transmission. The problem emerges between specification for separable interfaces such as cards, connectors, and chips. IEEE802.3 and OIF CEI standards often include a collection of common mode specifications measured at test points for these separations or interfaces. As 112 Gb/s PAM4 becomes more common place it is worth exploring common mode specifications and what they mean for performance. What matters more and what is not so important. Discussions include the nature of common mode, a historical perspective, procedures to compute the effects of CM, CM measurements, and practices to minimize the effect of common on differential signaling.