CI testing is a high frequency
noise injection into the system. CI testing can inject noise into the signal
lines of the PHY's system, sweeping over a wide frequency. The frequency sweep
can interfere with some digital blocks inside the PHY, resulting in failure or
link loss. There is importance to have a good common mode noise path from the
signal lines to connector ground to reduce the amplitude of the noise. A good
path for the noise to flow directly to earth ground can also help with CI
testing for the shielded cables.