IEEE 802.3's clause 184.108.40.206 specifies a certain a MDI return loss. The specification states that this return loss shall be maintained at all times when the PHY is transmitting data or control symbols. How does one measure the return loss, when the PHY is transmitting data or control symbols?
The Gigabit Ethernet Consortium (of the IOL from the University of New Hampshire) has published a reference PMA test suite, which lists MDI Return Loss as a test (found in page 11 of the document here). The test procedure there states to put the PHY in Test Mode 4 while performing the Return Loss measurement to satisfy the "when the PHY is transmitting data or control symbols" requirement. But wouldn't this mess up the VNA S11 measurement? If the PHY is already transmitting symbols, how would the VNA differentiate between what the PHY is transmitting versus a reflection of what the VNA is sending into the channel?