The patent says:
However, there are particular devices that do not follow the IEEE 802.3 Auto-Negotiation Protocol, and are instead forced into either 10 Mbps or 100 Mbps operation. Customers who use the Auto-MDI/MDIX algorithm have grown used to not requiring special crossover cables with products that implement the algorithm. When they force speed/duplex for some limited applications, the algorithm breaks down and link is not established. The root of the problem lies in the Auto-Negotiation state machine which turns off an auto-negotiating (AN) transmitter if it detects non-FLP (non Fast Link Pulse) energy on the wire (as occurring during forced mode). When that AN transmitter is turned off, it will cause the Auto-MDI/MDIX node to incorrectly determine its link state and cycle endlessly. The logical elements described below will allow operation of Auto-MDI/MDIX with nodes that are Auto-Negotiating even when the node in question is operated in forced mode.
So if you set the speed/duplex manually, that turns off autonegotiation, and changes the signal on the wire. That causes the other end to turn off autonegotiation as well. The Auto-MDI/MDIX algorithm can no longer work out whether it has a link or not and cannot function.
(The patent basically covers "try swapping the connection pairs, randomly, every few miliseconds, until a link is established")
(It might be that my use of wording wasn't clear, but...) As I wrote both end is manually set to fixed speed and fixed duplex. I'm looking for a technical reason, if any, why auto-MDIX depends auto negotiation. – Yasushi Shoji – 2014-02-20T06:52:00.633
My reading of it is that manual setting turns off the link pulses and therefore it's impossible to determine whether you've got the right pair or not, although I'm not entirely sure of that. – pjc50 – 2014-02-20T08:14:06.850