MII, GMII, RGMII, XGMII, XAUI, etc. are all MAC to PHY interfaces. They all serve a similar purpose, but have slightly different characteristics. MII = 4 bit parallel for 100M, RMII = 2 bit parallel for 100M, GMII = 8 bit parallel for 1G, RGMII = DDR 4 bit parallel for 1G, XGMII = DDR 32 bit parallel for 10G, XAUI = 4x3.25 Gbps serial for 10G. PHY to PHY would be the actual physical layer that goes over the Ethernet cable. The protocol signals (CLK, TX_EN, TX_ER, etc.) are only sent short distances as they re not intended to be shipped over many meters of cable. To achieve long distance transmission, the PHY chip will encode the signal so that it can survive the trip and be successfully decoded on the other end. This is different for different speeds of ethernet. 10M is simply machester encoded serial. 100M is encoded with a 4b/5b line code followed by MLT-3. 1G uses all 4 pairs in parallel in both directions with adaptive equalization and echo cancellation and PAM-5 encoding. 10G serial is simply 64b/66b encoded, but 10G over CAT-6 cable it quite a bit more complicated, using THP encoded PAM-16 with LDPC coding for forward error correction.