Ethernet PHY is a analog/digital mixed chip.So power must split.EMC/EMI is importance about Ethernet disign.MII/RMII/SMII/GMII is tradition parallel bus.BUT SGMII is a LVDS interface.LXT970 Datasheet is good for fast ethernet.Marvell&BroadCom's PHY documents is good for FE&GE ,too.