Fast Ethernet +/- pins go through a special 1-turn transformer for electrical isolation installed between the PHY and RJ45 connectors, commonly referred to as "

Fast Ethernet +/- pins go through a special 1-turn transformer for electrical isolation installed between the PHY and RJ45 connectors, commonly referred to as "the magnetics" in Ethernet world. It's physical ferrite and copper object molded like a chip, therefore it can't be trivially integrated into a silicon or in MCM. Hence most Ethernet chip require one external to the chip, and in this case it is indeed external to the chip, and integrated into the board at the same time. (yes a cap in-line works too if you know what you're doing)