What do pull-up and pull-down resistors do in an ethernet PHY?
What purpose do these resistors serve?


Those resistors are line terminators and also double-up as phantom power providers should phantom power be needed somewhere.

For the transmit connection, the likely explanation is that the output driver is a constant current (high compliance) driver and therefore needs pull-up resistors and a DC connection.

For the receiver, it is likely to be a diff amp that has its own biasing arrangement that should not be disturbed by an external dc connection hence, capacitors are used.