Different recommendations for ethernet front end terminaton?
It is a strictly hardware dependant issue. All of these PHY layer IC's have 50 ohm differential input and outputs to match the magnetics and ethernet cable. All of these IC's have PECL outputs but inputs can vary as to whether a bias current is needed or not. There are many PHY layer IC's on the market with various CPU/MPU connections, but at the magnetics layer the variations are minor, having mostly to do with bias currents. What is different is if the 3.3 volt source is built into the IC or is external, including receive and transmit. The slight differences are for impedance matching to a fine degree (including DC blocking caps). The goal is maximum frequency response with little DC bias current in the magnetics. Internal servo loops and 8bit/10bit conversion keeps DC imbalance in the magnetics to a minimum or bit errors could occur. There are designs where 48vdc is carried over the ethernet cable and taken off the tx and rx center taps for POE (power over ethernet).