In this case for AT&T FTTH there are two seperate devices a ONT and AT&T's router. The ONT is a media converter that converts the GPON (fiber) to Ethernet. AT&T

In this case for AT&T FTTH there are two seperate devices a ONT and AT&T's router. The ONT is a media converter that converts the GPON (fiber) to Ethernet. AT&T's router is superfluous in the scenario where the customer wants to user their own router. The only reason AT&T's router is necessary is to perform the 802.1x authentication. Living in Central Florida, the lightning capital of the United States, I take electrical isolation very seriously. This is what is so great about fiber. Instead of having a another separate conductor running into your house (twisted pair or coax) there is just a glass insulator (fiber). So no extra isolation needed.