BTW: in the EU there is movement towards mandating ISPs allow BYOD, including fibre ONTs.
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The benefit with an ONT (or even DOCSIS dumb modem) managed by the ISP is that they can do fleet upgrades much quicker as they don't have to keep all old protocols running. For instance the GPON -> XGSPON upgrade that some ISPs are running right now (or DOCSIS 3 upgrade) really only works well if you can turn off the old protocol which requires swapping out all ONTs/DOCSIS modems.
If customers bring their own stuff then you're stuck with these things for much longer.
But in cases where the ONT just looks like a media converter and you have a separate router I really can’t see any reason for the customer to provide their own ONT. Especially given PON is a shared medium so a misbehaving ONT can affect other customers.
I agree, and that is a problem. The rules and regulations are different in different countries. In Austria for instance the ISP can force you to use a specific DOCSIS modem or ONT but they have to provide you with a transparent way to connect to it (bridge mode etc.). Which from where I'm standing is a good tradeoff because it gives the ISP the flexibility to do mass migrations without having to consider very old deployed infrastructure.
With PON I think it doesn't matter all _that_ much but for instance people running ancient DOCSIS modems and limited frequency availability has been a massive pain for people stuck with DOCSIS infrastructure that want more upstream and can't.