I don’t have fiber access, but at least for cable, my provider (formerly Kabel Deutschland, now Vodafone) allows me to put the modem/router into "modem only" mode, which then allows me to use my own router. Outside of Fritzbox (which is again a whole integrated thing; with questionable features) there aren’t many DOCSIS modems freely available, and the no-name china devices don’t seem much better than my Vodafone Box.

In the U.K. you get a PON which gives you a cat5 gig or mgig port, you then connect your router and pppoe to your ISP. Most ISPs offer a managed router but the ISPs I’ve chosen have always allowed the pppoe option.

Faraday fabric is inexpensive, you can use ethernet to your own router and wrap the isp's in it.

The "glass fiber modem" is an inherent part of the GPON network. These are complicated. The "P" stands for "passive". Yours and and up to 127 other houses are all on the same "light domain" i.e. the downstream is passively split, and the upstream is passively combined, in optical boxes that don't even have electrical parts.

This needs crazy accurate timing for the upstream. The head end needs to know the exact delay to your particular box to give it a "grant" to transmit at exactly the right time so transmit bandwidth is not wasted by idle time or multiple boxes transmitting at the same time and corrupting each other.

You don't want brand X modems with dodgy configurations in this. Of course as a consumer you'd want "as little modem as possible" i.e. just give me an ethernet port running DHCP or PPPOE and let me do the rest.

You say "why should I have to pay", but they really haven't said or suggested anything about how they'd rather you paid for anything. They're talking about having an option to supply one's own device, not about requiring so.

The common rationale behind this I'm aware of is that an ONT device is technically a computer with persistence, hosting arbitrary code and data that you cannot (or at least not supposed to) audit or alter, despite being on your premises, operated on your cost (electricity, cooling, storage), and specifically deployed for your use. These properties hold for SFP modules too in general, not just SFP ONTs (they're all computers with persistence).

The catch is that this is further true for all of these kinds of modems.

The counter-catch is that despite that, for DSL specifically, you could absolutely bring your own modem, hw and sw both.

The counter-counter-catch is that with DSL, you were not connecting to a shared media, but point-to-point. This is unlike DOCSIS and GPON, where a misconfigured endpoint can disrupt service for other people, and possibly damage their or the provider's devices and lines.

That's all the lore I'm aware of at least.

I don't know if it's the case in Germany, but here in France consumer FTTH networks are of the GPON persuasion. These need to handle encryption and be able to properly register on the tree, so I'm not completely shocked they require some form of ISP-provided device to terminate the fiber connection.

There's also a EU law which says that users should be able to bring their own modems / routers, so AFAIK providers say that this particular terminal device is still "on their side of the network".

I've seen such devices come in two varieties.

One is a separate device which plugs on the optical network, does the encryption and stuff, and then exposes an ethernet port which is connected to the actual router which does wifi, etc. With SFR and Bouygues, it was trivial [0] to replace the ISP-provided router with one of your choosing. You get the normal external IPs and you do your thing. The ISP router sleeps in its box in storage. This was my setup up until a few years ago, with both these providers. Now SFR has moved to CGNAT, but the setup is the same, so I expect users to still be able to switch routers (but I haven't tested, since I'm not a client anymore).

Then there's Free, who provides a single device that connects to the fiber, does routing, wifi, etc. In this case, it's possible to flip a switch in its settings for it to act as a bridge (don't know how wifi behaves in this case, if it stays on). It then only accepts a single downstream client, which gets the external IP. SFR had a similar setup for DOCSIS.

I'm not familiar with how Orange, the biggest operator, functions. But I understand they have a general tendency to be a PITA so YMMV with them.

---

[0] For Bouygues, this device only talked on a tagged VLAN100 for some reason. On the SFR, the network expected you to send a client id in the DHCP request.

You’d need to be able to replicate whatever configuration the ISP provided device has, and they won’t give you that.

FTTH here in Australia is the same, you’re stuck using the network providers device, which just provides an Ethernet port, and a POTS port if you’re in to that sort of thing, with your LAN device connected behind it.

There was fierce lobbying back in the day (shout out to Simon Hackett / Internode) for our national broadband network to be simple dark fibre and that ISPs could build on top of that to provide innovation and differentiation.

Instead what we got was a bunch of ISPs that resell the National Broadband Network’s expensive wholesale plans with little in the way of either differentiation or innovation.

Edit to add: what the sibling comments said too.

Yes, with right kind of PON SFP stick this is possible.

Most kinds of PON sticks are still in the $150-300 range though for XGS-PON

(I use an XGS-PON stick with AT&T instead of their modem)

They most likely use GPON so the optic is going to see return traffic for your neighbors. So they make it hard (but not impossible) to bring your own optic or media converter.

If I recall, for something like GPON or XGS-PON, you end up having to clone the various attributes of the original for it to work properly. This typically includes serial number, hardware id, firmware identifiers, etc.