CityFibre ONT

I'm probably not the most educated to talk about this, but why wouldn't it be possible to remove the Calix 801Gv2 ONT and plug the Fibre straight into a SFP module using a Mikrotik router or similar?
Is whatever on the other end of the fibre locked to using the Calix ONT?

Edited by 0007 (Thu 30-Dec-21 23:47:25)

Who says it isn't possible?
It might well be possible. I've not seen anyone try it.

It works over Openreach FTTP if you spoof the details of your existing/registered ONT.

Give it a try and let us know.

It won't work. It's not point to point fibre but GPON which relies on the OLT permitting a device to connect to it.

While you do get "GPON ONT" SFP modules, the OLT needs to authorize the ONT before it will be allowed on the network.

There are a few issues to address:

The ONT is the demarcation point for the service. Replacing it with a third-party device may make the provider unhappy, especially if any vendor-specific remote management functions are not implemented.

GPON has a number of ONT authentication methods, most commonly just the serial number. Any replacement device would have to be programmed with the serial number of the existing ONT to fool the headend OLT.

You need an appropriate SFP. The cages on most switches and routers implement 1000BASE-X/10GBASE-X signalling for optical SFPs (which are just electical to optical media converters), and SGMII signalling for copper SFPs (which contain a PHY interface chip). Plugging in a regular GPON media converter SFP just won't work as the switch/router knows nothing about G.984 GPON protocols (PLOAM, GEM, OMCI, etc.) - you need one with an embedded processor which implements the various GPON protocols on the optical side plus an ethernet-like interface on the electical side to communicate with the switch/router.

You can find a number of threads on the Mikrotik and Ubiquiti forums discussing compatibility and issues with various embedded-ONT GPON SFPs in those switches/routers.

In reply to a post by k4a:

It won't work. It's not point to point fibre but GPON which relies on the OLT permitting a device to connect to it.

While you do get "GPON ONT" SFP modules, the OLT needs to authorize the ONT before it will be allowed on the network.

Which it probably does by serial number so simply spoofing that will likely work.

May be desirable to replicate a few other things, too, Generally not a major deal.

The bigger challenge is actually changing the serial number on the GPON ONT SFP.

In reply to a post by 0007:

I'm probably not the most educated to talk about this, but why wouldn't it be possible to remove the Calix 801Gv2 ONT and plug the Fibre straight into a SFP module using a Mikrotik router or similar?
Is whatever on the other end of the fibre locked to using the Calix ONT?

You mean like this? http://www.dslreports.com/forum/r32230041-Internet-B...

In reply to a post by 0007:

I'm probably not the most educated to talk about this, but why wouldn't it be possible to remove the Calix 801Gv2 ONT and plug the Fibre straight into a SFP module using a Mikrotik router or similar?
Is whatever on the other end of the fibre locked to using the Calix ONT?

If using Nokia or Huawei GPON SFPs, ensure the host MikroTik router is running an ARM based processor. Won’t work on MIPS or Tile proc.

In reply to a post by k4a:

It won't work. It's not point to point fibre but GPON which relies on the OLT permitting a device to connect to it.

While you do get "GPON ONT" SFP modules, the OLT needs to authorize the ONT before it will be allowed on the network.

I'm aware of that. Openreach is the exact same. That's why I said you need to spoof the original ONT details.

The fact it's GPON is why I believe it might work, not why it won't work.

There's only 1 way to find out.

Edited by j0hn83 (Fri 31-Dec-21 18:11:22)

In reply to a post by j0hn83:

I'm aware of that. Openreach is the exact same. That's why I said you need to spoof the original ONT details.

The fact it's GPON is why I believe it might work, not why it won't work.

There's only 1 way to find out.

Certainly works fine with Openreach.

Which have you done? Huawei or Nokia-Alcatel?

In reply to a post by Pheasant:

Which have you done? Huawei or Nokia-Alcatel?

Huawei. Took an Openreach 4 port, got console, took the customisation and cloned serial onto my own slightly later version Huawei 4 port with a 2.5G interface between optics and switch - the Openreach kit has a 1G backplane.

https://i.ibb.co/DpQLNx6/20211231-230222.jpg

Edited by CarlTSpeak (Fri 31-Dec-21 23:04:21)

Cool. Did you only spoof the serial from the donor ONT or did you need any other details to get auth working on the replacement? I’m reliably told Openreach just check the serial number and that’s it.

Took the BT customisations from the file system and amended some parameters as well to match the Openreach kit.

No idea if necessary or not but kept the equipment as close as possible to Openreach in every way possible. Later revision of hardware, matching configuration in every way.

When you SSH’d into the ONT were you still hooked up to the PON / in operation state 5? Presumably they use the same IP and username and passcode as the sticks?

In reply to a post by Pheasant:

When you SSH’d into the ONT were you still hooked up to the PON / in operation state 5? Presumably they use the same IP and username and passcode as the sticks?

The donor I acquired, opened up and connected to its console, just UART serial. Still had default Huawei credentials. Once firewall disabled could then log in using the same details and default IP address.

The newer kit is just logged into as a web console. It has no concept of routing the management traffic so you have to be connected to the same subnet but log in and you can change the serial number freely from the GUI.

EDIT: I can log in freely to the newer model, the firewall is a part of the customisations for BT in their own model. As it's dead equipment now no-one cares. The ONT reports the right things at the right time to the OLT, harms no-one and does no damage.

It was only required due to errors with the PON deployment here. For the cost of about £100 in ONTs and patch cabling it was a quite interesting intellectual exercise too.

Edited by CarlTSpeak (Sat 01-Jan-22 14:31:59)

This is on my list of things to experiment with, and there is a Huawei 1-port ONT where I currently have some time to work on it.

There is however no point my investing the time in figuring out a Huawei solution if my main location turns out to be Nokia. Is there a way to find out short of having a service installed?

In reply to a post by ft247:

There is however no point my investing the time in figuring out a Huawei solution if my main location turns out to be Nokia. Is there a way to find out short of having a service installed?

Not unless your neighbour on the same PON can tell you.

Some places are using the existing Huawei OLT that's in their exchange. Other areas are having new Nokia OLT's installed.

My PON (installed last year under the retro new site work) is Huawei.
My exchange is having a Nokia OLT installed and Openreach have my exchange marked as having FTTP by 2023 so I expect surrounding streets will be on Nokia kit.

There's also ADTRAN kit that could be used in future.

Short of stealing Mystic Megs crystal ball there's no way to tell for definite.

Edited by j0hn83 (Sun 02-Jan-22 12:18:58)

Thanks, I thought as much.

Take-up is very low on my street - Community Fibre beat Openreach to market by 6 weeks, but BT Wholesale are still showing 'planned', almost six months later. We are still on the list to go FTTP Priority at the end of April 22, so I guess they'll have to sort it by then.

The exchange (SW London, mostly residential) area is substantially complete (with the exception of one estate that is probably DIG copper) yet BTW are nowhere to be seen! I have seen one port used on a CBT a street over but apart from that there is minimal OR FTTP take-up.

In short sadly there's no neighbour I can ask.

I have read this thread with interest, having just signed up to Giganet/CityFibre. There is a lot of new geeky stuff for me to learn!

Naive question - does using a different ONT increase the speed at all? It seems a coincidence that the max speed of the CityFibre connections is ~900Mbps which is almost the same as the real speed of gigabit (and the speed of the GPON ONT that they provide). @CarlTSpeak, you mentioned your Huawei 4-port ONT has 2.5Gbe - does it enable you to get higher speeds, or are they limiting it further upstream?

In reply to a post by ft247:

This is on my list of things to experiment with, and there is a Huawei 1-port ONT where I currently have some time to work on it.

There is however no point my investing the time in figuring out a Huawei solution if my main location turns out to be Nokia. Is there a way to find out short of having a service installed?

Reckon it will be Nokia. They’ve pretty much now done away with Huawei ONTs for any new installs and Nokia appears to be the default even on a Huawei OLT. All due to Huawei being persona non grata and strict caps/limits on volumes and their future involvement in our 5G and broadband infra.

No it won’t make a difference.

Ultimately it’s the OLT in the headend exchange that sets the line rate and 1000 Mbps is the highest downstream profile on Openreach based GPON kit. In any event the 1000BaseT port on the box becomes the hard limit anyway. You physically cannot push more than ~940 Mbps (net) through such a port due to Ethernet framing and overheads.

Where other AltNet providers (not CityFibre or Openreach) are deploying XGS-PON based gear, then for customers wanting a greater then gigabit connection they will provision ONTs with port speeds with either 2.5Gbps or 10 Gbps capable copper LAN port(s) (indeed sometimes SFP+ cages) so that the Ethernet LAN port not a bottleneck.

Edited by Pheasant (Tue 31-May-22 22:10:15)

Thanks, although I don’t think you quite got my (CityFibre) question which whether they are using the limits of the 1Gbe port as a very crude bandwidth limiter. After all GPON is capable of just under 2.5Gbps down. Hence the question about the 2.5 Huawei equipment. It just strikes me as strange that the fastest speed available is exactly the same as 1Gbe. Anyway you sort of answered my question - the line speed is set at the exchange. Thanks!

No they are not using the port as any sort of bandwidth limitation device. GPON profiles are decided by the operator and set/configured the OLT.

Although GPON has a standards set 2.488 Gbps downstream and 1.2 Gbps upstream bandwidth limit. That is the total aggregate bandwidth available on the PON. There are no operators in this country which offer > 1 Gbps at the customer connection on GPON. There are several operators overseas that I'm aware of that do offer > 1 Gbps on GPON, but its not typical. This is generally because doing so compromises the available downstream bandwidth with other subscribers on the same PON.

Upload bandwidth on GPON is a slightly different matter. Recall than residential broadband users typically consume data on their connections in roughly 10:1 down/up ratio - therefore the upstream bandwidth can more 'safely' be over-subscribed - and that is exactly what CityFibre do on their network they oversubscribe their upload, such that they can offer a 1 Gbps nominal symmetric service.

On the other hand Openreach take a far more conservative view on upload and they will only ever allow up to a 220 Mbps customer connection upstream on their GPON net.

Back to the question related to Carl's "experiment", not that I want to step on his toes here, but the original 4-port Huawei ONT (no longer been installed since around 2019) as supplied by Opnreach had to my knowledge a shared 1 Gbps backplane connecting the optical GPON side of the device to the 4 Gigabit Ethernet ports, thus somewhat limiting the full throughput where there were more than 2 subscribers connected to the ONT. Swapping to a later model with a 2.5 Gbps connection on the backplane removes that impediment so that if your had 2 x Gigabit subscriptions they could 'pull' their full downstream bandwidth without this artificial limit die to the chipset design.

Thanks, that’s very insightful!

In reply to a post by kevinherring:

CarlTSpeak, you mentioned your Huawei 4-port ONT has 2.5Gbe - does it does it enable you to get higher speeds, or are they limiting it further upstream?

As Pheasant pointed out in his detailed reply, Carl only gets benefit from using a different ONT as the original supplied Openreach ONT is limited to 1Gb between the optical and Ethernet switch.
Carl has (or had) 3 separate connections running on that single 4 port ONT. 2 X 1 Gb and 1 x 330Mb.
With the Openreach 4 port ONT this would severely limit his throughput he can almost consume the entire PON with his own Huawei ONT.
Openreach allows all 4 ports to be used for 4 separate services.

As CityFibre don't allow multiple connections over a single ONT there would be no benefit to swapping it.

They will rate limit the line to your package speed at the OLT.

Edited by j0hn83 (Wed 01-Jun-22 17:40:02)

This ONT on a stick SFP from fs.com can allegedly be SSH'ed into and customised to your hearts content.

https://www.fs.com/products/133619.html

I have a feeling the Ubiquity ONT on a stick can also be customised too, but I have not looked into that. As time goes by unless the network vendors offer it as a solution it is inevitable that third parties will set into the void that they are leaving. A lot of it could IMHO be circumvented by offering PoE on the ONT. If Mikrotik can do it on a £25 wireless access point then it should be no problem to add to an ONT. I note that the Ubiquiti ONT can be powered via PoE. One suspects the option would solve a whole bunch of powering issues where there is currently no mains power at the most suitable location for the ONT. If it where a thing you would also start getting routers providing PoE on the WAN port too.

In reply to a post by Pheasant:

Although GPON has a standards set 2.488 Gbps downstream and 1.2 Gbps upstream bandwidth limit. That is the total aggregate bandwidth available on the PON. There are no operators in this country which offer > 1 Gbps at the customer connection on GPON. There are several operators overseas that I'm aware of that do offer > 1 Gbps on GPON, but its not typical. This is generally because doing so compromises the available downstream bandwidth with other subscribers on the same PON.

GPON (G.984.1) can also 2.4 Gbit/s up, 2.4 Gbit/s down, but most using profile 1.2 Gbit/s up, 2.4 Gbit/s down. Both Openreach and Cityfibre are using 1.2 Gbit/s up, 2.4 Gbit/s down.

BT was one main inventors of PON in 1980's. That fact everyone would had fibre in 1990's if BT had there way, there is a lot blame upon Margaret Thatcher who blocked that from happing.

I believe it’s EPON rather than GPON that you are referring to there. The former is an IEEE 802.3 derived standard. Not used at all in the UK in commercial rollouts but used extensively in places like Asia.

BT were indeed pioneers of PON and some of their early research papers are findable with a casual search. They also pioneered blown fibre, which for a long while they kept back from the competition. Some of the more (commercially) successful projects to have come out of Adastral Park.

GPON as 2.4 Gbit/s up, 2.4 Gbit/s down exists. I have no idea why no one using it. There is so little documented infomation about it. Even can not find anywhere there it is being used, which make it an even greater fascinating to me. What going on? Why?

You find it under heading 7 Bit Rate
https://www.itu.int/rec/dologin_pub.asp?lang=e&id=T-...

Never found commercial favour....

Plenty of standards get heavily pushed by vendors and simply wither and die on the vine. Countless examples abound. Here's one; even in the now utterly dominant world of Ethernet networking there were once competing standards for the same thing: 100 Mbps Fast Ethernet!

100baseT is now 100% universal - but back in the mid-nineties there was a three-way 'VHS/Betamax' war going on between 100BaseTX, 100BaseT4 and the standard pushed by the HP - yes I give you 100VG-AnyLAN... These were all defined IEEE 802. standards but only one ever made it through to dominant and lasting commercial success over the past nearly 30 odd years

Edited by Pheasant (Mon 06-Jun-22 21:07:59)

In reply to a post by amiga_dude:

GPON as 2.4 Gbit/s up, 2.4 Gbit/s down exists. I have no idea why no one using it. There is so little documented infomation about it. Even can not find anywhere there it is being used, which make it an even greater fascinating to me. What going on? Why?

More expensive optics in each home for no real gain.

To be honest I wanted to just get rid of the ONT but from everything you've guys have said it sounds like it's quite tricky, and has only been done on OR, and if it has been done it's undocumented.

There must be some other providers that have support for this sort of thing in the UK or no?
Any reason why ISP's etc are reluctant to provide support for this? Or they just can't be bothered?
Why even lock the OLT to specific ONT's I don't get it. There's PPPoE already for auth.
Do they just want a demarcation point or what is it?

Edited by 0007 (Sun 12-Jun-22 03:10:11)

In reply to a post by 0007:

Do they just want a demarcation point or what is it?

Certainly its a demarcation point on the Openreach network, its also worth remembering on a PON if you connect something directly to the fibre coming into your property that affects the light source it won't just affect you but all the other users of the PON so you can see why they don't want you doing it.

In reply to a post by 0007:

Why even lock the OLT to specific ONT's I don't get it

Makes sense to me, everyone's data is presented to every ONT on the PON, you have to have a method of addressing what ONT can read what data.

Edited by deleted (Sun 12-Jun-22 07:13:19)

Just say it is possible but I rather not do it. There are legal issues in which you could become in to a rather hot mess if you did. The main laws I could think of are:-
Computer Misuse Act 1990
Communications Act 2003

Thing like this have happen before, but with different intentions where people used to hack cable modems to get service level there wasn't entitled to. https://www.ispreview.co.uk/story/2009/03/19/virgin-...
It just something that comes to my mind. It the reason why I wouldn't do it. I wouldn't want to be the test case.

PS I am not saying you are thinking doing same thing.

There must be some other providers that have support for this sort of thing in the UK or no?

In the UK, not a single network. None.

Any reason why ISP's etc are reluctant to provide support for this? Or they just can't be bothered?

From the providers perspective there is zero benefit and more to lose than gain, due to the additional support complexity allowing third party devices into the network. It becomes yet another variable that could go wrong. There are also potential issues with physical network reliability and OH & safety if you give carte blanche on the optical interface. Fibre connectors are far less forgiving of abuse, soiling and mishandling than copper. On a shared PON architecture the operational integrity of all other users on the PON can be affected.

Why even lock the OLT to specific ONT's I don't get it. There's PPPoE already for auth.
Do they just want a demarcation point or what is it?

On a wholesale network like Openreach or CityFibre the network ISPs are able to decide on their own access control protocol whether that’s is PPPoE or IPoE/DHCP - this is separate from the essential lower level network access and security at the GPON level.

For this it is the network operator that must secure their network to ensure that there are no rogue devices that could intentionally steal bandwidth/access or simply other users data

In reply to a post by dect:

Certainly its a demarcation point on the Openreach network, its also worth remembering on a PON if you connect something directly to the fibre coming into your property that affects the light source it won't just affect you but all the other users of the PON so you can see why they don't want you doing it.

In reply to a post by Pheasant:

In the UK, not a single network. None.

Now that to me makes a lot of sense, but then my question to you would be how do other ISP's accommodate for this outside of the UK. They surely face the same issues that we would face by allowing others to have their own SFP modules etc.

In reply to a post by amiga_dude:

https://www.ispreview.co.uk/story/2009/03/19/virgin-...
It just something that comes to my mind. It the reason why I wouldn't do it. I wouldn't want to be the test case.

But that isn't really similar though is it, because you'd still need PPPoE info to be able to actually login to the network which is done at the OLT right, not the ONT?

Edited by 0007 (Sun 12-Jun-22 08:04:54)

PPPoE info to be able to actually login to the network which is done at the OLT right, not the ONT?

You’re confusing two completely different things.

Where PPPoE authentication is used by the ISP (not the network operator) it is negotiated outside of the OLT and ONT security. Neither of those devices take part in that authentication. PPP traffic is like any other traffic and is transparent. On the same PON network you can have multiple ISPs coexisting using a mixture of authentication both with or indeed without PPPoE

Yeah so even more to my point, having it go straight into your own SFP would be no issue for any ISP in terms of being able to get 'free internet' as another poster brought up. As there'd still be the process of authentication (PPPoE), which definitely isn't done at the ONT as you've mentioned.

The main points you all mentioned were:
1) Extra support/issues to accommodate for this w/ no benefit to the provider
2) Fibre connections at risk of being damaged due to connectors.
3) Can affect other Fibre users in the area due to shared PON architecture / light source
4) Eliminates 'Rouge devices' that could steal bandwidth/access data unintended for them.

My point is only to be able to have an ONT SFP Transceiver in my router and avoid having to use the ISP provided ONT.
My main question really now is how do ISP's out of the UK then accommodate for this when there's all these issues that are bought up by allowing the enduser to use a SFP module.

Edited by 0007 (Sun 12-Jun-22 08:33:09)

I’ve nothing more to add, feel like this is all rather circular…

If you feel that strongly about wanting to use your own SFP then you need to take it up with the infrastructure provider and see where it gets you, comments here have made it clear why its not a good idea.

In reply to a post by 0007:

Why even lock the OLT to specific ONT's I don't get it.

OLT manufacturers usually also sell ONUs. Out of the box most of them only have profiles for use of their own equipment as ONUs not a wide open one. Contracts likely have discounts on OLT hardware baked in and arranging an open profile would carry costs.

Some also use proprietary extensions for provisioning and monitoring which I think have already been mentioned. Openreach have streaming telemetry available from their ONUs using configuration in the firmware. Wouldn't happen with wires only.

Compatibility is also a consideration. Some ONUs just won't work with some OLTs. The link to a GPON SFP in this thread covers 3 vendors only.

On how some ISPs manage to offer wires only most don't. Even in the USA where people are used to bringing their own cable modems they require use of their own ONUs.

Provisioning systems, telemetry and monitoring, money, take your pick. This is how it is in the UK and for most PON customers worldwide.

In reply to a post by dect:

If you feel that strongly about wanting to use your own SFP then you need to take it up with the infrastructure provider and see where it gets you, comments here have made it clear why its not a good idea.

Doubt that'll get me anywhere really I think you probably know that aswell as me.
My main question now is, how do ISP's in other countries allow for people to use their own SFP's when all the issues mentioned previously can occur without an ONT.

If you use Init7 as an example how do they manage to allow customers to use their own SFP's?
Is there something different in their network that allows for it that we can't have for any reason?
Or is it just as simple as our ISP's here in the UK simply don't see a need to allow for something 'extra' and it's as simple as that since the majority probably won't use it.

Sorry if anyone took offense to what I said or if I was condescending at all.

Edited by 0007 (Sun 12-Jun-22 12:30:38)

In reply to a post by XGS_Is_On:

On how some ISPs manage to offer wires only most don't. Even in the USA where people are used to bringing their own cable modems they require use of their own ONUs.

Thanks for your reply, and yeah I've noticed most don't.
Originally I just wanted to first know if it was possible.
Now I know it's not possible, it's more of a why wouldn't the ISP allow for it. A number of issues have been mentioned in terms of why they don't allow for it. But then I can't see why other large ISP's in other countries have managed to get away with letting customers use their own SFP Transceiver.

Edited by 0007 (Sun 12-Jun-22 12:31:51)

In reply to a post by 0007:

If you use Init7 as an example how do they manage to allow customers to use their own SFP's?
Is there something different in their network that allows for it that we can't have for any reason?

Init7 don't use PON. Dedicated fibre and dedicated optics between home and their switch.

+1

Init7 aren’t your typical ISP either. Even in Switzerland! The self proclaimed “nerd ISP”. Mass market they certainly are not.

I think there is a notion here surrounding pluggable optics that needs to be challenged.

In a point-to-point network, pluggable SFP/SFP+ etc optics are essentially “dumb” - that is they are simply an Ethernet transceiver with any layer 3 stuff done on the box they’re being plugged into.

In a PON network, pluggable ONT/ONU modules are very much not just a transceiver but an actual bit of NTE in one - that is they have some limited processing etc capability on the pluggable itself AND they need to be configured at Layer 2 (GPON) to be accepted by and work/talk to the rest of the network.

Furthermore that configuration and update is 99.9% designed to be done with and in concert with the headend OLT and the providers network management setup. They can (by exception) be configured through local means (SSH etc) but this is not how Openreach will manage a 25 million+ network of devices. Not only that such access varies not only between vendors of this gear but also across versions/generations of gear. It’s utterly inconsistent. Attempting to manage a network in this way would be suicidal.

Definitely makes sense now, thank you the both of you, @XGS_Is_On, and @Pheasant.

Just another question - asking the differences in infrastructure when having one ISP vs another.
How does that work, the OLT is always owned/managed by CityFibre if I'm not mistaken? So at what point, or what comes after the OLT that is ISP owned?

And if you change between different CityFibre ISP's what changes, they have to plug you into a different port or something somewhere right? Or is it just different PPPoE info and that's it?

So basically to simplify, if I were with Vodafone for example, and switched to Zen under the same line, what would they have to change to make this happen on their end?

Edited by 0007 (Mon 13-Jun-22 19:51:51)

OLT is own by infrastructure provider ie Openreach OTL is own Openreach, CityFibre is OTL is owned by CityFibre. OTL has nothing to do with ISP. What owned by ISP is anything send to you. So it like router kit, some ISP want back end contract others dont care.

Think of PPPoE is in a way like a SSH tunnel from you home (Router) that connects your to your ISP. In fact you can not see any of the hops that happen between your router until get to your ISP.

Thank you for your reply

Fully aware that the OLT is on the their end & I only have an ONT & the router does the PPPoE.
Just wanted to know what goes on at the other end of the Fibre, my questions above are focused on that, rather than what happens on the customers side.

If anyone knows it'd be helpful, as it will also clarify things when it comes to understanding how switching ISP's on an existing line would work.

Edited by 0007 (Mon 13-Jun-22 22:09:46)

CityFibre change the VLANs your ONU maps to on their equipment and you end up being switched to the new ISP. That's all there is to it from their point of view, VLAN changes.

PPP is nothing to do with CityFibre, they send your data to the ISP in the same way whether it's there or not.

Thanks for your reply.
All makes sense now, think that answers all my questions haha!

In reply to a post by 0007:

In reply to a post by XGS_Is_On:

On how some ISPs manage to offer wires only most don't. Even in the USA where people are used to bringing their own cable modems they require use of their own ONUs.

Thanks for your reply, and yeah I've noticed most don't.
Originally I just wanted to first know if it was possible.
Now I know it's not possible, it's more of a why wouldn't the ISP allow for it. A number of issues have been mentioned in terms of why they don't allow for it. But then I can't see why other large ISP's in other countries have managed to get away with letting customers use their own SFP Transceiver.

It is very much possible.
Here you will find some useful info how to actaully do it:

https://akeyx.github.io/blog/ont-hacking/
https://chochol.io/en/computer-networking/how-to-con...