Single mode fiber cable: which SFP module? : homelab

[–] [email protected] 5 points 5 months ago* (last edited 5 months ago) (2 children)

Starting with brass tacks, the way I'm reading the background info, your ISP was running fibre to your property, and while they were there, you asked them to run an additional, customer-owned fibre segment from your router (where the ISP's fibre has landed) to your server further inside the property. Both the ISP segment and this interior segment of fibre are identical single-mode fibres. The interior fibre segment is 30 meters.

Do I have that right? If so, my advice would be to identify the wavelength of that fibre, which can be found printed on the outer jacket. Do not rely on just the color of the jacket, and do not rely on whatever connector is terminating the fibre. The printed label is the final authority.

With the fibre's wavelength, you can then search online for transceivers (xcvrs) that match that wavelength and the connector type. Common connectors in a data center include LC duplex (very common), SC duplex (older), and MPO (newer). 1310 and 1550 nm are common single mode wavelengths, and 850 and 1300 nm are common multimode wavelengths. But other numbers are used; again, do not rely solely on jacket color. Any connector can terminate any mode of fibre, so you can't draw any conclusions there.

For the xcvr to operate reliably and within its design specs, you must match the mode, wavelength, and connector (and its polish). However, in a homelab, you can sometimes still establish link with mismatching fibres, but YMMV. And that practice would be totally unacceptable in a commercial or professional environment.

Ultimately, it boils down to link losses, which are high if there's a mismatch. But for really short distances, the xcvrs may still have enough power budget to make it work. Still, this is not using the device as intended, so you can't blame them if it one day stops working. As an aside, some xcvrs prescribe a minimum fibre distance, to prevent blowing up the receiver on the other end. But this really only shows up on extended distance, single mode xcvrs, on the order of 40 km or more.

Finally, multimode is not dead. Sure, many people believe it should be deprecated for greenfield applications. I agree. But I have also purchased multimode fibre for my homelab, precisely because I have an obscene number of SFP+ multimode, LC transceivers. The equivalent single mode xcvrs would cost more than $free so I just don't. Even better, these older xcvrs that I have are all genuine name-brand, pulled from actual service. Trying to debug fibre issues is a pain, so having a known quantity is a relief, even if it means my fibre is "outdated" but serviceable.

[–] [email protected] 1 points 5 months ago* (last edited 5 months ago) (2 children)

Do I have that right? If so, my advice would be to identify the wavelength of that fibre, which can be found printed on the outer jacket. Do not rely on just the color of the jacket, and do not rely on whatever connector is terminating the fibre. The printed label is the final authority.

You got it right! On the cable unfortunately there is no wavelength printed (it's a cable made for my ISP), but I've read on a forum (that talks about this ISP):

GPON adopts WDM to transmit data of different upstream/downstream wavelengths over the same ODN. Wavelengths range from 1290 - 1330 nm in the upstream direction and from 1480 - 1500 nm in the downstream direction.

I think that I'll go with 2 SFP+ 10GBASE-LRM 1310nm 2km Module, Cisco that costs 29€

Edit: In the meanwhile, do you have any 2,5Gbe PCI card that you suggest (I need it to connect OPNsense to the ONT via PPPoE)? I've found only the QNAP QXG-2G1T-I225 that costs about 75€ or the Edimax EN-9225TX-E for about 41€ (but I haven't read much about this one).

[–] [email protected] 1 points 5 months ago* (last edited 5 months ago) (1 children)

In my first draft of an answer, I thought about mentioning GPON but then forgot. But now that you mention it, can you describe if the fibres they installed are terminated individually, or are paired up?

GPON uses just a single fibre for an entire neighborhood, whereas connectivity between servers uses two fibres, which are paired together as a single cable. The exception is for "bidirectional" xcvrs, which like GPON use just one fibre, but these are more of a stopgap than something voluntarily chosen.

Fortunately, two separate fibres can be paired together to operate as if they were part of the same cable; this is exactly why the LC and SC connectors come in a duplex (aka side-by-side) format.

But if the ISP does GPON, they may have terminated your internal fibre run using SC, which is very common in that industry. But there's a thing with GPON specifically, where the industry has moved to polishing the fiber connector ends with an angle, known as Angled Physical Contact (APC) and marked with green connectors, versus the older Ultra Physical Contact (UPC) that has no angle. The benefit of APC is to reduce losses in the ISP's fibre plant, which helps improve services.

Whereas in data center and networking, I have never seen anything but UPC, and that's what xcvrs will expect, with tiny exceptions or if they're GPON xcvrs.

So I need to correct my previous statement: to be fully functional as designed, the fiber and xcvr must match all of: wavelength, mode, connector, and the connector's polish.

The good news is that this should mostly be moot for your 30 meter run, since the extra losses from mismatched polish should still link up.

As for that xcvr, please note that it's an LRM, or Long Range Multimode xcvr. Would it probably work at 30 meters? Probably. But an LR xcvr that is single mode 1310 nm would be ideal.

[–] [email protected] 1 points 5 months ago (1 children)

Thanks for your precision! To the ONT arrives a single fiber with a SC/APC connector, but this is not a problem since I will be using the ONT provided and use the 2,5Gb copper port to connect it to OPNsense (looking for a 2,5 Gb PCI card). The 2 fiber that I've asked them to run (from OPNsense to server) are terminated with the same SC/APC connectors and I was thinking about using this SC female/female adapter and this SC/APC to LC cable that I've just realized that are still APC...I'll have a look if there are SC/APC to LC/UPC cables

As for that xcvr, please note that it’s an LRM, or Long Range Multimode xcvr. Would it probably work at 30 meters? Probably. But an LR xcvr that is single mode 1310 nm would be ideal.

Is it a LRM? Damn, I didn't realized since I've filtered for single mode. If the filter doesn't work, I've no idea which is LR. Would you be so gentle to point to a cheap one for single mode finer?

[–] [email protected] 0 points 5 months ago (1 children)

I've only looked briefly into APC/UPC adapters, although my intention was to do the opposite of your scenario. In my case, I already had LC/UPC terminated duplex fibre through the house, and I want to use it to move my ISP's ONT closer to my networking closet. That requires me to convert the ISP's SC/APC to LC/UPC at the current terminus, then convert it back in my wiring closet. I hadn't gotten past the planning stage for that move, though.

Although your ISP was kind enough to run this fibre for you, the price of 30 meters LC/UPC terminated fibre isn't terribly excessive (at least here in USA), so would it be possible to use their fibre as a pull-string to run new fibre instead? That would avoid all the adapters, although you'd have to be handy and careful with the pull forces allowed on a fibre.

But I digress. On the xcvr choice, I don't have any recommendations, as I'm on mobile. But one avenue is to look at a reputable switch manufacturer and find their xcvr list. The big manufacturers (Cisco, HPE/Aruba, etc) will have detailed spec sheets, so you can find the branded one that works for you. And then you can cross-reference that to cheaper, generic, compatible xcvrs.

[–] [email protected] 1 points 5 months ago (1 children)

Although your ISP was kind enough to run this fibre for you, the price of 30 meters LC/UPC terminated fibre isn’t terribly excessive (at least here in USA), so would it be possible to use their fibre as a pull-string to run new fibre instead? That would avoid all the adapters, although you’d have to be handy and careful with the pull forces allowed on a fibre.

The problem is the installation of the connectors. They've welded the fiber the SC/APC pigtails, I wouldn't be able to do that.

The big manufacturers (Cisco, HPE/Aruba, etc) will have detailed spec sheets, so you can find the branded one that works for you. And then you can cross-reference that to cheaper, generic, compatible xcvrs.

That would be very very generous of you; in the fiber section I'm pretty ignorant and I'm worried to purchase wrong items 🙈

[–] [email protected] 1 points 5 months ago (1 children)

I quickly looked up the HPE/Aruba transceiver document, and starting on page 61 is the table of SFP+ transceivers, specifically describing the frequency and mode. At least from their transceivers, J9151A, J9151E, JL749A, and JL783A would work for your single-mode, 1310 nm needs.

You will have to do additional research to find generic parts which are equivalent to those transceivers. Good luck in your endeavors!

[–] [email protected] 2 points 5 months ago* (last edited 5 months ago)

Thanks a lot! It seems that this from fs.com in the non Aruba compatible brands costs 32€ Thanks again!

Edit: And I've even found the LC/UPC to SC/APC cable!

[–] [email protected] 1 points 5 months ago* (last edited 5 months ago)

Re: 2.5 Gbps PCIe card

In some ways, I kinda despise the 802.3bz specification for 2.5 and 5 Gbps on twisted pair. It came into existence after 10 Gbps twisted-pair was standardized, and IMO exists only as a reaction to the stubbornly high price of 10 Gbps ports and the lack of adoption -- 1000 Mbps has been a mainstay and is often more than sufficient.

802.3bz is only defined for twisted pair and not fibre. So there aren't too many xcvrs that support it, and even fewer SFP+ ports will accept such xcvrs. As a result, the cheap route of buying an SFP+ card and a compatible xcvr is essentially off-the-table.

The only 802.3bz compatible PCIe card I've ever personally used is an Aquantia AQN-107 that I bought on sale in 2017. It has excellent support in Linux, and did do 10 Gbps line rate by my testing.

That said, I can't imagine that cards that do only 2.5 Gbps would somehow be less performant. 2.5 Gbps hardware is finding its way into gaming motherboards, so I would think the chips are mature enough that you can just buy any NIC and expect it to work, just like buying a 1000 Mbps NIC.

BTW, some of these 802.3bz NICs will eschew 10/100 Mbps support, because of the complexity of retaining that backwards compatibility. This is almost inconsequential in 2024, but I thought I'd mention it.

[–] [email protected] 0 points 5 months ago (1 children)

Regarding future proofing, I would say that anyone laying single pairs of fibres is already going to constrain themselves when looking to the future. Take 100 Gbps xcvrs as an example: some use just the single pair (2 fibres total) to do 100 Gbps, but others use four pairs (8 fibres total) driving each at just 25 Gbps.

The latter are invariably cheaper to build, because 25 Gbps has been around for a while now; they're just shoving four optical paths into one xcvr module. But 100 Gbps on a single fiber pair? That's going to need something like DWDM which is both expensive and runs into fibre bandwidth limitations, since a single mode fibre is only single-mode for a given wavelength range.

So unless the single pair of fibre is the highest class that money can buy, cost and technical considerations may still make multiple multimode fibre cables a justifiable future-looking option. Multiplying fibres in a cable is likely to remain cheaper than advancing the state of laser optics in severely constrained form factors.

Naturally, a multiple single-mode cable would be even more future proofed, but at that point, just install conduit and be forever-proofed.

[–] [email protected] 2 points 5 months ago

Regarding future proofing, I would say that anyone laying single pairs of fibres is already going to constrain themselves when looking to the future.

It could be right, but it depends on what people can run in the conducts. I was lucky to be able to pull those 2 cables. On the other hand, this is a rent apartment that I will soon leave :D

[–] [email protected] 5 points 5 months ago

You absolutely need the correct sfp. Multimode is not compatible with singlemode fiber.

[–] [email protected] 2 points 5 months ago (1 children)

Nobody is using MMF these days even for local runs. As to sfp, check https://fs.com and pick a matching pair that is cheapest. These days it makes sense to use 10G or 25G rather than 1G. Some people run 100G for their homelab, but even used it's pricy and noisy.

[–] [email protected] 2 points 5 months ago

Thanks, I didn't know that they could cost only 30€!

[–] [email protected] 1 points 5 months ago (1 children)

There definitely are optics available for single mode. The problem is that the wavelength will need to match what you are being sent from your provider. So we can't really answer your question, you will have to ask them.

1310nm is quite common but we can't assume your link is 1310nm.

[–] [email protected] 2 points 5 months ago (1 children)

I don't need to connect my provider fiber, but my router (OPNsense) to my server.

[–] [email protected] 2 points 5 months ago (2 children)

Oh, they just left you some extra patch cables? That was nice of them.

The wavelength of the fiber might be printed on it. I know some fibers are color-coded based on their wavelength, but I don't know if it's universal.

You'll need to match the modules to the fiber.

[–] [email protected] 2 points 5 months ago

I've checked on a forum and they say:

GPON adopts WDM to transmit data of different upstream/downstream wavelengths over the same ODN. Wavelengths range from 1290 - 1330 nm in the upstream direction and from 1480 - 1500 nm in the downstream direction.

So I guess that all the single mode SFP+ that have that range are ok.

[–] [email protected] 1 points 5 months ago

Unfortunately there is no wavelength on the cable, I'll check the ONT

[–] [email protected] 1 points 5 months ago

The cool thing about 1310nm light is that it will pass down either multimode or single mode fiber.

At that short of a distance, you should be able to use MM 1310nm SFPs.

As long as the SFPs can do the signalling speed needed by the link then I see no reason why it shouldn’t work.

[–] [email protected] 1 points 5 months ago

As long as both ends are a matching pair you should be ok. Of they ran single mode fibre use single mode sfps.

Check out FS.com Plenty of options for plugables.