PCI Express over Fiber [video]
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While at a higher level, thunderbolt and https://en.wikipedia.org/wiki/ExpEther can both of course work over fiber too!
(Q|O)SFP are basically just raw high speed serial interfaces to whatever - you see this a lot in FPGAs, you can use the QSFP interfaces for anything high speed - PCIe, SATA, HDMI…
> [snip]
> As explained in the intro, this doesn’t come without a host of compatibility issues, least of all PCIe device detection, side-channel clocking and for PCIe Gen 3 its equalization training feature that falls flat if you try to send it over an SFP link.
So, uh… what’s the benefit? How much overhead does Thunderbolt really introduce, given it solves these other issues?
Thunderbolt 5 offers 80Gbps of bidirectional bandwidth. PCIe 5.0 16x offers 1024Gbps of bidirectional bandwidth. This matters.
TB5 cables can only get so long whereas fiber can go much farther more easily. This means that in a data center type environment, you could virtualize your GPUs and attach them as necessary, putting them in a separate bank (probably on the same rack).
Good luck getting a 1Tbit tranceiver. Anydirectional. Also it's 512Gbitish per direction.
But yes I meant 512Gbps each way, to be clear.
One note: I believe the SMF max fiber length is 2km not 1m [1]. The data sheet [2] also says:
> - 2000m max on single mode fiber
[1]: https://www.vitextech.com/products/1-6t-osfp-2fr4
[2]: https://resource.fs.com/mall/resource/cn_osfp-2fr4-16t-data-...
There's already 800Gb transceivers readily available, 1.6 is probably getting preview deploys to some hyperscalers & other early adopters as we speak.
I couldn’t find any optical TB5 cables, but here’s a 4.5m TB4 one: https://www.owc.com/blog/the-new-superlong-40gb-s-owc-active...
And if TB3 is enough, Corning makes them in lengths up to 50m: https://www.corning.com/microsites/coc/oem/documents/ocbc/OE...
As for bandwidth, the medium transition seems to actually limit the author’s capabilities by losing some of the more advanced link-training features that are necessary for the highest-bandwidth PCIe 3 connections, never mind PCIe 5.
I don't think there is any reason TB couldn't do the same, beyond it would be even more niche to want non-modular/patchable cables+transceivers at those lengths (especially since fiber is often bundled dozens/hundreds of strands over a single trunk cable between racks).
And because of timings, my admittedly so-aonunderstanding that you can only get about 7m before you absolutely have to have a hub/repeater (unless you can speed up the speed of light considerably). This limit to how long a single length can be can't really be cheated without violating usb specs.
It's awesome if folks have packetized USB2. A pity it's not in the flipping spec though!!
That Corning made it 50m is wild. You need a virtual hub at the start that can pretend to be hubs 1-5 (so it's close enough to time well). Then a hub on the other side of the cable at (skinny) tree depth 6. Allowing for 4 devices under it (the number of ports on a usb2 hub in the spec. But you could work around by faking being not a skinny tree but a fat tree, maybe?).
- Learning : I want to learn about the lower level of PCIe and it's a good project. - Re-use of cabling : I have a bunch of single mode fiber bundle going around already. You can't find thunderbolt that just have a LC connector ... - Isolation : Active thunderbolt cable still often have copper for some low speed signals, they don't offer true galvanic isolation - Avoid dealing with thunderbolt. I want a custom chassis/pcb at one end and chips to convert from TB back to PCIe are not readily available to make custom stuff with ... (not as an individual anyway).
So yeah, if you want a ready to use solution, TB cable is absolutely a good choice, here I'm having some fun, learning in the process and hopefully sharing some of the knowledge.
Such a huge upgrade from the heavy thick 35 ft HDMI<->dvi cable I've used for so long.
Literally the only downside is figuring out how to roll it up, which I still haven't figured out how to do well with the 150ft cable I have.
It was astoundingly cheap too. I think the first one I got was under $60?! No one really knew the segment existed, they just needed to get some sales, I assume. I heard usb3 has been available but they've been bulky & expensive. Where-as the whole fiber optic cable seamlessly integrates the transceiver on mine. I like Cable Matters, they make some fine ones.
Worth noting too that well respected vendors have been selling optical thunderbolt cables for a while now. I wonder if they are length limited for latency reasons (& hello hollow core fiber)? I wonder if they are usb3/multiprotocol, or if they are usb4 only. I also wonder how they handle the incredibly jank usb4 requirement to also have a separate legacy usb2. As a usb-c enjoyer, I can still admit: sure seems like USB is a lot of work to support! I can't help but wonder how blissfully simple a future CXL over cable stack might look by compare. https://www.owc.com/solutions/usb4-cables
There is an interesting NSDI talk on the paper too - https://www.youtube.com/watch?v=kDJHA7TNtDk (2023)
still, it's incredibly cool for one guy to pull this off on his own. demonstrates mastery of the subject