4 ms·

Active optical (yes!) Thunderbolt cables can be much longer. After all, optical fiber was the original medium for Thunderbolt, back when it was still called Lig

by dcrazy 6mo ago

Active optical (yes!) Thunderbolt cables can be much longer. After all, optical fiber was the original medium for Thunderbolt, back when it was still called Light Peak. 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-optical-cable-delivers-supreme-flexibility-to-your-workspace 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/OEM-062-A4-AEN.pdf 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.

zamadatix 6mo ago
Hundreds of meters is considered short range in the world of *SFP. If you just plan on putting the GPUs in the same rack then I'm not sure it really matters, but you can really put anything anywhere in your DC and have things zoned with *SFP. 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).
dmitrygr 6mo ago
For the curious, that 50m cable is $500 MSRP. https://1sourcevideo.com/shop/corning-50-meter-thunderbolt-3-optical-cable.html https://1sourcevideo.com/shop/corning-50-meter-thunderbolt-3...
jauntywundrkind 6mo ago
Thunderbolt is kind of cursed. To insure maximum compatibility it mandates a legacy usb2 connection via separate connections. TB3/USB4/TB4 are packetized, but afaik there's no defined way to packetized usb2, it's expected there be a whole separate set of wires for it. 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?).
- dcrazy 6mo ago
  IIRC, USB-PD also requires USB 2.0 signaling. The idea of dedicated lower-bandwidth signaling wires isn’t uncommon in my very limited EE experience—level 3 charging reuses J-1772 signaling to control the charge available of the DC pins.

by dcrazy 6mo ago

zamadatix 6mo ago

Hundreds of meters is considered short range in the world of *SFP. If you just plan on putting the GPUs in the same rack then I'm not sure it really matters, but you can really put anything anywhere in your DC and have things zoned with *SFP. 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).

dmitrygr 6mo ago

For the curious, that 50m cable is $500 MSRP. https://1sourcevideo.com/shop/corning-50-meter-thunderbolt-3-optical-cable.html https://1sourcevideo.com/shop/corning-50-meter-thunderbolt-3...

jauntywundrkind 6mo ago

Thunderbolt is kind of cursed. To insure maximum compatibility it mandates a legacy usb2 connection via separate connections. TB3/USB4/TB4 are packetized, but afaik there's no defined way to packetized usb2, it's expected there be a whole separate set of wires for it. 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?).

dcrazy 6mo ago
IIRC, USB-PD also requires USB 2.0 signaling. The idea of dedicated lower-bandwidth signaling wires isn’t uncommon in my very limited EE experience—level 3 charging reuses J-1772 signaling to control the charge available of the DC pins.