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Optical PCIe 7.0 connection hits 128 GT/s

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204 points WithinReason | 5 comments | 17 Jun 24 11:42 UTC | HN request time: 0.963s | source
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yjftsjthsd-h ◴[18 Jun 24 00:02 UTC] No.40712649[source]▶
So I guess what this makes me wonder is: Why are we using electrical signals to connect the data lanes between components and computers these days, rather than moving everything to optical for data movement (obviously power would stay electrical, but that's already on separate lines)? I assume there's an element of cost, and once the photons get where they're going they have to be turned back into electrical signals to actually be used until such time as we get around to getting pure light based computers working (someday but not yet...), but that must not overwhelm the advantages or we wouldn't be looking at this being developed.
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AceJohnny2 ◴[18 Jun 24 01:17 UTC] No.40713112[source]▶
> I assume there's an element of cost, and once the photons get where they're going they have to be turned back into electrical signals to actually be used until such time as we get around to getting pure light based computers working (someday but not yet...)

You got it. We can't make optical transceivers as good as electrical ones. Not as small or power-efficient.

They require significantly different fabrication processes, and we don't know how to fab them into the same chip as electrical ones. I mean: you can either have photonics, or performant digital (or analog) electronics.

We've gotten really, really good at making small electronics, per the latest tech coming out of Intel & TSMC. We are... not that good at making photonics.

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hughesjj ◴[18 Jun 24 03:27 UTC] No.40713801[source]▶
> Not as small or power-efficient.

I wonder what the latency for switching medium is these days too (for the super small transceivers). To my understanding optical is better for attenuation than electric (less noise, and thus easier to shove more frequencies and higher frequencies on the same pipe), and can be faster (both medium dependent, neither yet approaching the upper bound of c).

I'm imaging the latency incurred by the transceiver is eventually offset from the gains in the signal path (for signal paths relevant to circuit boards and ICs)

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1. eqvinox ◴[18 Jun 24 11:03 UTC] No.40716271[source]▶
> To my understanding optical […] can be faster (both medium dependent) […]

The speed of light in optical fiber — for all types based on glass, ignoring miniscule differences — is 68% that of air/vaccuum. And that's not changing, and no state-of-the-art high speed applications are being developed on plastic fibre or free air.

So, latency wise, on runs with non-negligible length, optical will lose out to electrical, which is generally quite close to the speed of light. (Except of course after some point the electrical signal is just noise, and if you factor in delay caused by amplifiers/repeaters it becomes much harder of a question.)

There's this kinda-famous story of some HFT company pulling a copper cable across some bay, because they'd gain some nanoseconds compared to the fiber they had.

The transceiver latency for "long-range" links (well, call 100m long range for copper…) is actually worse for the copper links, as the whole DSP getup you need for that takes a few symbol times to process. Optical transceivers are just optodiodes and "simple" amplifiers, the latency is much less than a symbol.

(symbol = unit of transmission, roughly 1 bit on "old" fibre [≤ 25G lane rate], ca. 4 bit for 10Gbase-T, will vary more for faster connections.)

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2. formerly_proven ◴[18 Jun 24 11:12 UTC] No.40716339[source]▶
There's single-mode hollow-core fibers - which constrain the light via non-refractive / non-dielectric physics magic, i.e. without slowing down, to the hollow core. Not yet commercially available, I think.
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3. eqvinox ◴[18 Jun 24 12:05 UTC] No.40716791[source]▶
Nice! But even after they're commercially available, putting fibers into places has massive inertia, along the lines of ≥10 years… and on short ranges (inter-chip connections up to building/city networks) you really don't care :)
4. silizium ◴[18 Jun 24 14:01 UTC] No.40717992[source]▶
The speed of electrical waves in a chip is ~c/2.

The relative permittivity εr of SiO2 is ~4.

c = c0 / sqrt(εr)

c0 = 1 /sqrt(ε0εr × μ0μr) and in vacuum εr=μr=1.

But the frequency needs to be sufficiently high in order to observe wave propagation, let's say >10GHz.

For low frequencies the electric conductor behaves more like a RC chain.

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5. eqvinox ◴[18 Jun 24 17:20 UTC] No.40720080[source]▶
Considering the PCIe context, I was assuming we were talking about off-chip connections, i.e. diff microstrips/striplines in FR4 and air. εr is 2.6-ish there.

But you're right, I might have accidentally mixed in some radio connection bits, with the HFT company anecdote.