3 ms·

> To my understanding optical […] can be faster (both medium dependent) […] The speed of light in optical fiber — for all types based on glass, ignoring minisc

> 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.)

formerly_proven 2y ago
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.
- eqvinox 2y ago
  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 :)
silizium 2y ago
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.
- eqvinox 2y ago
  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.

formerly_proven 2y ago

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.

eqvinox 2y ago
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 :)

silizium 2y ago

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.

eqvinox 2y ago
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.