If you’ve owned a CD player or other piece of consumer digital audio gear manufactured since the 1980s, the chances are it has a TOSLINK port on the back. This is a fairly simple interface that sends I2S digital audio data down a short length of optical fibre, and it’s designed to run between something like a CD player and an external DAC. It’s ancient technology in optical fibre terms, with a lowish data rate and plastic fibre, but consider for a minute whether it could be adapted for modern ultra-high-speed conenctions. It’s what [Ben Cartwright-Cox] has done, and he delivered a talk about it at the recent 38C3 event in Germany.
if you’ve cast you eye over any fibre networking equipment recently, you’ll be familiar with SFP ports. These are a standard for plug-in fibre terminators, and they can be had in a wide variety of configurations for different speeds, topographies, and wavelengths. They’re often surprisingly simple inside, so he wondered if he could use them to carry TOSLINK instead of a more conventional network. And it worked, with the simple expedient of driving an SFP module with an LVDS driver to make a differential signal. There follows a series of experiments calling in favours from friends with data centre space in various locations around London, finally ending up with a 140 km round trip for CD-quality audio.
It’s an interesting experiment, but perhaps the most value here is in what it reveals to us about the way optical networking systems work. Most of us don’t spend our days in data centres, so that’s an interesting technology to learn about. The video of the talk itself is below the break.
Why? Becauseyou can?
Apparently so. Single mode fibre is single mode fibre, carries a single mode exceedingly well, so to send TOSLINK over standard long distance fibres isn’t reallly a surprise that it works. That’s how fibre networks are upgraded, simply plug more advanced optics on either end but the fibre stays the same.
Saying that SFPs are rather simple inside is dumbing it down a lot though (although 1Gbe SFPs these days are comparatively simple compared to SFP+ and SFP28)
SFP is simple because there are off-the-shelf chips to do the heavy lifting. The interfacing is fairly generic, so you don’t need a micro controller or anything.
I think a huge portion of the cost of “real” SFP modules is in the testing and qualification steps.
And the fact that there are only a few actual manufacturers but a lot of vendor markup.
The talk actually goes over all of this at length.
Best part of the article is right here:
“…calling in favours from friends with data centre space in various locations around London, finally ending up with a 140 km round trip for CD-quality audio.”
…as one does. Bravo, sir!
TOSLINK seems fairly low on the list of things that would be of interest in this case; but the fact that SFPs are apparently content to handle more or less arbitrary signals unrelated to the protocols they are intended for seems like it could be of use: thanks to vast economies of scale and upgrades on the datacenter side networking optics are cheap as chips for their level of performance, and there a variety of signals that go over differential pairs for short distances that you might want to extend further than copper makes convenient.