Tag Archives: communications

How Much Data in that Fiber?

In the “Age of Accelerating Returns” we are inundated with mega- giga- and tera-figures marking technological– and supposedly human– progress.  These numbers are now well beyond our capacity to comprehend.  It’s like the new finding that there are about 3E23 stars in the universe– I have absolutely no idea what that means.

One example is communications capacity.  We are often told “this link could transmit the equivalent of the Library of Congress in __seconds.”  Have you been to the LoC?  Do you have any idea how big it is?  Me neither.  Even if you knew there were 32 million books in the library, it doesn’t get you closer.

So I was thinking about a different way to explain the capacity of the latest fiber-optic transmission systems.  Long-haul systems (with reach of 1000’s of km) are getting to the point where they can shove 10 Terabits/second down the core of an optical fiber:

Light-carrying core of a long-haul optical fiber.

A side remark on optical fiber: if you shone a flashlight through a 10km thick slab of glass, how much light do you think would make it out the other side?  Modern optical fiber transmits 67% of (infrared) light over that distance!

A lot of very cool engineering goes into making this work.  The standard is DWDM DP-QPSK = dense wavelength-division multiplexing, (coherent) dual-polarization quadrature phase-shift keying.  You can get your tech jollies reading a pretty good overview written by the Optical Internetworking Forum.  The result– if you factor in all-optical amplification– is that you can transmit data for thousands of kilometers entirely optically.

What is 10 Terabits per Second?
I was thinking about ways to visualize this without resorting to the Library of Congress.  One thing we understand pretty well is video.  And in fact, much of the need to light up fibers with ever more capacity is driven by video. You can transmit a 1080p HD video stream with about 5Mbit/sec capacity.  That means a single fiber can push 2,000,000 HD video streams.

What do 2 Million Video Streams Look Like?
Again, 2 million is a number that is too high for humans to visualize.  I thought about what it would look like if you stack up that many streams.  For that exercise, let’s use 42″ flat panel TVs… sort of the “standard” flat TV size these days.  Each (generally LCD) panel measures 0.93m x 0.5m, with 52dpi pixel resolution.

I wondered what it would look like if you stacked (without bevel) that much display capacity– how much live, HD video a single fiber could carry.  For comparison with the fiber, let’s keep it in a circular format.  Here is what I get:

HD teradisplay driven by single optical fiber.

So a 9-micron diameter fiber could feed a very high-resolution 1110m-diameter display… a factor of 1.5E16 larger area (OK, not possible to comprehend… just picture a piece of glass the diameter of a red blood cell perched on top of the Burj Khalifa there).  And if you need help understanding how tall that building is, here’s a video.

4 Terapixels— now that has some serious potential!  My favorite use would be to make large sections of park, city, university into telepresence walls (video, sound, maybe 3D) to a sister city on the other side of the world.

By the way, the fact that you can feed 4 Terapixels with 10 Terabits (2.5bits/pixel/frame) for high-quality video is a testament to the efficiency of H.264 compression.  The ability to run H.264 on even handheld devices is a direct result of Moore’s Law.  Without such good compression, the cost of transporting video would be unsustainable (though I’m sure optical component suppliers wish it were a little less efficient).

Are There Enough TVs for That?
Easily.  The world already buys 180 million LCD TVs per year.   A single plant (Sharp’s Sakai City/Osaka site) has a capacity of 7.8 million square meters of LCD per year– and it is being eclipsed by new plants in China.  The visual comparison of that display area I’ll leave for another day…