Why are signals from passive optical networks split into 32?

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I attended a course on fibre technology recently. The presenter was Dr. Jeffrey Bannister from Orbitage.

He was talking about fibre optics being a relatively old technology, and is now being used as a means of transporting the vast amounts of information that’s the Internet. Remember the earthquakes near Taiwan, which halted Internet traffic in Asia?

There’s an interesting point he made, that there are only 4 of these hair-thin optical fibres supporting the Asian Internet traffic. And if I remember correctly, these optical fibres run in between Vietnam and Philippines, to Taiwan, and to Japan. I can’t find any reference on the number of fibres used, but 4 seems incredulous. I mean, it takes a lot of money, time and effort to set those submarine cables. It makes sense to use more, since optical fibres are cheap (as cheap as fishing lines, so says Dr Bannister). Maybe there are dark fibres.

Another interesting point is that optical fibres do not rely on electricity to convey information. Shine a light at one end of a fibre, and it’s interpreted as a “1” at the other end. No light means a “0”. Voila! Zeroes and ones for digital use. Using a physics property called Brewster’s angle, light can be transmitted for long distances with little loss of energy or result in data corruption (light keeps bouncing around along the optical fibre).

A third interesting point is that the light used in transmitting our Internet data is not visible at all! It’s actually infrared light, because it has the best result for the single-mode optical fibres used.

A fourth interesting point is that upstream and downstream data use the same optical fibre. It’s accomplished by using different wavelengths of infrared light, using a technique called wavelength-division multiplexing.

So where do passive optical networks come in, and what are they? If I understand it correctly, it’s an architecture for housing splitters, and

Each splitter typically splits the signal from a single fiber into 16, 32, or 64 fibers, depending on the manufacturer, and several splitters can be aggregated in a single cabinet.

Remember the data travelling along just 4 optical fibres mentioned before? There are many endpoints for that data, so somewhere along the line, the data have to be split up. That’s where the splitters come in.

By now, your coder senses should be tingling. Let me highlight the source of the tingling:

single fiber into 16, 32, or 64 fibers

16? 32? 64? They look familiar…

In his talk, Dr Bannister mentioned that splitters split the signals into 32 or 64. Now if he mentioned only “32”, I might have waved it off. But he mentioned “64” in the same breath too, and that’s what triggered my coder senses.

And in case you haven’t caught on,
16 = 2^4
32 = 2^5
64 = 2^6

So after his talk, I went up to ask him about this. At first, he misunderstood my question, and explained more on how the splitting was done.

My question was actually something else. Fibre optics do not need electricity to transmit data. The splitters do not need electricity to split signals. Basically everything is analog. Why is the binary concept, the basis of digital, used in the number of splits?

His answer was actually very simple. It’s easy to calculate the efficiency. (Or light energy. Or wavelength.) Splitting a signal into 2 means it’s a simple 50% divide.

He thought about it, and said the engineers could probably split signals into 10 or powers of 10. But splitting in powers of 2 is easy for the math.

Frankly speaking, I didn’t expect such a simple and logical answer. I was actually stunned for a few seconds.

Does that make you feel computers have a completely efficient understanding of the world?

[Update: Commenter John Bartell has more information on splitters and passive optical networks.]

[image by ktsimage]

  1. Leong Hean Hong

    Since everything is analog, there is no need to adopt the binary concept. It doubt if it is for calculation of efficiency.

    Instead of “computers have a completely efficient understanding of the world”, I think computer has started to influence our understanding of the world.

  2. Vincent

    “computer has started to influence our understanding of the world”

    That is definitely true. Even the non-tech people are affected by the events on the Internet (to some degree).

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