Based on the volume of mail from East Coast-based cable-system employees over the last few months, it appears that there’s plenty of interest about the actual carrying capacity of fiber-optic cable, versus coaxial cable.
One reader, based in Virginia, tagged her fiber curiosity to “a bad case of FiOS-in-the-face,” referencing Verizon’s triple-play brand. In case you haven’t seen it, here’s a sampling of how Verizon fluffs fiber into a series of bouncy refrains: “Jump into a brilliant fiber optic future!” “Internet and TV transformed through the power of fiber optics!”
20 YEARS ON
If you, too, are feeling FiOS-weary, remember this handy fact: Cable operators jumped into a brilliant, fiber optic future nearly 20 years ago. They started transforming TV through the power of fiber optics in the late 1980s.
That’s why there’s that “F” in “HFC.” It stands for “fiber.”
And speaking of the “F” in “hybrid fiber coax” — ever wonder where fiber drops off to coaxial cable, relative to your house? We hear loads about Verizon’s “fiber-to-the-home.” But where does cable’s fiber actually drop off, other than “at the node”?
Answer: Probably about a quarter-mile from your house, if you live in a city or its suburbs. Translation: Not far.
The decision about how far to pull fiber toward homes is a big one. Not going far enough can attract interference, noise or distortion. Going too far can get real expensive, real fast — especially if digging into streets or lawns is required.
Back in the days when cable operators were first making these “how far” decisions, they put a giant amount of energy into design tests about how far fiber should go. It was the stuff of technical papers at industry conferences. (In fact, what’s now the SCTE’s annual “Conference on Emerging Technologies” began life, in the late 1980s, as its “Fiber Optics” conference.)
It turned out that the best place to hand fiber off to coax was at the center of a circle with a one kilometer radius. Figuring average densities of around 80 homes per mile, and typical subdivision layouts, there were about 500 homes inside that circle. That’s why you hear about the “500-home node.” The distance from the outer edge of the circle to the middle is about a quarter of a mile.
DRUM ROLL PLEASE
There is a way to do the math on fiber vs. coax carrying capacity, but know going in that the answer is misleading. The type of fiber used for sending video — known in tech circles as 1310 nanometer, single-mode — can move as much as 10 Gigabits per second, compared to around 5 Gbps for coax.
MILEAGE MAY VARY
Actual carrying capacity does vary, depending on how long the fiber is, and what you put on either end of it. But in general, it’s safe to conjure fiber as the fire hose, coax as the garden hose, and twisted-pair copper (meaning telephone wires) as the sipping straw.
Don’t despair, because that’s the simplistic way to look at raw capacity. Let’s put aside, for a moment, the notion of reusable bandwidth, node splitting, and switching — all of which correlate into nearly unlimited bandwidth potential for cable operators.
Instead, let’s look at what all that bandwidth (real or perceived) gets you. Consider a high-end home in the future — maybe four or five years out. Inside are eight high-definition TVs, four cable modems and four voice adapters. Let’s even say the HD streams were squished by an advanced compressor, so they take up even less space.
If all of the HD displays were on, and all of the cable modems were streaming video, and all of the voice adapters were handling calls, you still only get to about 72 Mbps, needed by that house. (“Only.”)
The math goes like this: Eight HD displays times 8 Mbps per HD stream is 64 Megs; four cable modems, streaming video, 8 Mbps; four phone calls, negligible. All in, 72 Mbps.
YOU'VE STILL GOT TO FILL IT
Think about that, the next time someone dangles 100 Mbps in front of your nose.
Both fiber-to-the-home and any handful of cable techniques — channel bonding and switching come to mind — can blast that much bandwidth to a house. More, almost certainly. But what will you do with it?
Think of it this way. Your car’s speedometer probably goes up to about 150 miles per hour.
When was the last time you pushed the needle up to 150? Have you ever?
Somewhere along the way, fiber became sexy. Verizon made it so.
So much for not using industrial terms in consumer marketing.
Stumped by gibberish? Visit Leslie Ellis at www.translation-please.com.
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