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Upstream Bandwidth and Symmetry

Recently, a childhood friend wrote to express his outrage at the U.S. cable industry, for “deliberately precluding people from getting symmetrical broadband speeds.” Several angry blogs bludgeon this topic, too. Salty opinions abound.

This week’s translation will attempt to explain (hopefully without damaging the friendship) why this particular accusation is incorrect — by reasons of regulation, physics and present reality.

Let’s take it from the top. “Symmetric,” in a bandwidth sense, means the same amount of speed goes toward the computer as away from it. Most of today’s operators provide asymmetrical packages — so many Megabits per second toward the computer; fewer Megabits per second away from it.

This fits the pattern of most early and ensuing Internet traffic. Your request for a Web page, video stream or voice call is substantially smaller than the resultant page, stream, or conversation.

This changes, of course, with affordable HDTV cameras. That clip of the weekend at the beach is a massive file to move upstream, compared to typing in a Web address and pressing “enter.” Peer-to-peer traffic also changes the scene. We’ll get to that.

The total available bandwidth of a contemporary cable system is likewise highly asymmetrical. It goes like this: Upstream traffic moves within a tiny gash of spectrum between 5 MHz and 42 MHz. Downstream stuff moves over a path that starts at 54 MHz, and goes as high as 1 GHz (or, 1,000 MHz).

Why 54 MHz? Why not just move that boundary up higher, to make the upstream path wider?

Here’s what veteran cable engineers say, when asked the question: Because that’s where channel 2 starts.


What they mean is this: Long, long ago — as in 74 years ago, in 1934 — the Federal Communications Commission was empowered to develop, maintain and enforce a table of radio frequency allocations for non-governmental use. (The National Telecommunications and Information Association handles the spectrum used by the government.)

This industry’s monthly engineering trades actually print a “Frequency Chart,” produced annually and in coordination with the FCC. Walk into any headend, and you’ll probably see one tacked to the wall somewhere. They’re a colorful bit of “tech art.”

As a direct result of the FCC’s frequency allocations, the U.S. cable industry is required to contain its upstream (or “reverse”) path traffic within that little gap, located between 5 and 42 MHz.

(The 12 MHz separating the top of the upstream band, at 42 MHz, and the bottom of broadcast channel 2, at 54 MHz, is called “guard band.” It prevents the two segments from colliding and making a mess.)

It’s true that cable’s spectrum is contained within shielded wires and doesn’t free-wheel through the air, bumping willy-nilly into broadcast channels. Still, the FCC decided that cable operators should display off-air channel 2 at the identical 54 MHz spot as the broadcasters — so that ordinary people could find it, when they tuned in. (Remember — this was 60 years ago.)

So that’s the regulation part of why cable’s broadband plant isn’t symmetrical.

Let’s back up even farther — to the 1860s. That’s when a Scottish physicist named James Clerk Maxwell found a way to combine the properties of electricity with the properties of magnetism. His discovery was foundational to the “electromagnetic spectrum” that is the basis of all modern telecommunications. (To put this in people-context, Albert Einstein had two hero shots on his wall — Newton, and Maxwell.)

The electromagnetic spectrum is invisible and vast. It’s not just about radio waves. Microwave is there, as is infrared. So are ultraviolet rays, X-rays, and gamma rays. In varying degrees, and if you could see them, they all look like the letter S, on its side. The sine wave.

The RF (radio frequency) portion of the electromagnetic spectrum can be made to do stuff — like carry radio and TV — by manipulating its frequency (the number of times the sideways S recurs) and its amplitude (how big the sideways S is). This manipulation is called modulation, the imprinting of a signal onto a wave.

In the cable upstream path, the type of modulation commonly used is deliberately and necessarily sturdier than what’s used in the downstream path. That tiny slash of upstream spectrum is a tough environment, bristling with noise and impairments. It’s the road with potholes big enough to swallow a Mini Cooper: You just have to slow down.

So that’s the physics of it.

Then there’s the empirical evidence, which says that people, on average, receive four times more information than they transmit. Maybe this will change, with user-generated video. Peer-to-peer traffic will, by its very nature, occupy all unused space. It behaves like a gas, that way.

The bottom line is this: Nobody is deliberately precluding symmetrical bandwidth. In the beginning, nobody even knew what to do with the upstream spectrum provided to them by the FCC. TVs were still black and white and analog. Two-way plant wouldn’t emerge for another 40 years.

But, you say: If the broadcasters are going all-digital in February, doesn’t the channel 2 thing go away, or at least present a way to expand the upstream boundaries?

Ah, wouldn’t that be grand. Alas: Going digital is one thing. Outfitting hundreds of thousands of amplifiers, taps, and in-home TV tuners to know that the upstream got wider is another.

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