Translation Please: Data Speed Vs. Throughput

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From last week's mail emerged this timeless question: “I've heard people talking about broadband speed vs. throughput. Is there a distinction between these two that I'm missing?”

It's not an obvious answer. Both are measured in Megabits per second. Both involve the gargantuan volume of bits hurdling through the Internet and its tributaries.

Here's the distinction: Usually, when people talk about broadband speeds, they're talking maximums — also known as “peak burst rates.” That's the point where you're running your modem flat out, squeezing every last bit of that 6 Megabits per second (or whatever speed) connection you're paying for.

Testing for peak burst rate involves measuring spikes — not unlike how the on-demand side of the house knows, empirically and anecdotally, that Friday nights are big VOD nights.

If it were just you on the network, speed and throughput would look the same. But, alas, it's not just you. Which brings us to throughput.

When people talk about throughput, they're talking averages — how many bits get to their destination, over time. Peak burst rate depicts heaviest activity; throughput depicts averaged activity.

Testing for throughput often involves measuring the time it takes to transfer a large file (as opposed to, say, a stream of video). The throughput number is the file size divided by the time it took to get from departure to arrival.

Conversations about burst rates and throughput often invoke a sister question: How fast is fast enough? Consider a house with three HDTVs, all on, and a cable modem giving everything it's got (of that 6 Mbps tier) to a P2P exchange. Back of the envelope math, assuming MPEG-2 compression, puts the consumption number — meaning the aggregate throughput needed at that address — at around 50 Mbps.

If you're a telco seeking to round out that triple play with some video, which you plan to run entirely over DSL lines (AT&T's IPTV plan so far), and you need to bump up your broadband speeds, you're probably paying pretty close attention to peak burst rates and throughput.

John Malone, chairman of Liberty Media, which owns DirecTV, implied as much in a recent interview in the Denver Business Journal. His remedy (paraphrased): Leave the DSL line alone. Bring the video goodies in over a satellite dish. That way, the DSL line has some elbow room to compete on the basis of speed.

Not necessarily a new idea, but somehow sharper (and threaded with irony) because of who said it.

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