Bandwidth

Posted on July 26, 2013 by Tommy McGuire
Labels: electronics, protocols

Way back in the early days when DEC's and minicomputers roamed the landscape, growling and snarling and scaring small mammals, there was an adage:

Never underestimate the bandwidth of a station wagon loaded with 9-track tapes.

And it just occurred to me to ask, what is the bandwidth of a station wagon loaded with 9-track tapes?

To back up a bit, a tape is a data storage mechanism, made of very thin plastic coated in iron oxide onto which you could impress bits through the loving application of magnets and steam. A 9-track tape was 1/2-inch (a centimeter and a quarter, more or less) wide and 2400 feet (or 730-ish meters) long, rolled up into a plastic reel. With the cover, it was about an inch (2.54 centimeter) high and 12 inches (.0015 furlongs) in diameter. A 9-track tape stored 1 byte across the 9 tracks on the width of the tape, at a density of 6250 bytes per inch, for a total capacity of about 150 megabytes, including overhead. Given that size, let's say you could get 12 tapes in a cubic foot

On the other side of the equation, a station wagon is an automobile with some expanded cargo capacity. Picture a large hatchback or short SUV, or maybe a crossover without whatever it is that makes them not station wagons. Some random site on the Internet alleges that my old Mercury Sable wagon may have had 81.3 cubic feet of cargo space (I couldn't find the specs on my 1985 AMC Eagle wagon, unfortunately), which works out to about 140 GB. (Not bad, that's 30 DVD's, more than I would have thought.)

Say you could make one round-trip across town in an hour, including loading and unloading the tapes. (Loading and unloading would be more of a problem than you think; when handling 9-track tapes, it is almost impossible to resist the urge to go bowling.) That works out to 325 Mbps, significantly faster than the 100 Mbps ethernet that was common the last time I ran across a 9-track tape. So, there you go.

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