Re: Station wagon full of bits


Stu Grossman (GROSSMAN@Sierra.Stanford.EDU)
Thu 26 Mar 87 11:43:12-PST


I did a few calculations of the bandwidth of Wagonnet, and heres what I
came up with:

The most readily available magtape technology happens to be 2400' reels.
These have a common maximum density of 6250 bytes/inch. This results in
a value of 6250*12 = 75000 bytes/foot, and therefore we get 75000*2400*8 =
1.44e9 bits/tape. Now, your average 2400 foot reel is about 10.75 inches
in diameter by 1 inch high, so (squaring off the corners) you get a volume
of 115.6 cu inches/tape, and therefore you can get 12^3/115.6 = 14.98 tapes
per cubic foot. This amounts to (rounding up to whole magtapes) 15*1.44e9=
21.6e9 bits/cu ft. Now, your typical station wagon holds anywhere from
30 to 65 cubic feet of stuff. I'll presume that this station wagon is
large, since it has to deal with Bitnet. Therefore it holds 65*21.6e9=
1.404e12 bits/station wagon.

Now, at a rate of 55 mph (I don't want to break the law), this station
wagon can get across the country in 3000/55 * 3600 +10% = 216000 seconds.
The 10% is for dealing with changing drivers, food amd gas stops, etc....

This results in a baud rate of 1.404e12/216000 = 6.5e6 bits/sec. This is
comparable to typical Ethernet thoughput with a good controller and few
collisions. Of course, this network has the advantage that congestion
doesn't make it lose data. It just gets there kinda slowly. However,
the RTTs are rather long (on the order of 216000*2= 5 days.

But despair not, there's some hope! If congress raises the speed limit to
65mph, then the typical RTT will be reduced to a mere 3.8 days. You could
probably convince congress that it would be in the best interests of national
defense to do this!

Note that a speed limit of 216 million mph would result in a much more
reasonable RTT on the order of .1 seconds!

                        Stu Grossman
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