how does one define "capacity"

[David Farber <dave () farber net>]

________________________________________
From: craig () aland bbn com [craig () aland bbn com]
Sent: Saturday, April 19, 2008 4:01 PM
To: David Farber; bob37-2 () bobf frankston com
Subject: how does one define "capacity"

[Dave F - for IP if useful]

Bob F asks:

    How does one define "capacity"? To use a trivial example -- what's the capa=
    city of a copper wire? We know it can usually carry a single phone call or =
    maybe 1GB Ethernet or ...

Which is an excellent question, but the way it is phrased here suggests
it is unanswerable.  However, it is answerable.

Fiber has three optical passbands in which light passes (relatively) cleanly.
The available capacity is about 25 terahertz per passband.   We've been
stuck at 1 bit per Hz for something like 20 years, so one fiber's capacity
is about 75 terabits.  We get far less today due to limitations of
transmission equipment -- so Bob's point applies that we can expect
to see continued improvement -- but somewhere around 75 terabits
(or 750 x 100 Gbps Ethernet connections) per fiber, we are forced
to use multiple fibers in parallel (a technique that fails somewhere
short of a yottabit [10^24] per second, at which point your
fiber bundles are the same diameter as subway tunnels, which seems
a good definition of implausible).

Copper is tougher, as there's crosstalk from other wires and other signals.
I'm not well briefed on copper, but when I talk with folks who do ADSL
signalling and the like, it is clear the basic rules are similar to fiber --
just with lower data rates.  Someone well versed in copper could probably
(for an ideal copper strand) do calculations similar to the ones for
fiber above for copper.

Summary: we can compute capacity.  We're not close.

Craig

PS: There's another version of this calculation that involves depreciation
schedules for installed multiplexing equipment, current ratio of terminated
and unterminated fibers in conduits, and ability to pull more fibers.
Don't go there :-).

*****************
Outreach Director, GENI Project Office
Chief Scientist, BBN Technologies

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