[ih] Bandwidth v Capacity [Early Internet Report when Vint was at Stanford (and DARPA PI)]

[Steve Crocker]

I was in a small meeting in the Pentagon several years ago.  The speaker
was known to be one of the candidates for the next ASD for C3I (or maybe
C4I).  I was quite surprised when he commented there was a
fundamental distinction between analog and digital systems, implying that
analog systems can dothat can't be done by analog systems.  Not the sort of
person I'd want to see in charge of the DoD's transition to an all digital
world.  Fortunately, he wasn't selected.

Steve


On Sun, May 31, 2026 at 8:51 AM John Day via Internet-history <
internet-history at elists.isoc.org> wrote:

> I should have said, when I encountered the analog/digital misconception, I
> was shocked, especially since it came from an ECE student.  Too much C and
> not enough E! ;-)
>
> Sometimes these little things that we don’t think we need to mention are
> really important.
>
> > On May 31, 2026, at 06:48, John Day via Internet-history <
> internet-history at elists.isoc.org> wrote:
> >
> > Brian,
> > I totally agree with you. In fact, I have been known to use a question
> like that on exam. We are sloppy in our use of language.
> >
> > Besides bandwidth vs capacity, I have found that there are professionals
> out there who believe that analog and digital signals are fundamentally
> different. Not that a digital signal is one that hasn’t traveled far enough
> to betray that it is really analog underneath.
> >
> > And as long as we are cleaning up language, a network graph is not a
> topology. One could refer to a set of network graphs with a common
> invariant as a topology, but a single network graph is just a graph.
> >
> > Take care,
> > John
> >
> >> On May 31, 2026, at 06:32, Brian Carpenter via Internet-history <
> internet-history at elists.isoc.org> wrote:
> >>
> >> I'm sorry if it came across as criticism; I genuinely wonder when the
> usage
> >> originated.
> >>
> >> (via tiny screen & keyboard)
> >> Regards,
> >>       Brian Carpenter
> >>
> >> On Sun, 31 May 2026, 21:30 vinton cerf, <vgcerf at gmail.com> wrote:
> >>
> >>> Rant received; thanks for the tutorial.
> >>> v
> >>>
> >>>
> >>> On Sat, May 30, 2026 at 11:55 PM Brian E Carpenter via
> Internet-history <
> >>> internet-history at elists.isoc.org> wrote:
> >>>
> >>>> Hi,
> >>>>
> >>>> Yes, that's a very nice slice of history. It leads me to one of my
> hobby
> >>>> horses: when did the solecism of using "bandwidth" to mean "capacity"
> first
> >>>> arise? This is something that should annoy every physicist, or anyone
> who
> >>>> has read Shannon's foundational paper [1]. Unfortunately, it's become
> >>>> firmly established in the Internet community and beyond. And it
> matters
> >>>> because it quite often creates confusion, particularly in media
> reports.
> >>>>
> >>>> * Bandwidth is measured in herz (cycles per second) and is the
> frequency
> >>>> range that a communication channel can transmit.
> >>>>
> >>>> * Capacity is measured in bits per second and is the amount of binary
> >>>> information that a communication channel can transmit.
> >>>>
> >>>> There is no fixed relationship between the two (which is more or less
> the
> >>>> main point of Shannon's paper). Anyone who ever heard the startup
> screeches
> >>>> of a modem should know this.
> >>>>
> >>>> To illustrate the issue, page 1 of Vint's report says:
> >>>>
> >>>> "During the months of December (1975) and January (1976), we undertook
> >>>> extensive and detailed timing measurements of the ELF VDH behavior to
> >>>> ascertain the degree to which VDH performance affected total TCP
> >>>> bandwidth and delay."
> >>>>
> >>>> That (and the other 14 occurrences of "bandwidth") should be
> "capacity."
> >>>>
> >>>> Page 8 says:
> >>>>
> >>>> "The actual
> >>>> line utilization is about 20% in each direction, assuming a nominal 50
> >>>> kbits/second available full-duplex capacity between ELF and the IMP."
> >>>>
> >>>> That's correct usage.
> >>>>
> >>>> My favourite sentence is on page 45:
> >>>>
> >>>> "Many of the experiments have been
> >>>> frustrating, owing to a bug of some kind in UCL's buffer allocation
> scheme
> >>>> causing them to crash irrevocably when attempting to achieve high
> >>>> bandwidth."
> >>>>
> >>>> If only they had tried to achieve high capacity!
> >>>>
> >>>> So, my question is: when did this inaccurate use of "bandwidth" to
> mean
> >>>> "capacity" first arise? It was clearly well established by 1975.
> >>>>
> >>>> I looked in Donald Davies's book [2], and it only uses "capacity" (I
> >>>> would expect no less of him). Baran in 1964 [3] used "capacity"
> correctly
> >>>> many times. He also used "bandwidth" correctly once [5] and debatably
> a
> >>>> second time [6].
> >>>>
> >>>> Pierce [4] in 1961 was completely clear on the difference between
> >>>> bandwidth and capacity (and he learned directly from Shannon).
> >>>>
> >>>> Regards/Ngā mihi
> >>>>   Brian Carpenter
> >>>>
> >>>> [1] C. E. Shannon, "Communication in the Presence of Noise,"
> Proceedings
> >>>> of the IRE, vol. 37, no. 1, pp. 10-21, Jan. 1949
> >>>> [2] D. W. Davies, D. L. A. Barber, W. L. Price & C. M. Solomonides,
> >>>> "Computer Networks and their Protocols," Wiley, 1979
> >>>> [3] P. Baran, "On Distributed Communications Networks," IEEE
> Transactions
> >>>> on Communication Systems, Vol. 12 No. 1, 1964, pp. 1-9
> >>>> [4] J. R. Pierce, "Symbols, Signals and Noise," Harper, 1961
> >>>>
> >>>> [5] "In a conventional circuit-switched system each of the
> >>>> tandem links requires matched transmission bandwidths.
> >>>> In order to make fullest use of a digital link, the post-
> >>>> error-removal data rate would have to vary, as it is a
> >>>> function of noise level."
> >>>>
> >>>> [6] "Most importantly, standardized data blocks permit
> >>>> many simultaneous users, each with widely different band-
> >>>> width requirements to economically share a broad-band
> >>>> network made up of varied data rate links."
> >>>>
> >>>> On 30-May-26 15:13, Barbara Denny via Internet-history wrote:
> >>>>> While poking around on DTIC,  I found this interesting early report
> on
> >>>> the Internet while Vint was still at Stanford and Jon Postel was
> still at
> >>>> SRI (How many of you knew that? :-))The period of performance is Nov
> >>>> 15,1975 to Feb 15 1976.
> >>>>> https://apps.dtic.mil/sti/tr/pdf/ADA024823.pdf
> >>>>> You can find a little bit more info by using the contract number in
> >>>> dtic.
> >>>>> Happy Reading,barbara
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