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

Rant received; thanks for the tutorial.
v


On Sat, May 30, 2026 at 11:55 PM Brian E Carpenter via Internet-history <
[email protected]> 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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