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

[Brian Carpenter]

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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