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

[John Day]

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
>>> --
>>> Internet-history mailing list
>>> Internet-history at elists.isoc.org
>>> https://elists.isoc.org/mailman/listinfo/internet-history
>>> -
>>> Unsubscribe:
>>> https://app.smartsheet.com/b/form/9b6ef0621638436ab0a9b23cb0668b0b?The%20list%20to%20be%20unsubscribed%20from=Internet-history
>>> 
>> 
> -- 
> Internet-history mailing list
> Internet-history at elists.isoc.org
> https://elists.isoc.org/mailman/listinfo/internet-history
> -
> Unsubscribe: https://app.smartsheet.com/b/form/9b6ef0621638436ab0a9b23cb0668b0b?The%20list%20to%20be%20unsubscribed%20from=Internet-history