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

[John Day]

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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