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

John Day jeanjour at comcast.net
Sun May 31 06:36:24 PDT 2026 

You are undoubtedly right and it is to some degree just an annoyance.

The other misconception that goes with this is that if the ISP advertises X ‘bandwidth/capacity’ should the customer expect to have X available to them all the time? How many other customers could the ISP support if that were the case?

> On May 31, 2026, at 08:59, Bill Woodcock <woody at pch.net> wrote:
> 
> I do understand the difference between bandwidth and capacity and throughput and goodput and so forth, as used as terms-of-art within our industry.  But I also recognize that “bandwidth” is also a term-of-art in the _business_ of the Internet, meaning “the thing that ISPs sell.”  It doesn’t make sense to complain that they need to change or use a different word or use a whole paragraph to describe it.  And we can’t really expect that usage not to leak back into the rest of the industry, when it’s a more commonplace usage than the “range of frequency” term-of-art.
> 
> So I sympathize, but I don’t think this is a hill worth dying on.
> 
>                                -Bill
> 
> 
> 
> 
>> On May 31, 2026, at 12:48, 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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