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

[Brian E Carpenter]

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