[ih] Nit-picking an origin story

[Steve Crocker]

It's very common for people to assume the Arpanet lines were 56Kbps, so it
might be helpful to explain why they were 50 kbps instead of 56 Kbps.  I'd
guess a large fraction of the people on this list are familiar with the
following, but for youngsters who didn't grow up before the telephone
system became fully digital, here's the brief story.

*Digital* lines are 64 kbps, with 7 bits out of 8 used for data.  Hence 56
kbps.

Previously, the lines were analog, and data was sent by encoding bits into
sounds.  The encoder/decoder was called a "modem," standing for
modulate/demodulate.  The encoding improved over time from 300 bps to 1200,
and then to 2400 bps.  With careful "conditioning" of the lines and further
advances in code, the data rates were pushed up to 9600 and then to 19200
bits per second.

When the Arpanet was designed, one of the offerings from AT&T was 50,000
bits per second.  They achieved this data rate by using twelve(!) voice
grade lines and spreading the data across all twelve.  The modem, Western
Electric series 303A, I believe, was a complicated piece of equipment.  The
box that held could accommodate four of these modems and had a footprint
about the same as a refrigerator and stood about half as tall as the IMP.
There was no relationship between the techniques used to create a 50,000
bit channel using twelve voice grade lines and the later technique of using
digital lines to transmit 56,000 bits per second.  Nonetheless, because the
numbers are close and 56,000 bits per second became the universal building
block for digital transmission, it's very common for people to assume the
Arpanet had 56 kbps lines.

(I expect others on this list know more about the details than I do and may
be able to add some color or correct any errors I've made.)

Steve


On Sun, Aug 24, 2025 at 8:44 AM Vint Cerf via Internet-history <
internet-history at elists.isoc.org> wrote:

> lars is correct
> v
>
>
> On Sun, Aug 24, 2025 at 8:25 AM Lars Brinkhoff via Internet-history <
> internet-history at elists.isoc.org> wrote:
>
> > Jack Haverty wrote:
> > > Performance was also an issue as the ARPANET grew and traffic
> > > increased.  One of the limiting factors to performance was the routing
> > > algorithm.   Packets were always sent on the "shortest" path.   But
> > > that meant that the aggregate performance was also limited to
> > > 56kb/sec, which was the maximum line speed of any path.
> >
> > I beleve the correct number is 50,000 bits/second.
> >
> > ARPANET Information Brochure, from 1980.
> > "The complete network is formed by interconnecting the nodes through
> > wideband communication lines, normally 50,000 bits per second (50KBPS),
> > supplied by common carriers."
> > https://apps.dtic.mil/sti/pdfs/ADA096798.pdf#page=19
> >
> > BBN Report 1928, from 1970.
> > "In the last quarter, we designed and implemented a test program to
> > obtain data on the performance of the fifty kilobit communication
> > circuits"
> >
> >
> http://bärwolff.de/bbn-arpanet-reports-collection/BBN%20(1970)%20Interface%20Message%20Processors%20for%20the%20ARPA%20Computer%20Network%20(Report%201928,%20Quarterly%20Technical%20Report%204).pdf#page=10
> <http://xn--brwolff-5wa.de/bbn-arpanet-reports-collection/BBN%20(1970)%20Interface%20Message%20Processors%20for%20the%20ARPA%20Computer%20Network%20(Report%201928,%20Quarterly%20Technical%20Report%204).pdf#page=10>
> > <
> http://xn--brwolff-5wa.de/bbn-arpanet-reports-collection/BBN%20(1970)%20Interface%20Message%20Processors%20for%20the%20ARPA%20Computer%20Network%20(Report%201928,%20Quarterly%20Technical%20Report%204).pdf#page=10
> >
> >
> > This was due to the Bell/Western Electric 303C wideband modem using a
> > group service of 12 voice circuits.
> >
> >
> https://bitsavers.org/communications/westernElectric/modems/303_Wideband_Data_Stations_Technical_Reference_Aug66.pdf#page=6
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