[ih] Nit-picking an origin story (touch at strayalpha.com)

[John Shoch]

As I finished writing this I just saw Guy Almes' comment on 50 Kbps analog
modems vs 56 Kbps digital lines -- thanks, Guy.

Allow me to offer a bit more context about how (I think) this evolved --
and try to separate some apples from oranges.

When Tom Marill of CCA first explored a cross-country data experiment in
1965 he listed some telecomm choices available at the time (although the
discussion mixes in issues of technical matters and tariff considerations):
--AT&T Direct Distance Dialing (DDD).  Dialed phone calls via the analog
voice network, with an AT&T "data set" (modem), typically 2-wire with
per-minute charges.  A 103A data set could get 300 bps.
--Wide Area Telephone Service (WATS).  This was really a tariff overlay,
allowing discount levels for high use, with some tariffs for unlimited use.
--Western Union Broadband Exchange:  This was a dialup service offered by
Western Union built upon their telegraph/Telex infrastructure, NOT on the
AT&T long distance network.  It was meant for data use, had 4-wire
circuits, but still needed a modem.  The other party had to have a WU
connection, as well.
--There were also private, dedicated, leased lines available from the phone
companies.  These were a lot more expensive.  You still needed a modem, and
a tariff plan.  He mentions the Telpak tariffs which allowed you to bond
together, for example, 12 voice channels for a total of 50 Kbps.
--He also notes the need for auto-dial and auto-answer devices, to allow
unattended dialup connections.

By the time of the Marill and Roberts paper at the 1966 FJCC they describe
the experiment they propose to undertake with dial-up, on demand, serial
links providing terminal access between 2 machines:  ""Initially, a 4KC
four-wire dial-up system will be used with 1200-bit-per-second asynchronous
modems."  The reference to four-wire access is a good hint that they will
not try to use AT&T dial-up lines.

In a (final?) report several years later CCA described what they actually
did:  dial-up access from a user on the MIT TX-2 to the SDC Q-32:
--They connected to the Western Union Broadband Exchange service with
dial-up access.  Auto-dialer at MIT, auto-answer at SDC.  (SDC did not
install an auto-dialer, and thus could not initiate a call to MIT.)
--4KC analog channel, data sets at each end, tariff of $.75 per minute.
They report using 1200 bps Western Union 2121B modems (but I can't find any
information on those units).
--It provided terminal access for a single TX-2 user to the Q-32 time
sharing monitor (no addressing to machines nor to processes at the Q-32).
--In terms of actual usage they reported over an 8 week period:  94 calls
attempted with 78 calls completed successfully. Assuming 5 work days/week,
that's about 2 terminal sessions per day.  Average time for call set-up on
successfully dialed calls was almost 20 seconds,
[An editorial note:  AT&T was selling lots of modems for
terminal-to-computer access, and this was a great demonstration of remote,
serial, terminal-to-computer-to-computer access from one time-sharing
system to another.  It has been asserted that they "...connected the TX-2
computer in Mass. to the Q-32 in California with a low speed dial-up
telephone line creating the first (however small) wide-area computer
network ever built. The result of this experiment was the realization that
the time-shared computers could work well together, running programs and
retrieving data as necessary on the remote machine...."  When you read the
final report, though, I find that description of a "wide-area computer
network" a bit...generous.]

During this period Roberts was working on the ideas for the Arpanet.  As
well reported, it's Scantelbury (from the NPL in the UK) who urges him to
seek out higher-speed connections.  As I recall, there are reports that
Roberts realized that the government had access to better tariffs for use
of leased lines from AT&T.  Telpak provided a tariff for 12 analog voice
channels bonded together to provide a total of 50 kbps.
AT&T describes the use of the 303 wide-band data station/modem (sort of the
size of a 2-drawer file cabinet):  "The next lower convenient breakdown is
the "group" channel which uses the bandwidth of 12 voice circuits. The
303-type equipment can transmit at a synchronous speed of 50 kilobits per
second over group facilities."
https://bitsavers.trailing-edge.com/communications/westernElectric/modems/303_Wideband_Data_Stations_Technical_Reference_Aug66.pdf

The BBN papers on the Arpanet design at the 1970 FJCC, by Heart, Ornstein,
et al., describe the 50 kbps links between the IMPS (presumably through the
AT&T modems).
"Implementation of the subnet involves two major technical activities:
providing 50-kilobit common carrier circuits and the associated modems...."
That's 50 kbps through multiple analog channels -- and the basis for
the "50 kbps" line rate for the Arpanet.

This was all followed later, in the 1970's-80's, with more direct access to
the underlying digital hierarchy in the US.  Recall that in the US a T1
digital circuit carries multiple voice channels with a total capacity of
1.544 Mbps.  The smallest unit is carrying one digitized voice channel --
8K sampling at 8 bits/sample = 64 Kbps for a DS0 link.  To directly carry
data, though, they have to steal some bits for overhead -- taking out 1
bit/sample the available throughput is 8K sampling at 7 bits/sample = 56
Kbps.  Not that this is an entirely digital link, requiring a special
DSU/CSU at each end (not an analog modem).

Of course, modem technology continued to advance.  By the 1990's (?) they
had reached about the limit for what you could cram into an analog voice
channel -- a 56 Kbps modem.  (As I recall, though this was not 56K in both
directions.). Note that this goes from digital in the computer, to analog
out of the modem and into the dial-up line, to digital for carrying through
the digital voice network, back to analog, out to the receiving modem, and
back to digital for the computer.  Phew!

John Shoch

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