TELECOM Digest OnLine - Sorted: Re: Very Early Modems

Re: Very Early Modems
25 May 2005 11:01:57 -0700

Jim Haynes wrote:

> The Bell System didn't allow any "foreign" (meaning
> customer-provided equipment) attached to the switched network. They
> vigorously defended this position until it was overturned by the
> Carterfone case.

Our school system originally used Teletypes rented from Bell with the
built in modem and dial equipment. IIRC they ran about $100 a month.

Then they bought or leased Teletypes from private vendors. The modems
were supposed to go through the Bell "DAA" box (the protection* unit)
but often that was forgotten. Although the phone system was still
mostly hard wired in those days (excepting the 4-prong home jacks), we
got around that by using a special transmitter cup that had a tiny
jack for our modem to connect into.

*Some said the "protection" was really to protect Bell System
revenues, not the network. However, see the garbage people sometimes
hook up today, despite supposedly being certified, and knowing the
shortcuts we took back then, I'm think having that protection wasn't
such a bad idea after all. Recall that Bell was responsible for
everything in those days so if an illegal attachment hurt something
Bell was still stuck to fix it. (Most illegal users knew to hide
their gear before calling Bell in.)

> And since it did not need the entire bandwidth of a voice-grade line
> IBM designed the modem with four different frequency bands so that
> up to four systems could operate simultaneously over a voice-grade
> line.

The IBM book says they used the four frequency bands to get
an effective 1200 baud rate, which seems good for the 1950s.

> The earliest modems were not really called that but were the carrier
> systems installed in telephone and telegraph company offices to
> allow multiple telegraph transmissions over a single voice-grade
> circuit.

The Bell history says that telegraph signals could be carried on the
low end of a voice grade circuit--apparently this was done even in the
1930s with simple electronics. I believe pre-WW II Bell carrier
systems were pretty limited in deployment and capacity; it wasn't
until postwar microwave and widespread coaxial cable could they get
high volume. Coax did exist before WW II, but I suspect it was quite

> Then in the early 1960s the Bell System opened things up by leasing
> modems that allowed the customer to connect business machines to the
> modem and transmit data over the switched network.

The Western Union history describes advanced switching and
communications networks for telegraph traffic, including special
networks for govt and business. It looked to be state of the art for
its day (1960s). I'm still hazy on how Western Union missed the boat
on data communication which was after all their specialty. Some say
WU had a very limited transmission network and depended on Bell for
that "final mile" although in cities WU had quite a broad network.
Or, their Telex wasn't as good as AT&T's TWX.

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