Chapter 1
Data Communications: Emergence 1956-1968
Modems and Multiplexers
1.6 Multiplexer Innovation: American Data Systems 1966-1968
The same dynamics that were driving the potential buying
demand for modems were having a more immediate impact on multiplexers,
the second great product of data communications. Multiplexers make it possible
for many users to share the same communication channel at the same time.
Multiplexers date to the days of the telegraph, and, although
incrementally innovated over time, performed the same task as when first
created: subdividing a communication channel into a larger number of channels.
State-of-the-art in the early 1960’s were frequency division multiplexers
(FDMs). Developed by the telephone and telegraph companies in the 1930’s
and 1940’s, FDMs partitioned the frequency of an analog voice channel into
multiple lower speed subchannels with each subchannel occupying a defined
range of frequencies separated by frequency guard bands or safety zones. (See Exhibit 1.2 Frequency Division Multiplexer.)
Exhibit
1.2 Frequency Division Multiplexer
back to top
The need for guard bands imposed a practical limit on the bandwidth of a FDM to 1800 bps to 2000 bps. However, as computer terminals were particularly
slow (75-150 bps), a large number of terminals could be supported
by FDMs. FDMs did not require modems - another advantage. Even
so, as the number of terminals that needed to be connected to computers
increased, a new solution was needed - one to be innovated by a
new start-up.
In 1966, Art Wilkes received an offer too good to pass
up, and resigned from AT&T to work for his best customer, Scantlin
Electronics (SE). His job was to sell their newest FDM
capable of connecting 150 terminals to one 2400 bps circuit. SE’s business of leasing on-line
stock-quote terminals to brokerage firms and other financial
organizations was booming and Wilkes had no problem getting
them interested in the new FDM. Concurrently, Wilkes’ boss,
Bob Schaaf, was writing a business plan to launch SE into the
product business, presumably to sell their FDMs. Schaaf, however,
surprisingly recommended selling a yet to be developed line
of time division multiplexers (TDMs), a recommendation that
was summarily rejected. Around this same time, late 1967, Wilkes
was let go for not meeting sales quotas, despite the fact that
the FDM did not work. Wilkes and Schaaf would strike deals
leaving them free to pursue any ideas developed at SE in return
for foregoing any owed compensation and stock options from
the company. Immediately they began planning to start of a
company selling the TDMs they would develop. In January 1967
they incorporated their new company: American Data Systems
(ADS).
Time division multiplexers did away with the notion of
frequency sub-channels inherent to FDMs and instead gave every connecting
terminal, or device, a fixed time slot on a rotating basis. (See Exhibit
1.3 Time Division Multiplexer) The effect was to dramatically increase
the number of terminals or devices that could be serviced by a factor of
four, and with incremental innovation over time, by a factor of ten. TDMs,
unlike FDMs, required modems for they needed a digital circuit made possible
by modems.
Exhibit
1.3 Time Division Multiplexer
In need of someone to engineer and build some prototypes,
Wilkes called his college roommate, William Norred, a site engineer with
a technical support company for the Apollo moon program.
Norred:
"I was responsible for everything from sweeping the
floors up through the storage of rocket propellants."
Norred remembers:
"Art called me one day and said: "I'm
thinking about starting a company. Would you like to participate in it?" I
said: "Yes. What are you going to do?" He said: "We're going
to make modems and multiplexers," and I said: "What's a modem?
And what's a multiplexer?"
Ignorance did not deter Norred who readily said
"Yes." He and his wife gathered their few belongings, packed
them into their car and, in February 1968, drove from Texas to California:
"When I arrived, Art told me that the money
had fallen through. I said: 'That's fine. I'm moving in with you,' and
I actually lived in his den for a number of months. In fact, the first
multiplexer was built in his garage.”
Norred began building the TDM, while Wilkes and Schaaf
searched for funding.
back to top
