Tymnet

~ the last words ~

Packet networks were national before anything about them was global. Turkey's was TURPAK, an experimental X.25 network the PTT put up in 1986, and it had about 2,100 users in 1992 and around 9,100 by 1998. Nine thousand people, in a country of sixty five million. That is the whole story of why I never touched one. By the time I had a modem, in 1998, the thing to dial was an ISP that put you on the internet, and the packet-network era went past my part of the world as a line item in a ministry budget rather than as something anybody used.

Tymnet was the American version of that idea, built earlier and much bigger. It grew out of Tymshare, a time-sharing company founded in 1964 that had an expensive problem: its customers were spread across the country and its computers were not, and long distance calls in the 1960s cost more than the computing did. So LaRoy Tymes and Norman Hardy designed a network to move the terminal traffic cheaply. Work began in 1968 and the first Supervisor program came up in November 1971.

What Tymshare then noticed is the thing that turns a cost center into a company. The network it had built to serve its own customers was worth more than the time-sharing. Tymnet was sold as a service in its own right, and by the middle of the 1990s it was carrying about five thousand nodes across thirty foreign countries, hauling traffic for airline reservation desks and for consumer services like CompuServe. If you dialed a local number in 1987 and reached a computer two thousand miles away, something like this was doing the carrying, and you were never told its name.

~ the record ~

~ how it worked ~

The design was centralized in a way nothing built after it would be. A program called the Supervisor sat above the whole network and computed the route for every connection before it was established, then loaded that route into the nodes along the path. The internet routes each packet independently and locally. Tymnet decided the whole path up front, from one place that knew everything, and then the path stayed put for the length of the session.

It sounds fragile and it was surprisingly not, because the Supervisor could be moved and re-elected if the machine running it failed. What it gave in return was character-level efficiency. Statistical multiplexing meant an idle terminal cost the network almost nothing while a busy one got what it needed, and on links that expensive that arithmetic was the entire product.

~ who owned it ~

Tymnet passed through more hands than most infrastructure survives. McDonnell Douglas bought Tymshare in 1984, which is how an aerospace company briefly ran a public data network. British Telecom bought the network business in 1989. Then came the Concert joint venture with MCI in the 1990s, and after the WorldCom merger the customer base ended up split across two owners who each had their own opinion about how long to keep a legacy X.25 network alive.

Splitting a network between two owners is how networks die. Neither half is worth investing in and each side is waiting to see what the other does. That is roughly what happened here, slowly, across the second half of the nineties, while every customer on it was being sold a frame relay or IP replacement by the same salespeople.

~ the day it died ~

MCI disconnected its portion on 31 March 2003. British Telecom finished migrating the last of its customers in February 2004, and in early March 2004 the final Supervisors were switched off, thirty two years and change after the first one came up.

Almost nobody wrote about it, which is correct for what it was. Tymnet was never a product anybody chose. It was the thing under the product, invisible when it worked, and by 2004 the traffic had all moved to a network that charged nothing per minute and asked nobody's permission to carry a session. The last public X.25 networks outlived their usefulness by about a decade purely on the strength of the terminals still bolted to walls in places that had not been renovated yet.

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