In the past six years of operation, TYMNET has increased from 30 computer nodes to 240 computer nodes spread throughout the United States, Canada and Mexico. Tymnet also provides the equipment, technology and programming for the data base services offered by the local Postal Telephone and Telegraph administrations in England, France, Netherlands, Germany, and Switzerland. Tymnet currently serves over 100 host computers. During these six years the mean time between failures of the TYMNET computer nodes has gone from 1.14 hardware failures per node per year to 0.5 failures per node per year, with a mean time to repair of under four hours. This increase in hardware reliability is a result of equipment modification in response to failures, improvement in monitoring tools, a centralized network control, and decentralized board repair.

TYMNET is a large full duplex terminal oriented network that has been developed by Tymshare, Inc. The network is composed of over 100 interconnected computers serving as hosts or nodes. The evolution of this network was made possible by resolving a variety of hardware, software, and telecommunications problems. This paper describes the steps that took place in this evolution, the problems that were encountered at each of these steps, and how they were solved.