The entities providing value-added network (VAN) service make the implicit assumption that by leasing private lines from the telephone carriers, they can make more efficient use of the lines than can the telephone carriers. If this is proven true, the VAN's should have scale economies at network sizes much smaller than the telephone network, while achieving other desirable technical and economic properties. Why the VAN carriers think this assumption is true is explained.

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.