l demand due to increased marginal cost would exceed the mandated demand for capacity due to network access. Thus, unbundled network access would cause total network usage to fall. Regulators should not conclude from this outcome that incremental usage did not affect the network because the reduction in usage is the result of increased costs due to unbundled access. Consider the impact of unbundled access on the three service layers of the network. At the transmission layer, the network must provide access to transport, including dedicated transport connecting the network switches or wire centers. At the sortation layer, the network must provide various types of services using its switching facilities. Also, the network must provide unbundled access to signaling networks that are used to operate the network. Finally, at the distribution layer, the network must provide the services of local loops to the customers' premises. Unbundled access is likely to have significant effects on the configuration of the network. Recall that the formal representation of a network involves nodes, links, and values assigned to the links that can represent such information as capacity and costs. By granting competitors access to particular network elements or combinations of network elements, unbundled access affects the characteris- tics of those network elements from the point of view of the network firm. If another firm has access to a particular link, for example, the capacity available to the network firm is reduced. The network firm would encounter additional costs if it were to expand the capacity of the link. The capacity and cost values assigned to the link must therefore be modified to reflect access to that network element. A change in the cost of one or more links can significantly alter the design of a network to connect a set of nodes. A change in the cost of one or more links can change the configuration of the least-cost minimum spanning tree network or the least-cost cycle network. Because the design of a network re