Design of telecommunication networks

We consider the case when several users should be connected to each other at the lowest possible cost, under the constraint that the time required to transmit information between any pair of users is always less than a threshold given a priori. The proposed architecture is based on hubs, which are special users to which the information sent by their neighbors is directed before being transmitted through other hubs to the final recipients. We assume that at most one failure may happen on the connections between a pair of hubs at any time