We consider the problem of installing a two-level telecommunication network. Terminal nodes communicate with each other through hubs. Hubs can be installed on terminal nodes and they are interconnected by a complete network. Each terminal is connected to a hub node by direct links. The aim is to minimize the cost of installing hubs and links. We present valid and facet defining inequalities for this problem.
SIAM Journal on Discrete Mathematics, Vol. 19, 501-522, 2005.