Hub location problem in a tree-star network

Hubs are special facilities that are used as intermediate switching points. In the hub location problem we want to design a network contains a central node and locate hub facilities and allocate nodes to hubs in order to route the traffic between origin-destination pairs. In this paper origin and destination are terminal nodes .The path form origin and a destination should path through at least one hub. We consider the case that hubs are not necessarily connected directly to the central node. We present a programming model for this problem.