Efficient interconnection networks are critical to the performance of large communication systems and parallel and distributed computing systems with hundreds or thousands of communication components. The well-known Clos (1953) network, or v(m,n,r) network, can be extended to provide full one-to-many or multicast capability. In this paper, we consider several typical routing control strategies for Clos-type nonblocking multicast networks, and derive the necessary conditions under which this type of network is nonblocking for arbitrary multicast assignments in the strict sense as well as under these control strategies. The necessary conditions derived are represented as the number of middle switches, m/spl ges//spl Theta/(n log r/log log r). These results match the sufficient nonblocking condition for the currently best available explicitly-constructed, constant-stage nonblocking multicast network, and provide a basis for the optimal design of this type of multicast network.
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