ice, that the network manager sets a minimum time interval between successive new connection setup in the network to allow central- ized operation to be carried out. In this condition, requests arrive at the OXC in such a way that if a sufficiently small time interval is selected the probability that two or more requests will arrive in this interval is zero. Let us call Tin the probability that in the considered interval a new connection request is delivered to the OXC through a specific input direction. This probability can be called "traffic intensity" and we assume that it is independent from the specific port considered and from the state of the other ports. If all the channels are statistically independent and all the destinations have the same probability to be selected when a new connection is required, the probability p(j) that a generic channel setup request arrives in the considered time interval and is directed toward the jth direction among the possible N is p(j) = Tin/N SO that the index j can be sup- pressed calling it simply p. The new channel provokes blocking, if it belongs to an SP where output is no longer free toward the channel destination while other SPs have a free output but are unreachable by the new channel. Let us call m(j) < M, j =1, ... SP, the number of channels routed through the jth SP. The probability P that the SP receiving the new channel cannot route it toward the correct direction coincides with the probability that C channels are already routed in that direction in the same SP. This configuration requires that, in the considered plane (let us first imagine without the loss of generality and let us drop the useless index) m > W n and its probability writes (7.39) Switching Systems: Architecture and Performances where Ps(N/m) represents the probability that a blocking condition arises in the given time interval due to the arrival of a new channel conditioned to the fact that, before the new arrival, the SP processes m channels. Saturating all the acceptable values of m it can b