 Hence the probability of it being unavailable is 1−(1−"p")2 = 2"p"−"p"2.The probability of blocking, or the probability that no such path is free, is then [1−(1−"p")2]"m".The Jacobaeus approximation is more accurate, and to see how it is derived, assume that some particular mapping of calls entering the Clos network (input calls) already exists onto middle stage switches.