You're analogy is incorrect. In a star topology, you claim the mail carrier can't go from house to house, he goes from hub->spoke 1->hub->spoke2->hub, etc. Whereas in a ring he can go spoke 1->spoke 2-> etc. Thus his steps taken are N for ring, and 2N for star. The problem is for a cluster box, star as you call it, that serves N homes, the postal carrier only goes to 1 place, where as for a ring of N homes he must go to N places. There is no need for the postal carrier to visit the spokes with cluster boxes. So going door to door is O(n) whereas cluster boxes is O(1).