derations we will refer in general to the maximum throughput, SO that when we say that a router has a capacity of 300 Gbit/s, this means that its maximum throughput is 300 Gbit/s. Naturally, the maximum throughput is generally smaller than the sum of the bit rates of the lines exiting from the switch. A router having 100 in/out ports at 10 Gbit/s (for a total transmission capacity of 1 Tbit/s) could have a maximum throughput of 600 Gbit/s (about 6 109 packets out per second). This is due to the fact that statistical fluctuations of the traffic load create a fluctuation of the load on each output line. Thus, the transmission systems have to be dimensioned on the maximum possible load and not on the average one. The ratio between the maximum throughput and the overall capacity of a packet machine output system is called channel utilization and it is often used besides the maxi- mum throughput to characterize the performances. Switching Systems: Architecture and Performances 7.6.2 Scalability A core router or a core OXC is a long-lived equipment, corresponding to a relevant CAPEX and operational cost (OPEX) expended from the carrier to buy and operate them. Thus, they will remain in the network well beyond the time a commercial strategy lasts or a particular service is implemented and also much more than the edge machines that are more service related. Thus, they have to provide the carrier with a real future proof equipment, characterized ideally by a small first installation cost and a cost of successive upgrades as proportional to the capacity of the upgrade as possible. The choice of the switch fabric architecture is a key from this point of view due to the fact that the different modules of the switch fabric have to interact with each other if the nonblocking characteristic is to be maintained while expanding the router, while different line cards are almost independent, but for a loose coordination carried out by the central controller. 7.6.3 Interface Cards Density Core OXCs and routers, especially Tbit-route