he quantitative mea- surement of their performance. 7.6.1 Capacity, Throughput, and Channel Utilization Even if switching equipment has the common role of routing the information carrying sig- nal through the network, different classes of equipment are characterized by a "capacity" that often is defined ad hoc for the specific equipment. Since confusion can derive from this situation, in this section we want to define clearly the capacity definitions we will use in the following. In the case of circuit switching (e.g., OXCs), the machine capacity critically depends on the switch fabric. If the switch fabric is nonblocking, every permutation can be done among the input and output channels. In this case, it is natural to call switch capacity the maximum amount of signal in terms of bit/s that can be delivered to the system. In the circuit layer (both OTN and SONET/SDH) all the connections are bidirectional, SO that it is usual for this kind of switching machines to define the bidirectional capacity. For example, if a maximum of 100 channels at 10 Gbit/s structured in 50 bidirectional connec- tions converge on an OXC, the capacity of the OXC will be 5 Tbit/s (and not 10 Tbit/s as it was taking into account the whole input information). We will adopt the bidirectional capacity definition since it is almost universally used in technical literature, but also advise that it creates a few problems when comparing routers' capacity with OXC and MSSP capacity due to the fact that routers do not have the restric- tion of bidirectional connections. If the inner switch fabric is blocking, the capacity of the switching element can be evaluated multiplying the earlier definition multiplied by (1 - b), where b is the blocking probability. Telecommunication Networks Considering now packet systems, namely, routers and carrier class Ethernet switches, there is not a universally accepted measure of the performance and there are several pos- sible definitions for throughput. Among the possible definitions of throughput, we want to evi