 only up to the moment in which congestion manifests itself inside the switch. At that point, the output growth is less than the input since part of the arrival traffic is rejected by the switch. Moreover, in dynamic fabrics, the time delay from the arrival and the departure of a packet is approximately constant only within the uncongested working state, while it starts to grow more and more when congestion starts. Evaluating throughput and fabric crossing delay is another type of quantitative perfor- mance evaluation of a switch fabric. Here we will analyze all the qualitative elements, while some quantitative evaluation is covered to Appendix E. 7.2.1 Crossbar Switch Fabrics The simpler switch fabric scheme is the so-called crossbar switch that is shown in Figure 7.3 [1,4]. The crossbar switch is constituted by a set of 2 x 2 switches placed at the crosspoints of a matrix of connections. The control of the switch fabric operates on each single switch independently by setting it to one of its four states, as shown in Figure 7.4. Switching Systems: Architecture and Performances Out 2 Out 1 Elementary 2 X 2 switch FIGURE 7.3 Functional scheme of a crossbar Output ports switch fabric. Bar state Cross state FIGURE 7.4 States of the elementary space division 2 X 2 Upper broadcast state Lower broadcast state switch constituting a switch fabric crosspoint. If the switch fabric is designed only for unicast traffic, only the bar and the cross state are used, otherwise all the states are exploited. In this way, any input signal can be routed toward any free output port. It is easy to see that a crossbar switch fabric is strictly nonblocking even if multicast con- nections are considered, if all the elementary 2 x 2 switches have multicast ability. Moreover, a crossbar fabric is modular at every layer. As a matter of fact, greater crossbar fabrics can be built starting from a base crossbar with a recursive construction whose first step is the buildup of the basic crossbar from the 2x2 switches. In particular, starting from