 fabric output port can always leave the fabric. The switch fabric input queues are infinite SO that the queue output process is also a Poisson process. The electronic elaboration duration dominates the round trip time, being 2t 22 Te = 100 ns. The inner bus speed is 2.5 Gbit/s. The line card speed is 125 Mbit/s. The packets are 512 bytes long. The effect of the persistent property of p persistent CSMA protocols for different values of the persistence probability is shown in Figure 7.21, where the normalized throughput is shown versus the average number of access points contending the transmission medium during a packet time. This parameter is naturally related both to the number of access points, that is of input/output ports of the switch fabric, and to the offered load. As obvious, decreasing the persistence probability, the performance is quite improved in regime of high collisions up to the disappearance of the congestion regime when the persistence probability is very low. The effect of the use of several transmission wavelengths is shown in Figure 7.22. The interesting aspect of this figure is that the saturation throughput almost remains the same increasing the number of available wavelengths; this is due to the fact that we use relative measurement units for the throughput SO that the real throughput is obtained multiplying n by the offered load. Switching Systems: Architecture and Performances FIGURE 7.21 Comparison among the throughput depen- dences on the average number of conflicting access points in the case of different bus archi- tectures. A, Unslotted random access; B, slotted random access. From C to F: p persistent CSMA. Average number of attempts in a packet time C, p = 1; D, = 0.5; E, p = 0.1; F, p = 0.01. FIGURE 7.22 Relative throughput of a WDM bus-based switch fabric versus the offered traf- fic for different values of the number of Offered traffic (Tin) wavelengths. Thus, having a constant value of n that is reached at higher and higher values of the offered load by increasing the number 