g packets at the input, the result is quite differ- ent with respect to the well-known LAN case. Let us assume to have a 16x16 switch fabric whose bus is located in the backplane of an equipment and implemented with an optical star coupler. Assuming that each transmitting laser transmits at 10 Gbit/s in the second window (see Chapter 3) emitting a modulated power of 1 dBm. The overall loss of a 16x16 optical star coupler is about -15 dB for the coupler and 1 dB for propagation, SO that a standard PIN detector is enough to detect the signal at 10 Gbit/s without noise problems (compare Section 5.4 from Chapter 5). The round trip time, assuming the star coupler to be L=2m long can be written as (7.25) where C is the speed of light is the electronic elaboration time, that is, the time the electronic processor of the access point needs to carry out the carrier sense Since 2L/c=6.6ns, the round trip time is dominated by the electronic elaboration time. Assuming that the access point needs to observe up to 5 bits to decide if the carrier is pres- ent, we can write Switching Systems: Architecture and Performances Let us also assume that the packets are 512 bytes long, that is, :41 us, we have, from Equation 7.22, Nmax = 0.994. Following the same line already considered for the random access bus-based switches, we can say that the maximum real traffic that a bus-based switch fabric can manage is half the saturated throughput divided by the number of ports. In our case, we can assume that the line card bit rate is 622 Mbit/s while the average traf- fic manageable by each input port is 0.5nmg R/N 22 300 Mbit/s. This number can be compared with the figure evaluated in the example at the end of Section 7.3.2.1, where a 16x16 switch fabric based on a random access slotted bus with an internal speed as high as 1.2 Gbit/s and line cards transmitting at 1.2 Gbit/s were able to manage only a traffic on the order of 200 Mbit/s per input port. The comparison between these simple examples clearly shows the superiority of the CSMA-CD 