bus when used to build a switch fabric designed to work in a condition of high traffic. Naturally, if the high traffic condition is not verified, having a higher throughput is less important and in these situations other parameters can influence the design of the switch fabric, like the cost. In this case, it could be possible that the simple random access bus is the preferred choice. 7.3.2.4 Switch Fabric Based on Variations of the Carrier Sense Multiple Access Bus Up to now we have analyzed the base version of the switch fabric based on CSMA-CD bus, but several variations exist to optimize the switch fabric performances. These variations are deeply studied in the field of LAN networks and we will simply describe some of them qualitatively, commenting on their performances without present- ing an analytical derivation. The reader interested in the mathematical models is encouraged to look at the immense repertory of technical literature on LANs, taking into account, however, that some ele- ments are quite different in the case in which a bus protocol is used in a switch fabric, as demonstrated in the previous section [29]. In order to improve the CSMA-CD performance in the case of bus-based switch fabrics, it is possible to adopt different strategies: Exploiting the fact that the propagation time along the bus is particularly short: We have seen that this is a key element allowing the collision probability to be reduced in a standard CSMA-CD bus. In the case of a switch fabric, however, the propagation along the bus is SO short that a coordination among the bus access points could be conceived. This coordination can be based on the transmission of an access point of a "transmitting bit" in a reserved slot of the bus frame. This allows the other access points to be informed of the intention of one of them to transmit, SO as to avoid collisions. Naturally, when more than one access point has booked a transmission, a round- robin mechanism is needed to state the order in which they have to access the bus. p Persist