opagate its leading edge over all the bus length and SO inform all the access points of its presence. This time is called bus round trip. Switching Systems: Architecture and Performances Since the collision probability can be written as the probability of collision during the round trip multiplied for the ratio between the round trip and the packet duration, increas- ing the packet duration in a given CSMA bus causes the saturated throughput to increase monotonically up to 100% for negligible round trip with respect to the packet duration. Moreover, if a collision occurs, any access point detects it within a round trip time and can stop any active transmission to end the collision as soon as possible and recover media availability. This last technique is called collision detection (CD) and the protocol adopting CSMA and CD is called CSMA-CD. As in the case of random access busses, CSMA-CD busses can also be slotted or unslot- ted, depending on the fact that the input packets are synchronized on a slotted bus frame or are transmitted on the bus in an asynchronous way. From now on we will consider switch fabrics based on slotted CSMA-CD busses due to their high performances. To evaluate analytically the throughput of a CSMA-CD bus-based switch fabric, we will assume again that the input queues are of infinite length and that high traffic conditions are verified SO that a packet is always present at an input queue head [30]. Moreover, in order to evaluate the worst case, we will assume that all the access points are at one of the bus extremes and all the detection points at the other, SO that any packet, independent from the origin and the destination, experiences a round trip of 2t. We will also assume that the time slot on the bus frame is exactly equal to 2T. This only means that the length of a packet in the bus bit rate has to be an integer multiple of 2T, that is, practically no restriction, considering that the packet length will be designed to be much larger than the round trip time. At the beginning of each