put interfaces FIGURE 7.1 Functional scheme of a space diversity switch fabric. In case of packet network machines, queues are located in the interface cards to perform the storage and forwarding of incoming packets. The interfaces are connected with a switch fabric that has the role to create a correspon- dence among input and output ports to send the incoming signal in the correct direction. Finally, a centralized or distributed processor manages the equipment. The architecture of large switch fabrics is complex, due to the need of finding a trade- off between the ability of creating whatever permutation between input and output with fast switching times and using simple control algorithms and the need of avoiding a huge hardware structure with great costs and low reliability. For this reason, we will devote a preliminary section to switching fabric architectures. This allows us to deal in a systematic way with this key point, reviewing a set of general results that will be useful several times dealing with different network equipment. After the section dedicated to switch fabrics, we will also discuss the particular prob- lems a platform for a switching machine has to face, which does not exist in platform for transmission systems. This is another general discussion that adapts to almost every kind of switching machine. 7.2 Space Division Switch Fabrics In all the switching machines that are deployed in the telecommunication networks, there is a part of the machine devoted to transfer the signal from the input port to the correct output port. This subsystem is called switch fabrics [1,2]. The first kind of switch fabrics that comes under consideration is the class of the so-called space diversity switch fabrics, and we will concentrate for the moment our attention on them. In this section, we will review a few schemes of space diversity switch fabrics that are both simple and important for applications. However, this is only a tiny part of the huge amount of different architectures that have been proposed and 