the technology with which they are realized (synchro- nous digital hierarchy [SDH], Ethernet, IP, etc.). Moreover, it should be considered that almost all the signal processing that is needed in the network is carried out in the switching elements, since for their nature, they generally have powerful signal processors. This contributes to differentiating further the switching systems, not only with respect to the technology and the layer where they are inserted, but also for the particular imple- mentation of signal processing functionalities. For all the aforementioned reasons, it is almost impossible to create an exhaustive review of all switching network elements. As we have done in all the chapters of this book, we will try to give a clear idea of the main switching machines that will be the core of the IP convergent network; regarding other categories of equipment we will not be SO accurate. Dealing with the optical layer, we will classify the main architectures of optical cross connect (OXC) and optical add-drop multiplexers (OADM) introducing their properties and performances; moreover, we will also analyze new SDH/SONET ADMs. As far as packet switching is concerned, besides a discussion about carrier class Ethernet switches, we will dedicate an important attention to Internet protocol (IP)/multiprotocol label switching (MPLS) routers. All the aforementioned machines have in common a high-level architecture that is shown in Figure 7.1. The switching machine is interfaced with transmission systems or with other machines, in case at a lower level, through input/output interfaces. The role of the interfaces is not only to regenerate the signal, but also to condition it to be processed by the system. This conditioning comprises not only signal regeneration, but also, for example, the ter- mination of the header at lower layers, possibly resynchronization of the incoming signals, error detection. Telecommunication Networks Equipment central control Input signals Output signals Input interfaces Switch fabric Out