have an impact on several aspects of real switch fabrics design. Among the relevant references to start a detailed analysis there are [21-25]. 7.3 Time Division Switch Fabrics Up to now, we have considered space division switch fabrics. This means that in the switch fabrics we have considered the connections between a set of spatially divided input ports and a set of spatially divided output ports that were realized by configuring a switch fabric where signals coming from different inputs are routed through spatially separated routes. This is not the only way to perform switching, that is, to establish and tear down connec- tions among sets of input and output ports. Another possible way is using time division multiplexing (TDM) and demultiplexing, that is, operating in the time domain [7]. 7.3.1 Time Slot Interchange-Based Switch Fabrics In order to introduce time domain switching, let us introduce with a bit of formality the simpler type of TDM multiplexing and demultiplexing. Much more complex methods are used when multiplexing is used to better exploit the transmission channel, and the reader is encouraged to look for the SDH/SONET description in Chapter 3 and Appendix A and to the related references. In order to introduce time domain switching, our simple description is sufficient. A TDM frame is a timely periodic sequence of N time slots, each of which is divided into a header field and a data field. We will assume that the header's field is composed of NH bytes and that it contains the identifiers (IDs) of all the tributary channels multi- plexed in the frame besides other information, for example, tributary channels nature (IP, Ethernet, ...), FEC, and SO on. We will also assume that the data field is composed by ND bytes, divided into N blocks, each of which hosts data belonging to one of the N raw channels multiplexed in the TDM frame. The frame efficiency is us = =nd/(nD+nH), indicating the amount of bytes of the frame devoted to data among the overall frame capacity. If each raw channel has a bit rate