h the speed at which the memories have to work is the multiplexed frame bit rate that is given by Equation 7.9 and that scales linearly with the number of input ports. Thus, the limit of a time division crossbar based on TDIs is the speed of the used memories. Just to give an example, using CMOS circuits with a clock at 20 GHz, it is possible to real- ize a 16 X 16 switch where the tributary channels have a bit rate of 1.2 Gbit/s and the frame efficiency is 0.96. Telecommunication Networks 7.3.2 Bus-Based Switch Fabrics The use of TDIs is not the only way to realize time domain switching; it is a technique largely diffused in static switching applications, like SDH/SONET ADM, but practically unused in dynamic switching machines. As a matter of fact, even if there is no general principle against the use of a switch based on TDIs in a dynamic switching application, the need of managing completely asynchro- nous incoming packets of different length creates a lot of additional complications in the switch design. For this reason, in almost all dynamic switching applications, the bus-based time domain switch is used. This type of time domain switch fabric, reproducing the architecture of a bus network, is schematically represented in Figure 7.18 [29]. The input packets are stored in the switch fabric input queues, every queue is connected to a bus terminal that either in a slotted or in an asynchronous way transmits the packets in the queue through the bus after tagging them with the address of the output port where the output queue hosts the packet before sending it out of the switch fabric. Naturally, the performance of such a time division switch is strongly dependent on the type of bus used inside the switch. However, a few key elements are independent from the particular bus implementation. The most relevant of these characteristics is probably the behavior of the bus through- put. Let us indicate with n(Tin) the relative throughput of the adopted bus, that is, the ratio between the number Tout of packets per seco