s the book Broadband Packet Switching Technologies by H. Jonathan Chao, Cheuk H. Lam, and Eiji Oki,6 the article "Fast Packet Technology for Future Switches" by J. J. Deegan, G. W. R. Luderer, and A. K. Vaidya, the article "Survey of Switching Technologies in High-Speed Networks and Their Performance" by Y. Oie et al., 8 and the article "Fast Packet Switch Architectures for Broadband Integrated Services Networks" by F. A. Tobagi, T. Kwok, and F. Chiussi.9 We will not cover the multiple possibilities here. However, we will look at the most common architectures. A straightforward switch fabric is the crossbar switch. We have mentioned that the crossbar switch also requires a scheduling module, which at each moment in time will tell the fabric which inputs it must connect to which outputs. If the connections are functioning at a constant bit rate, the schedule can be computed in advance SO the fabric simply executes the results and sequencing of operations as dictated by the scheduling algorithm. If the connections are supporting variable bit rate links, how- ever, the schedule will have to be calculated by the fabric on-the-fly. This already points at significant intelligence that must be present in the fabric circuitry block. If two incoming packets from different inputs compete for the same output at the same time, output blocking will occur. The problem is addressed either by running the switch fabric much faster than the inputs, which is usually hard and very expensive to accomplish, or by placing buffers inside the crossbar switch fabric at each junction. The latter approach is also characterized by the pres- ence of an arbitration module that must be incorporated at each egress port and that decides at each moment which one among multiple buffers should be allowed to output a packet (or more) for rout- ing to that specific egress line. Despite its simple concept and structural elegance, the fundamental problem with the crossbar- based switch fabric is that it unfortunately does not scale well with the number 