 condition a DWDM signal and to the more compact aspect ratio of gray interfaces. For example, DWDM interfaces at 10 Gbit/s would need XFPs, while gray interfaces, either extended or normal small form factor pluggables (SFPs) (compare Chapter 5). Supposing that the difference in electronics can be neglected, calling U the platform cost of a router card, AR the card area devoted to interfaces, and As the overall card area, we can write (7.35) At this point, the correct inequality that can drive the choice in the absence of other issues, is (7.36) At this point the solution is no more obvious. As a matter of fact, an optimized DWDM platform has in general a platform cost much smaller that a router platform, SO that PGR and Pin int ; moreover at least two gray interfaces are present on a line card in the space of one DWDM interface, that is, NGR 2 2Nint. Switching Systems: Architecture and Performances Just to see even better the effect of the platform cost, let us incorporate PTX and CTX in C'TX SO to rewrite Equation 7.36 like (7.37) In effect, the result is now much less obvious and with the decrease of the prices for tran- sponders and the increase in complexity of the routers back plane, it is possible to say that the trend is that using external transponders will get more and more convenient. 7.6 On the Performances of Core Switching Machines Core switching machines, mainly OXCs and multi service switch platform (MSSP) in the transport layer and routers in the upper layer, are the most complex systems in the net- work. Core routers can have routing capabilities up to 109 packets per second, while the larger OXCs manage a total capacity on the order of hundred Tbit/s. In order to achieve these performances the most advanced architectures have to be adopted, with a great attention to the impact that the characteristics of these big machines have on the network. In this section we would like to carry out a brief discussion on a few key requirements that are common to core switching equipment and discuss briefly t