t real-time connectivity requirements. Under this dynamic connectivity environment, a lightpath is taken down after a defined period of time (known as the connection-holding time). The most important problem in wavelength-routed networks is the routing and wavelength assignment (RWA) problem, which, however, has been proven to be mathematically NP- complete.7 The multicast RWA problem is even harder to solve than the unicast RWA problem. Optical switches or switching matrices must have the proper optical characteristics such as low insertion loss, high isolation, and low back reflection. In addition, they need to be reliable, fast, compact, low power con- suming, and scalable. As discussed in Chapter 2, "Optics Primer," technolo- gies that are suitable for optical-switching matrices include, but are not Figure 7-8 IP over a lambda- switched WDM Core LSRs Edge LSRs network. ADM ports WDM Network ADM ports Edge LSRs LSRs = Label Switching Routers RWA = Routing and Wavelength Assignment Chapter 7 restricted to, Micro-Electro Mechanical Systems (MEMS), some MEMS-like technologies (such as thermocapillary and piezo-electric), liquid crystals, bubbles, thermo- and acousto-optics, and holograms. Beyond technology, there are always issues related to price point affordability. In general, as discussed elsewhere in this book, MAN technology of any kind needs to meet different optimality criteria than that for long-haul applications. For example, while looking at OXCs for metro networks, the cost of crosscon- nects needs to be much lower. This is because the total link costs in the met- ropolitan core are not necessarily dominated by fiber costs, which is the case in the long-haul domain. In fact, the opposite may be true for metro access. As an observation, it is not likely that OXCs with wavelength conversion will be used in metro networks in the near future. One key question is how rapidly the lambda establishment can be set up. If subsecond connections are required, then a full-fledged optical switch is needed. If a car