situation in Europe and in most of Far Eastern countries. The traffic pattern in which all the traffic is to be transported to the core (there is no traffic directly routed from a metro node to another metro node) is called hub traffic; the core nodes interfacing the metro network are called hubs, and the access nodes are called sources (even if the traffic is bidirectional SO that it also routes from the hubs to the sources) [8]. Since metro networks provide connectivity in all cities, there is a higher probability of cable break with respect to long haul systems deployed along great infrastructures like roads and railroads mainly in the countryside. Metro network connections transmit high-capacity signals where a great number of cus- tomer signals are aggregated; thus, effective survivability has to be provided. Ring topology is suitable for both hub traffic and fast survivability mechanisms [10] and is practically the standard topology in the metro or regional area. Since the evolution to broadband network started from a telephone-centric network gen- erally maintaining the same central offices, a two-layer architecture is frequently adopted, as shown in Figure 3.8. A first ring set connects the access nodes with metro intermediate nodes, constituting the so-called metro access network. All the signals processed by the intermediate nodes are collected by a single ring, called metro core ring [11], delivering it to the hubs. It is quite common, especially in big cities, to have two hubs physically separated in different buildings. If the hub equipment is correctly dimensioned, this assures network survivability in case of a catastrophic event completely destroying a hub. Networks Fundamentals and Present Architectures Metro core nodes Metro core ring Metro access rings Metro access nodes FIGURE 3.8 Two levels multi-ring metro network. The presence of intermediate nodes also provides a further level of traffic consolidation increasing the exploitation efficiency of the high-capacity metro core and core transmis-