new topology elements are added. This last operation is particularly simple if no switching/routing machine is installed at the physical layer. As a matter of fact, if either SONET or OTN are present, new net- work elements have to be added to the network manually not only at the hardware level, where installation of new equipment is needed, but also at the management system level; as a matter of fact, TMN has to be configured manually to recognize the new network configuration. On the other hand, MPLS has the automatic network discovery feature that makes the network aware of the presence of new hardware even without a manual intervention on the central management system. 3.4.1.3 Network Scalability: Metro Network If a metro network is considered instead of a core one, the issues related to network scal- ability change a bit. The topology of the network in a highly populated metro area is practically standard: a two-level hierarchical ring topology that is represented in Figure 3.8. In this context, scaling the network has two possible meanings: increasing traffic and adding a new access ring having as a hub an already existing metro core node. Apart from exceptional situations like catastrophes or an abrupt development of the metro area as that happened in some area of China like Shenzhen in the last 10 years, the addition of a new metro core node is a quite improbable event. Adding a new access ring, provided that the hub node is not saturated and that fibers are present in the field, is quite easy. It requires the individuation of the central offices in which to install the access equipment and the deployment of new metro access systems; after that the network has to be configured either through the management system (in case of next generation SONET) or through the control plane (in case of carrier class Ethernet). For this reason, many scaling issues that are present in a core network are not applicable in the metro area, and both next generation SONET and carrier class Ethernet seems more than suitable to 