es it off by sending suitable commands to the element manager to release the cor- responding network resources. Due to the fact that we are considering either SDH/SONET or OTNs, TMN paths are semi-static. This means that, once a path is set up, generally it remains up until some great network restructuring: it is quite rare that paths are tear down just because the traffic request generating them is ended. Moreover, the path setup via the TMN is not a very fast operation, due to the complexity of the procedure and to the fact that sometimes even manual intervention of the operators required. On the other hand, due to the centralized algorithm used (Figure 3.46), the operation is generally optimized. Workstation FIGURE 3.46 TMN functional scheme. Telecommunication Networks Besides paths-related functions, the network manager supervises all the "abstract" char- acteristics of the network: for example, MS routing if it exists and SO on. 3.2.8.4 Service Management Layer The service layer manages the services delivered to the end customer, providing custom- ers authentication and billing, storing in its database the customer profile in terms of ser- vice fruition history, and providing a software platform to implement in the network the commercial offering of the carrier both in terms of service bundle and in terms of paying solutions. Also security is implemented in this layer, by access keys and customers information separations. Last but not least, this layer is the master of the procedure for the introduction of new services in the network. 3.2.8.5 Business Management Layer This is the most abstract layer, whose role would be to take under control the business aspects of the network: costs and service related revenues, up to the evaluation of a sort of network profit-and-loss sheet. From the earlier discussion, it is clear that the TMN is a very complex software, com- posed by several logically separated elements and also by physically separated parts. Thus, for the TMN to work, it has to be provided with a commun