that the vertical structure of the core, and often also of the metro, network is quite complex. The network equipment communicates with each other and with the central network management to carry out complex operations. In order to go deeper into the description of the network architecture, a more formal method needs to be introduced: the so-called layering. In the next section we will describe in more detail the network control system. After that, using the layering method, we will go in deep into the functional structure of the netowrk starting from the service/packet/ transport levels model. 3.2.1.4 Control Plane and Management Plane In order to coordinate their working, network elements exchange control information both among elements of the same network (e.g., transport elements) and among elements of dif- ferent networks. These exchange of information and commands follow set of rules that are called protocols. Such control information have to travel, much like data do, from one network element to the other; thus, a transmission capacity is needed to implement coordination among network elements. Such transmission capacity can be either embedded into the same network transmit- ting data (e.g., in the headers of the TDM frames) or provided by a completely separated control network. In both cases, it is possible to define two logical networks having the same nodes locations and the same network elements, but using a different transmission capacity and transmitting different data. The logical network transmitting data is called "data plane" and the logical network conveying protocol-related information is called "control plane." The control plane is necessary to assure network operation, but generally it is not suffi- cient. As a matter of fact, the control plane comprises procedures that coordinate elements of network operation, but do not comprise functions for reporting the network status to a central control, where network management operators supervise network working. Such reporting is needed for various 