umber of other functionalities that we will detail in the following chapters; the packet switches process the signal in small segments, called packets. At their output the packets are aggregated in continuous data fluxes (called chan- nels) that are passed for transmission to the network entities of the transport layer. The core node architecture formed by these three equipment groups is represented in Figure 3.10. The architecture of Figure 3.9 is not yet a formal functional layering as it will be intro- duced in the next section, but it is frequently considered as the basic structure of the core network. The functional layering can be derived by structuring the three networks with suitable layer models and formally defining opportune interfaces among them. 3.2.1.1 Service Network The service network is constituted by the machines that customers interface to request the various telecommunication network services. For example, when requesting a VoD, a customer has to communicate with a video server where all the content is stored to ask the transmission of the desired video. Service equipment is naturally strongly dependent on the service under consideration, but in any case they interface the packet network by packet interfaces. This means that the service equipment fragments the information that has to transmit through the network into data packets that are delivered to the packet network as individual entities. Each data packet has a data field, in which the information to be transmitted is encoded, and a header field containing information about the packet itself, such as origin and Telecommunication Networks destination, length and type of service, number within the communication between the user and the service source, and SO on. A typical packet format used by service machines to interface the packet machines is the Ethernet frame, which we will introduce later in this chapter. 3.2.1.2 Packet Network The main functions of the packet network can be summarized as follows: Packet delivery to the correct desti