 through multiplexing and groom- ing, and hosting the service sources like VoD servers and soft switches. In order to implement such functionalities, the core network is composed of several dif- ferent equipment, called in general network elements. The way in which different functionalities are divided among the network elements and the way in which they cooperate to carry out collective tasks constitutes the so-called verti- cal or functional network architecture. The remaining of this chapter is devoted to analyzing this architecture, the different alternatives and the standards that have been introduced to allow compatibility among products of different vendors and to realize a quality standard on which carriers can build their business. In this section, we introduce a synthetic but complete description of the whole architec- ture to give the reader a reference frame where more detailed information is collocated to make the reader familiar with the main network functionalities. The network entities are organized in three superimposed networks: the transport net- work, the packet network, and the service network. This vertical architecture is repre- sented in Figure 3.9. Service content provider Service Logical connections Packet Packet switch Channel transmission request Transport Circuit Transmission switch systems FIGURE 3.9 Vertical architecture of the core network evidencing the transport, the packet, and the service networks. Networks Fundamentals and Present Architectures Metro switch network server Service Packet switch Packet Transport switch Transmission terminals Transport Transmission systems FIGURE 3.10 Scheme of the node of a core network where there are present service, packets, and transport equipment. At each node of the core network are potentially present three groups of node equip- ment. One or more network entities belonging to the service network generate service signals. These feed one or more network elements of the packet network that performs statistical multiplexing, besides a great n