gram packet network. Appendix A the structure of one of the most used TDM frame formats, the synchronous digital hierarchy (SDH)/synchronous optical network (SONET) frame. The information field contains the packet to be transported by the frame. Once the transport network assures the correct transmission of the channels, the packet network sees them as the transmission resources at its disposal to deliver packets to their destination. Using channels, the packets can be delivered by the packet network following two main strategies: datagram and virtual circuit. In a datagram network each packet is managed individually, with no relation with the packets constituting the same communication between the service equipment and the customer. Thus, consecutive packets of the same communications can be routed through the network via different routes, depending on the network state at that moment (see Figure 3.12). This technique allows the channels to be exploited at best, since a packet is multiplexed in a specific channel among those useful to transmit it to destination with the only task of saturating the channel SO as to optimize its use. On the other hand, due to the independence of packets in a datagram network, packets from the same connection between user and server can arrive in the wrong order experi- encing different routes in the network. Moreover, if a packet gets lost, it is evident only at the end of the communication and duplicated packets (that can exist as a consequence of routing errors) are identified only at destination. This lack of control on transmitted packet by the datagram causes the service network (or the receiving node in the packet network) to carry out a supplement of work in reorder- ing the arriving packets and asking for retransmission of the lacking one while disregard- ing the duplicated one. All these operations can be closed only when all the messages have been delivered, making it difficult to have a short delivery time for messages. Moreover, the delivery time also manifests a stron