nation in the order in which they are transmitted Individuation and, if possible, correction of errors due to packet processing or transmission Services requirements fulfillment in terms of, for example, delay and reliability Packets recovery in case of failure in the network Packet network performance monitoring and control In order to deliver packets to the destination, the packet network uses a set of virtual con- nections among its nodes created by asking permanent TDM connections to the transport network. In particular, a packet network machine, creates, at its interface with a transport net- work, a TDM channel and asks the transport network to transport the channel from its origin to its destination. A TDM channel is a continuous stream of information constituted by a periodical repeti- tion of the TDM frame. An example of TDM frame is reported in Figure 3.11: in the simpler case, it is constituted by a frame header and an information field. The frame header contains all the information that are needed for the correct transmis- sion of the channel from the origin to the destination in the transport network, for exam- ple, the destination address, the frame length, a frame synchronization word allowing receivers to individuate the frame start, an error correcting code (ECC), the position inside the information field of the packets transported by the frame, and SO on. We will detail in Data field Header field Frame TDM channel Header FIGURE 3.11 Scheme of the TDM frame of a channel. In the lower part of the figure the TDM channel is represented as a peri- odical sequence of identical frames. In the upper part, one frame is analyzed evidencing the data field, where the packets are located, and the header field. In this part of the figure, pa indicated the data part of a transported packet and h the packet header, while the frame header is represented by FH. Networks Fundamentals and Present Architectures Packet 3 lost Route of packets 1 and 3 Route of packet 2 Packets from A to B FIGURE 3.12 Routing in a data