g random variability, due to the fact that duplication, lost, or wrong order are random phenomena depending on instantaneous network status or on errors that cannot be forecasted. This lack of control on the packets delivery time renders difficult to support real-time ser- vices like TV or telephone, rendering datagram difficult to implement in public networks. Telecommunication Networks Connection between A and B in three packets Virtual circuit Packets from A to B FIGURE 3.13 Routing in a virtual circuit network. If all the packets belonging to the same communication are routed through the same route in the network, the network is said to implement virtual circuit routing (see Figure 3.13). The path the packets have to follow has to be set up when the communication starts by advertising all the traversed machines that packets with a certain communication ID have to be routed through a certain path. At the end of communication the network has to tear down the path by advertising the network entities that the communication has ended. This means that a more complex signaling is needed with respect to a datagram network. Moreover, since the packet routes are stated at the beginning of a communication, it is impossible to change routing to follow the network status changes and channels are exploited less efficiently. On the other hand, the packets delivery time can be controlled much more effectively and monitoring of a communication in terms of delivered packets, errors done, and SO on can be performed much more effectively. Real-time services require a virtual circuit network and core carrier networks imple- ment virtual circuits, even if IP is datagram oriented, using for example multiprotocol label switching (MPLS). Returning to the packet network functionalities, error management is carried out either through error correction or through error detection codes whose redundancy is carried out by the packet header. Fast recovery of data disrupted by a failure is carried out with different methods. In datagram netwo