rks, the packets involved in the failure are simply lost. They will lack at the destination causing a retransmission request. For the datagram routing properties, retransmission will naturally use available resources, avoiding failed equipment. In a virtual circuit network, virtual circuits will be disrupted by a failure and they have to be recovered. Several methods are available that can be divided into two classes: protec- tion and restoration. In the protection case, every time a virtual path is set up, another path is also set up for the same connection. This second path has the constraint of not having network resources in common with the first (request that has to be passed to the transport network since the packet one does not have transport equipment visibility). Networks Fundamentals and Present Architectures The second path is used as backup of the first in case of failure. Both hot and cold backup techniques can be used, depending on whether the packet is always sent contemporary on the working and protection path or if the protection path is used only in case of failure. In case of restoration, spare capacity is planned in the network without connecting its use to a particular failure. When there is a failure, the affected virtual circuits are rerouted using the spare capacity. Also in this case, different types or restoration exist, as detailed in Chapter 8, depending, for example, if the spare capacity is reserved for restoration or, in case of failure, circuits to be rerouted compete with the requests on new circuits. Restoration is largely the most used recovery method of packet circuits, since protection is left to the transport network. 3.2.1.3 Transport Network The functionalities of the transport network can be summarized as follows: Transmission of channels received from the packet network Optimization of transmission systems exploitation via TDM hierarchical multi- plexing and grooming Verification of the absence of errors in the transmitted TDM frame Fast recovery of lost channels due to a