exploited, since their potentiality would be to carry up to four lower speed channels. In a certain instant, a new connection request arrives from the packet network, consist- ing in five STM4 connections from A to D, as shown in the traffic matrix present in Figure 3.15b. If all the capacity would be routed through the route A-C-D, two STM16 would be needed, one of them greatly underutilized since it transports only one tributary. On the contrary, four STM4 can be routed through the route A-C-D as shown in the figure, the fifth is joined to the three channels directed in B from A, saturating the corre- sponding STM16 whose destination is node B. In order to optimize the use of the transmission resources, the STM16 is demultiplexed in B, the channels directed to B are passed to the packet network, and the channels directed to D are joined to the STM16, multiplexing the channels from B to D, thus improving its exploitation efficiency. In this way, by using a redistribution of the tributaries among the transmitted channels in an intermediate node, the transmission capacity is fully optimized. A similar operation can be performed by returning the channels at the packet network asking to redistribute them among already existing channels. The grooming at the packet level is clearly the most efficient, but it requires cooperation between packet and transport network. Several methods are used to guarantee transmission quality at the transport level. Errors are corrected using Forward Error Correcting Codes (FEC) while channels lost after a fail- ure can be recovered either with protection or with restoration procedures. Due to the structure of the transport network and to the possible presence of restoration in the packet network, protection is by far the most used survivability technique. There is a fundamental difference between protection in the transport network and in the packet network. In the packet network, even if virtual circuit routing is used, the chan- nels are used simultaneously by different virtual circu