front of these types of platform failure. Telecommunication Networks 7.7 Circuit Switching in the Transport Layer The transport layer, either the OTN or the SDH/SONET layer, is characterized by long last- ing connections, called circuits. Even with the evolution of the traffic generated by the dominance of data related com- munications, the duration of a transport layer connection is much longer that the duration of connections at upper layers, for example, MPLS LSPs. Thus, switching machines working at the transport layer have to switch rarely after remaining a long time in the same configuration: they are static switches. Both in SDH/SONET and in OTN standards (see references from Chapter 3) the con- cept of switching at the multiplex section layer is defined as the operation carried out by a machine that accepts at the input a set of STM or OC signals in the TDM case or a set of WDM combs in the WDM case and routes them toward a defined output port without demultiplexing. In practice, however, this type of physical layer switches are rarely used in the SDH/ SONET case and never passed from labs to a product stage in the WDM case. The physical layer switches that are deployed in the field operate on paths and optical channels (i.e., tributaries of the TDM multiplexed frame or of the WDM comb), SO that the switching functionality is located at the OCh layer in the WDM case or at the path layer in the TDM case. In this chapter we will deal only with optical channel/path layer switches. The functionalities of a physical layer switch can be divided in three classes [26,55]: 1. Connection switching 2. Connections management 3. Connections survivability 7.7.1 Connection Switching This is the set of functions needed to transmit within the equipment the signal incoming from an input port toward the correct output port. Functions belonging to this class are as follows: Demultiplexing the incoming signal to separate the tributaries Separation of the service channels carrying control plane and management plane information