ion of the service channel in the OTN case and of the MS header in the SDH/SONET case and to their recreation at the output of the switch when the multiple sections are recreated. 7.7.3 Connection Survivability All the functionalities related to protection and restoration mechanisms are collected under this class of functionalities [57]. In particular, depending on the type of survivability mechanism implemented, the pro- tection switching can be operated by the switch or can be the responsibility of a separated card that could also be part of the transmission system. In any case, all the physical layer procedures and protocols related to survivability are managed also by the switch. 7.7.4 Optical Cross Connect The OXC is the physical layer switch used in mesh OTNs; the layering structure of the functionalities of an OXC was already introduced describing the OTN standard in Chapter 3 and it is repeated here in Figure 7.30 for reference. OCh-OH OMS-OH OMS-OH OTS-OH OTS-OH FIGURE 7.30 OTN layered architecture of an OXC. Telecommunication Networks In the figure, an OXC directly connected with the WDM line is considered SO that multi- plexing and demultiplexing functions are performed by the OXC. The specific characteristic of the OXC is that the switching function is located at the OCh layer; this means that the OXC does not access the signals multiplexed into a wavelength (e.g., the LSPs, if the client layer is an MPLS network), but manage the wavelength channel as a whole. As a consequence, the OXC terminates both the optical transport section overhead (OTS-OH) and the optical multiple section overhead (OMS-OH), both encoded into the service channel. The OCh-OH is on the contrary transmitted transparently at the output. There are several different types of OXCs that can be classified considering the following elements: How the OXC is connected with the WDM transmission system The technology used to realize the OXC switch fabric The routing properties of the OXC switch fabric 7.7.4.1 OXCs with WDM or Gray Interface