he obvious differences between the func- tionalities of equipment working on wavelengths with respect to equipment working on timeframes [46,47]. The definitions of the three layers composing the OTN are given in Figure 3.33 with the nomenclature defined in this standard. In particular, the layers have the functions described in this section. 3.2.7.1 Optical Channel Layer This layer receives requests of transport from the client layer in the form of a TDM frame to transmit through the network. Starting from these requests, the layer creates the optical channels (OChs) that are the elementary structures transmitted by the optical network. In creating OChs starting from client requests, the OCh layer performs multiplexing and grooming of client signals and enveloping them into the OCh frame. Once the frame is created, the layer controls the OCh's transmission through the net- work performing routing and restoration, monitoring the transmission quality and report- ing warnings and alarms to the central management system. The OCh layer has several adaptation functions to accept as input various possible cli- ents such as SDH/SONET, Ethernet, IP, and SO on. 3.2.7.2 Optical Multiplex Section The optical multiplex section (OMS) is the layer below the OCh. Its task is to perform wave- length division multiplexing and demultiplexing and to transmit the WDM signal. Thus, its functions are as follows: multiplexing and demultiplexing, monitoring of the end-to-end WDM comb transmission quality, and a few networking functions related to the survivability of the whole DWDM signal and to its routing as a single element. Telecommunication Networks OCh-A OCh-A source OCh AP OCh AP OCh-TT OCh layer operates on optical OCh-TT source channels OCh TCP OCh TCP OMS-A OMS-A source OMS AP OMS AP OMS-TT OMS layer operates on WDM OMS-TT combs source OMS TCP OMS TCP OTS-A OTS-A source OTS AP OTS layer operates on optical OTS AP OTS-TT signals performing transmission OTS-TT source OTS TCP OTS TCP FIGURE 3.33 Scheme of the standard layers of t