circuit switching of OChs and protocols to control automatic protection. Moreover, synchronous encapsulation of asynchronous signals is also implemented, in order to allocate into the OTN every pos- sible client. The OCh encapsulation is divided into several steps, each having its own envelope. In the first step, the client signal is mapped onto the optical path unit (OPU) with the addition of an OPU overhead. The OPU is then enveloped into the optical data unit (ODU). The ODU is equivalent to the VC in SDH or the VT in SONET. It is the elementary frame of the optical path that connects OXC client ports after the OCh header is terminated thus managing electrical connection between clients via the OTN. All information relative to optical path end-to-end routing between OXC client ports and, in case it is applied, optical path protection having as end ports the client ports is coded into the ODU header. Different ODUs are multiplexed into the ODU TMC. This is essentially a multiplexing stage, allowing different tributaries to be transmitted together into the same OCh. As a matter of fact, a single OCh can have a transmission capacity much higher than the capac- ity of an ODU (lets think, e.g., of a 40 Gbit/s OCh) and in order to better exploit an OCh it is convenient to define a multiplex stage allowing multiplexing of different client channels onto the OCh. Finally, the ODU TMC is enveloped into the OCh frame adding a header with all the information on the end-to-end OCh transmission between the optical terminations of the OCh inside the OTN and a tail with the forward error correcting code (FEC) redundancy. It is important not to confuse the OCh with the optical path. The OCh is an all optical channel, transmitted on a chain of wavelengths (or on a specific wavelength if wavelength conversion is not present in the network) between optical interfaces into the OTN. The optical path is terminated, possibly in an electrical manner, on the client interfaces of the OXCs or the optical add drop multiplexers (OADMs). Jus