gure) and since the signals at different wavelengths travel together in the same fiber link, the higher attenuation channel practically deter- mines the link reach. This is the reason why several times CWDM systems are realized selecting only eight channels to be multiplexed on the first eight frequencies of the standard. The situation completely changes if no water peak fibers are used (whose attenuation curve is also reported in the figure). In this case, all the standard CWDM channels can be used. Last, but not least, the optical transmission section (OTS), carries out physical transmis- sion of signal through optical fibers. The structure of a trail along the optical layer and its model are represented in detail in Figure 3.36. Each layer has a signaling protocol; OCh protocol is embedded in the frame created by the adaptation function that create the OCh from the client signal, the OMS Protocol and the Optical Transmission Section Protocol are carried by the so-called service channel, a dedicated communication channel on a frequency on the edge of the useful spectrum. Telecommunication Networks F LEM F_OLA FIGURE 3.36 Layered model of a trail through an OTN: OXC, optical cross connects; OLA, optical line amplifiers; LEM, line extension module, that is, the electronic regenerator. The structure of progressive client signal encapsulation needed to create the signal to be transmitted in the OTN is represented in Figure 3.37. From Figure 3.37, it is clear that the process of producing the OCh frame from the client signal is quite complicated, taking into account all that is needed both to envelope differ- ent types of signals and to transmit all the needed network protocols. As a matter of fact, SONET, SDH, Null, PRBS,. Encapsulation Client signal Client Client signal ODU TCM TCM OH level ODU (including TCM) OCh OH OMS OH OTS OH FIGURE 3.37 Progressive encapsulation of a client signal into the OTN frame. Networks Fundamentals and Present Architectures the OTN foresees both protocols to control the control plane 