 edge machine where the TDM frame is decoded and the error is detected. At that point, however, the OCh arriving at the wrong edge could be switched in the wrong direction in any of the traversed OXCs and the error would be much more difficult to detect. In principle OCh header could be coded into a TDM frame with the data over the wave- length carrier. In every node where the OCh header has to be read or regenerated, part of the signal is split and sent to a receiver to extract the OCh header from the frame. This method is, however, not practical, since it would require a receiver at the data speed for each channel to be monitored in every monitoring point. In order to be feasible, a solution for the OCh header coding has to require either analog narrow band components, or a low bit rate receiver. Several techniques have been proposed to carry the OCh header with the wavelength data, including Pilot tones [37,38] Low-frequency subcarrier modulated data channels [39] Optical code division multiplexed data channels [40,41] 10.3.3.1 Pilot Tones This technique is simple and targets the association to the wavelength channel only of an identifier. The association is realized by modulating the amplitude of the data carrying signal with a slow carrier whose frequency is exactly the channel ID. Thus, all the channels Next Generation Networking: Enabling Technologies, Architectures, and Performances FIGURE 10.24 Time behavior of the signal after pilot Time (A.U.) tone modulation. in the network are associated to a pilot tone frequency that allows each network element to recognize them. After modulation, the time shape of the signal looks like the plot of Figure 10.24. The modulation depth is generally between 1% and 10% to maintain the unavoidable SN penalty within 0.5 dB, moreover, when erbium doped fiber amplifiers (EDFAs) are present, in choosing the pilot tone frequencies, the long time response of EDFA is to be taken into account. As a matter of fact, if a pilot tone frequency is chosen below 50KHz, this starts to be