 sufficiently slow enough to modulate the excited ion population into the amplifier, thus causing intermodulation between the pilot tones of different DWDM channels traversing the amplifier. Using pilot tones, the presence of the wavelength channel along the correct route in the network can be monitored even at an EDFA site, with the monitoring system shown in Figure 10.25. An unbalanced tap coupler (e.g., a 1:10 coupler) can split a small amount of signal and send it to an electrical spectrum analyzer. The spectrum analyzer is set to filter out the Transmission fiber Isolator Isolator Single Filter Single flattening stage stage filter Receiver Spectrum analyzer FIGURE 10.25 Scheme of the monitoring system that can be used to detect the transit of the correct WDM channel through a line site using pilot tones. Telecommunication Networks data carrying modulation and to retain the pilot tone from which the channel ID and the channel power can be deduced. Pilot tones technique has the main advantages of being simple and inducing a little penalty on the signal. Disadvantages are the impossibility of monitoring the quality of the signal or including in the header attached to the signal other information as the QoS class, the payload, and SO on. Due to this last characteristic, when the pilot tone technique is used, an OCh header is also included into the service channel to carry all the information that is not supported by pilot tones. 10.3.3.2 Low Frequency Subcarrier Modulated Data Channel This technique relies on the fact that the header occupies a much less bandwidth with respect to the data; thus, if it is transmitted somehow in parallel to data, the bit interval is much longer. This fact is exploited by modulating the header data onto a low frequency carrier and then the resulting analog signal onto the wavelength channel signal. Generally, phase or frequency modulation is selected to avoid the sensitivity penalty implied by intensity modulation. The available bandwidth for this signaling method depends on the wa