218 CHAPTER 7 Spectral power fluctuations in DWDM networks etc., depend on modulation format, Forward error correction (FEC) type, and the receiver type (PIN or APD) and are therefore different for any transmission system. Signals at higher data rate require better receiver OSNR and therefore are more impacted by the power excursions for the same transmission distance. 7.7 SUMMARY AND CONCLUSION Failure events in transparent optical DWDM networks can cause sudden changes of network load conditions. While EDFAs with fast AGC can efficiently suppress excursions in the total channel power, SHB in the amplifiers and SRS in the transmission fibers can lead to fast spectral power fluctuations which can cause BER burst or channel outages. The effects are nonlinear and their impact depends on span count, channel drop, and amplifier saturation level. The influence is low for typical metro/regional networks. Large networks with links consisting of more than eight spans can be severely affected. As long wavelength channels suffer from more degradation, a wavelength- dependent light path assignment can help minimize impairments. Also, a channel replacement circuit that maintains a specific spectral power profile can help. ACKNOWLEDGEMENTS The authors wish to thank Roman Hartung for his valuable assistance in carrying out the experimental studies. The work reported here has been funded in part by the European Union within the FP6 IST project NOBEL (contract number FP6-506760). ACRONYMS AGC APD BER DWDM EDFA FEC MD-RODAM NRZ O/E/O OSA OSNR OTN PIN ROADM Automatic gain control Avalanche photo diode Bit error ratio Dense wavelength-division multiplexing Erbium-doped fiber amplifier Forward error correction Multi-degree reconfigurable optical add/drop multiplexer Non-return to zero Optical/electrical/optical (conversion) Optical spectrum analyzer Optical signal-to-noise ratio Optical transport network p-insulator-n (photodiode) Reconfigurable optical add/drop multiplexer