11.4 Raman amplification in WDM networks 323 An alternative to the above described adaptive PMD compensation schemes and their problem of dynamic transient response is the choice of modulation formats that lower the baud rate by multilevel coding and thus enhance the receiver robustness against PMD. In some Western European countries the DQPSK modulation format has become the primary choice for 40Gbit/s transmission, as its dual-level modulation reduces the baud rate by a factor of two. This enables doubling of the link distances. For transmission at 100 Gbit/s, other modulation formats are under consideration since DQPSK is not sufficient. One option is DSP- based coherent PM-QPSK which is very robust against PMD. Modern DSP tech- nology is evolving to reach the calculation speed needed for mitigating the fast DGD transients. In practical networks, even if a system cannot compensate for all impair- ments due to PMD, a certain residual outage probability may be acceptable. Service providers generally have to guarantee a certain QoS to their customers. This has a direct impact on the required overall survivability design of all OSI layers of the network and the physical layer including layer L1 resilient archi- tectures. So, it is important to know the outage performance of certain systems on a given fiber infrastructure. Deutsche Telekom and NTT have undertaken