334 CHAPTER 11 Transport solutions for optically amplified networks 25 20 Mean = 72.9 km Max. = 109.0 km Min. = 30.0 km Count 15 10 5 0 95-100 30-35 35-40 40-45 45-50 50-55 55-60 60-65 65-70 70-75 75-80 80-85 85-90 90-95 105-110 110-115 >115 Amplifier section length [km] FIGURE 11.17 Histogram of the length distribution amplifier sections for a typical national German transport backbone network based on EDFA supported optically transparent DWDM links [15] amplifiers with sufficient spacing, there is no real demand for the introduction of low noise Raman amplifiers in such a network structure. Nevertheless, some of the emerging trends in different carrier network strategies might lead to new issues which could be addressed by hybrid EDFA/ Raman amplification. One of those trends is the implementation of ROADM based optically transparent meshed or ring structures. To deliver real benefits, any restrictions on optically transparent system reach should be minimal to enable flexible lightwave paths through the network, not only supporting the shortest paths connecting pairs of nodes but also allowing for detours which may occur in protection scenarios or due to limited capacity along the shortest route. At the same time the increase of line rates continues and with it the introduction of more complex modulation formats with more demanding OSNR requirements. Depending on the mix of modulation formats and symbol rates along the link, there might be increased OSNR require- ments or the need for solutions to mitigate impact from fiber nonlinearities. Another point could be increased span lengths in the case of hut-skipping, e.g., driven by the need to save OpEx from power or space. The potential for high data rate trans- mission over a transmission link with high-loss fiber sections was shown for a link typical of German fiber infrastructure within the scope of a field trial [16], [17]. This work reports the transmission of 170 Gbit/s over 185 km of field fiber plus 25 km of lumped SSMF, with a total loss of 61 dB, using Raman assisted transmission. 11.5 SUMMARY Telecommunication services are subject to fundamental changes, due to the growing displacement of circuit switched traffic by packet switched traffic for residential customers, and new broadband services. It is a challenge for the