Cross-layer Optimization: Considering Physical Impairments While Routing

Often, in networks it is not enough to consider the available resources, but it is also necessary to consider the impairments that affect the signal quality at the physical layer and cause increased Bit Error Rate for services. This is a kind of cross-layer optimization, where the services are optimized with physical layer constraints.

The first use of grooming to repair the impaired signal was presented in [88]

where such VPNs were configured, where the signal quality was satisfactory since the physical impairments were considered. In [120] the results were extended for routing in general. [107, 119] present deeper results on the same topic, while [46]

gives an overview of the problem, and proposes an additional method for improving the signal quality by increasing the power level of signals that have to go far while decreasing the power levels of signals that go to closer destinations in order to avoid the harmful effect of nonlinear distortions.

2.6 Conclusion

The objective of this chapter was to present an overview of traffic grooming in connection-oriented networks (mainly in WDM networks) and the wide variety of mathematical tools used to address this issue. Traffic grooming refers to techniques used for an efficient sharing of the bandwitdh offers by, e.g., a wavelength, using Time Division Multiplexing. It is usually associated with the routing of the requests and the survivability issue in single or multiple failure scenarios. Furthermore, traf-fic requests might be uni- or multicast, the traftraf-fic pattern may evolve with time, and the network could be multilayer. Therefore, traffic grooming is only part of the con-cerns addressed when designing and optimizing a network. But even when restricted to simple physical topologies (unidirectional path or ring) where the routing is fixed and with small grooming factor, the traffic grooming problem is difficult to solve and to approximate. Also, when all aspects have to be taken into account (traffic grooming, routing, survivability, and so on), problems to solve are so difficult that exact solutions are usually no longer expected, and it is essential to develop effi-cient heuristic algorithms. Some of them were presented in Section 2.5. Chapter 3 presents the state-of-the-art regarding exact approaches for this problem.

In this research area, several important questions are still open and further re-search are needed. In particular, when optimizing only the number of ADMs in SONET/SDH networks, practical values of the grooming factor are 3 and 4, but this is reapeated several time from the slower 55 Mbit/s streams to the current 10 Gbit/s wavelengths. So, it is important to develop efficient optimization tools for grooming factors 3 and 4, but also to consider hierarchical problems in which unitary requests are combined by four into streams that are themselves combined by four, and so on.

In the general context, where traffic grooming is associated with routing and sur-vivability issues, existing heuristic algorithms provide upper bounds without

guar-antee on the quality of the solution. Furthermore, the size of practical problems is too huge for existing mathematical tools. Therefore, research effort has to be put into the development of new mathematical tools allowing us to address large instances and to obtain optimal or near-optimal solutions.

Acknowledgements This work was partially supported by EU COST action 293 – Graphs and Algorithms in Communication Networks (GRAAL), COST action 291 – Towards Digital Optical Networks, and IST FET AEOLUS.

The authors would like to thank specifically European Action COST 293 GRAAL, which of-fered us a fruitful scientific framework to share ideas and expertise with other researchers.

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