Routing Strategy

Routing was carried out along the shortest path, the metric being distance (in km).

Candidate shortest paths that did not have enough capacity for the arriving demand were filtered out. Node-disjoint working and protection paths were chosen. Block-ing resulted when no path could be found to satisfy a demand with a given protec-tion scheme. Dijkstra’s algorithm was used to find shortest paths, and a two-step approach was applied to select protection paths.

5.4.4.5 Results

Table 5.4 presents the average connection availability under both DPP and SPP. In the case of continental-size network topologies (Cost266, KL, Janos-US-CA, and NSFnet), the average availability values are very different compared to the small or medium size networks, where no differences can be observed, independently of the application of DPP or SPP. In the case of SPP, however, the worst-performing one (Janos-US-CA) has more than six times less availability than the best (KL) one. Nonetheless, these larger topologies can only achieve a maximum of “three nines” of availability, even when DPP is applied. On the other hand, results show that the dominant component in the network availability model is the fiber optic cable, due to the frequency of cable cuts and the relatively long duration of repair times.

Therefore, connection availability is dependent on the lengths of links, or in general terms, on the link length distribution of its network topology. Figure 5.7 shows how the mean downtime of connections in two of the studied topologies (Germany50 and COST266) increases when the link lengths are multiplied by a given factor;

Figure 7(a) shows that the average downtime increases linearly when connections are unprotected, with different slopes for different topologies. When protection is applied, however, the degradation rate is lower (see Figure 7(b)). With respect to the restoration overbuild, it can be seen in Table 5.4 that DPP requires almost two times the working capacity for protection, while DPP requires approximately the same capacity for both working and protection paths.

Table 5.4 Average availability and average restoration overbuild under DPP and SPP Figure of Merit DfnGwin Germany50 Cost266 KL Janos-US-CA NSFnet DPP - Conn. Availability 0.999983 0.999985 0.999790 0.999838 0.999723 0.999620 DPP - Restoration Overbuild 2.45 2.78 2.70 2.65 2.73 2.96 SPP - Conn. Availability 0.999932 0.999884 0.998272 0.999483 0.996525 0.998842 SPP - Restoration overbuild 1.69 1.85 2.02 2.24 2.06 2.30

Another interesting result concerns the average minimum distance, evaluated in hops between node pairs. For topologies with a small average minimum distance, a short total hop count can be expected in both DPP and SPP, yielding a suitable availability value. However, other factors such as link length distribution and shar-ing rules can modify the expected values. It has also been observed in SPP that the sharing group size can change the expected availability values if only topology fea-tures are considered. With respect to average node degree, it can be noted that under DPP it improves restoration overbuild because it helps in finding disjoint paths for backup.

(a) Unprotected connections

(b) Protected with DPP and SPP

Fig. 5.7 Average downtime as a function of scaled link length

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Routing Optimization in Optical Burst