## 6 Conclusions  
In this paper we take a TE approach which substantially reduces contention using only topological information. This approach uses static routing and has benefits in both network architecture, and network QoS.  
Routing is preferentially done online and dynamically, based on \"current\" network resource availability. However, to support dynamic routing, the network may need to be flooded by frequent resource update messages, and additional hardware and software may need to be deployed at each OBS node hampering scalability. Pre-planned static routing can avoid both such signaling overhead and more complicated architecture condition. In addition, optimization techniques can be applied in pre-planned routing such that contention can be minimized, or substantially reduced, from a global point of view.  
Considering the architecture, several advantages of our approach deserve to be highlighted: no extra hardware or software components are required on the core nodes for dynamic routing, no network flooding with signaling messages resulting from (over)active link state update protocols, no place for out-of-order arrivals, which are a disadvantage of some dynamic contention resolution schemes, also found on multipath routing schemes, typically requiring large memories at the edge nodes for re-ordering operations. These are important characteristics that permit to keep simple and cost-effective the architecture of the core nodes.  
At this level, the service to be taken into account is the delivery of bursts to the correct destination and the quality of this service is described by its performance in terms of burst loss, _i.e._, the lesser the better. We demonstrate that burst loss can be minimized by appropriately choosing the paths that bursts must follow. That is, an effective choice of paths can lead to an overall QoS improvement for the network. The resolution of an ILP problem is involved in the process, which makes this approach mainly tailored for offline static routing. However, since eligible paths can be provided as input for the strategies, the solutions can be promptly reached. Taking into account the computation times involved (which have been found in a range of seconds for some instances presented in Table 2), the relatively infrequent update requests expected from changes in the OBS backbones whose topologies typically last for long time scales, and the quasi-stationary aggregate traffic demands at optical backbones, which are expected to change relatively slowly [15], this approach can be considered feasible for the real production of OBS networks by means of an operation process to be executed during the initial setup phase of the network.  
Our results also show that the achieved performance improvement depends on the physical connectivity of the network, with more highly connected networks showing better performance. This happens because the proposed algorithms take advantage of more, short, alternative paths. Our strategies can provide an initial stage of improved performance, measured in terms of burst loss reduction, without incurring state dissemination protocol penalties. The routes obtained can be used alone, as single-path static routes to provide load-balancing without the need for additional control messages with regard to link status, or combined with other dynamic contention resolution schemes (deflection or segmentation, for example) and used occasionally as a default routing to assume whenever the network needs to recover from instability, favoring network resilience. Considering that any service performance at higher level is directly affected by the network performance, this strategies can also serve as a basis for the development of QoS policies.