Spare Capacity Dimensioning for different Re-Routing Strategies in Transport Networks

Bandwidth variable transponders (BVTs) have the capability to adjust the line-rates of lightpaths in optical transport networks. This feature is relevant for the restoration of IP over optical networks where the capacity of the IP links is provisioned by lightpaths. When a fibre cut occurs, the affected IP links are restored by re-routing the disrupted lightpaths over usually longer protection paths. In most cases, to adapt to the physical limitations of these paths, the BVTs that implement the affected lightpaths have to reduce their line-rates. This results in IP links with reduced capacities which may cause congestion in the IP layer. Today, due to the lack of information exchange between the IP and the optical control planes, the IP layer is not aware of the capacity reduction and cannot trigger automatic traffic re-routing so as to avoid congestion. Therefore, the policy applied today is to shut-off all IP links affected by a line-rate reduction. In this paper we study alternatives to this policy by investigating re-routing strategies in the IP layer that avoid shutting-off all IP links affected. The assumption is made that in future a software defined network (SDN) control plane enables coordination between the IP and optical layers. The efficiency of the strategies is assessed by calculating their spare capacity requirements, i.e. the extra capacity required in the optical layer to carry the IP traffic in case of single link fibre cuts. In our case study we consider a network scenario from Deutsche Telekom. The results show that the rerouting strategies lead to significant capacity savings compared to the traditional shut-off approach.