Double-link failure models, in which any two links in the network fail in an arbitrary order, are becoming critical in survivable optical network designs. Optical networks that are over-dimensioned and purposely preplanned for full protection against any single-link failure may not survive well in an event of a double-link failure. It is therefore important to understand how much impact a subsequent second link failure may have upon traffic recovery of both failed links. This paper primarily aims to investigate traffic recoverability when a double-link failure occurs. We propose several methods for improving traffic recovery without using additional backup capacities other than the existing backup resources available for single-link failure recovery. In each proposed method, integer linear programming formulation is derived to optimize the backup resource allocation. Numerical results on various network configurations have shed some light on how network operators can choose one of five alternative methods to better handle a double-link failure. From numerical results, Method 1 offers some improvement on the traffic recoverability of the second failed link without disturbing the recovered traffic of the first failed link. Method 2 reveals that the traffic recoverability of the first failed link can be improved if the backup routes of the unrecovered traffic of the first failed link can be reassigned. Method 3 is useful for maximizing the traffic recoverability of the first failed link. Method 4 further increases the recoverability of the second failed link while maintaining the same recoverability of the first failed link as Method 3. Finally, Method 5 accomplishes the maximum overall traffic recoverability.

双链路故障模型对网络的生存能力至关重要，在这种模型中，网络中的任何两个链路都以任意顺序发生故障。为避免任何单链路故障而提供全面保护的超尺寸光网络，如果发生双链路故障，可能无法幸免。因此，重要的是要了解随后的第二条链路故障可能对两条故障链路的流量恢复产生多大影响。本文的主要目的是研究发生双链路故障时的流量可恢复性。除了可用于单链路故障恢复的现有备份资源以外，我们提出了几种无需使用额外备份容量即可提高流量恢复的方法。在每种建议的方法中，推导了整数线性规划公式，以优化备份资源分配。各种网络配置的数值结果为网络运营商如何选择五种替代方法之一更好地处理双链路故障提供了一些启示。从数值结果来看，方法1在不干扰第一条故障链路的已恢复流量的情况下，对第二条故障链路的业务可恢复性进行了一些改进。方法2显示，如果可以重新分配第一个故障链路的未恢复流量的备份路由，则可以提高第一个故障链路的流量可恢复性。方法3对于最大化第一个失败链路的流量可恢复性很有用。