IEEE COMMUNICATIONS LETTERS, VOL. 11, NO. 2, FEBRUARY 2007 201 Second Phase Reconfiguration of Restored Path for Removal of Loop Back in P-Cycle Protection Rachna Asthana, Student Member, IEEE, and Y. N. Singh, Senior Member, IEEE Abstract— P-Cycles are one of the most promising techniques used for span protection in optical networks. However when p-cycle is used to provide protection to any failed span, there may be overlapping of some nodes in the working path and in the restoration path provided by the p-cycle. These nodes are repeated in the restored path, hence redundant link capacity is used. These repetitions will form loops at the overlapping nodes. To remove these loops and release the redundant capacity, an algorithm is developed to reconfigure the restored path. This aspect of the p-cycle has not been discussed so far in the literature. The capacities required without reconfiguration and after reconfiguration have been compared. It has been found that with reconfiguration, considerable amount of capacity can be released, and the restored path length can also be reduced significantly. Index Terms— P-Cycles, path restoration, reconfiguration of restored path, capacity saving. I. I NTRODUCTION I N the field of optical network protection and restoration, p- cycles have gained tremendous momentum in recent years, due to their ability to provide ring like speed and mesh like efficiency and flexibility [1]-[2]. P-Cycles, a technique for span protection, are preplanned and fully pre-connected closed loop structures of spare capacity; hence real-time switching actions are required only at the two end nodes of the failed span. P-Cycles provide protection to on-cycle spans and unlike rings, they provide protection to straddling spans (chords of the p-cycle) also. Hence, their efficiency can be as good as the efficiency of mesh networks [2]. Since p-cycles are formed only in the spare capacity, routing of primary paths is not affected. Therefore the flexibility of mesh networks is retained with p-cycles. In recent years, a lot of work has been reported in literature [3] on various issues in p-cycles. To achieve the theoretical efficiency of mesh networks, extensive work has been done with Hamiltonian p-cycles [4]. Strategies have been proposed to provide dual or multi failure network survivability [5] with re-configurable or shared p-cycles. To obtain the optimal solution for the configuration of p-cycle, many heuristics have been developed [6]-[7]. Recently the span protection technique of p-cycle has been extended to provide node [8] and path protection [9] also. Manuscript received October 7, 2006. The associate editor coordinating the review of this letter and approving it for publication was Prof. Maode Ma. This work was supported in part by the Department of Science and Technology, Govt. of India under Fast Track Scheme for Young Scientist vide Grant No SR/FTP/ETA-31/2003. R. Asthana is with Harcourt Butler Technological Institute, Kanpur, UP, 208002 India (email: rachnas@iitk.ac.in). Y. N. Singh is with the Electrical Engineering Department, Indian Institute of Technology Kanpur, UP 208016 India (email: ynsingh@iitk.ac.in). Digital Object Identifier 10.1109/LCOMM.2007.061635. Fig. 1. Primary path (sequence of arrows), restored path (dashed line), and final path after removal of loop back (thick line). However, the loop back in the restored path of p-cycle has not been discussed so far in the literature. In the next section, we explain the issue and in Section III, the mathematical model [10] and technique to remove the loop back is given [11]. Section IV discusses the simulations and results and finally, conclusions are given in Section V. II. LOOP BACK IN RESTORED PATH Consider the hypothetical networks shown in Fig. 1. AB..D..FA is a p-cycle formed in spare capacity in Fig. 1(a). (Although we are showing the network connections in one direction, in reality, the same will exist for reverse direction also.) Let us consider the failure of on-cycle span AF. A working path XBAFY (shown by sequence of arrows), from source ‘X’ to destination ‘Y’ and passing through the failed span, will now be restored by the p-cycle AB..D..F. The restored path will now be XBA-B..D..F-Y(shown by dashed line). If any node is common between working path and the p- cycle, except the end nodes of the failed link, then the restored path will pass through that node twice. This is called loop back in the present work. In the restored path of Fig. 1(a) the node B is visited twice; hence there is a loop back at node B. If the number of common nodes in working path and the path provided by p-cycle is more, then there will be more loop backs. Refer to Fig. 1(b), the working path is DGFAKJL. In the event of failure of AK, the restoration path, provided by the p-cycle (A..DEFGH..IJK), is DGFA-DEFGHIJ-KJL (shown by dashed line), and common nodes are D, F, G, and J (A, and K are the end nodes of the failed link). There are loop backs at all the common nodes. Due to the loop backs in the restored path, extra capacity will be used. If these loop backs can be removed from the restored path then all the redundant capacity will be released. The released capacity can be used for routing of other working paths or for protection. The removal of loop backs will also reduce the number of hops and the restored path length. Hence propagation delay and signal degradation 1089-7798/07$20.00 c  2007 IEEE