OPTOELECTRONICS LETTERS Vol.6 No.4, 1 July 2010
Two dynamic reconfiguration approaches for optimizing the restoration path length in p-cycle protection network Raghav Yadav1*, and Rama Shankar Yadav2 1. Computer Science and I.T., Sam Higginbottam Institute of Agriculture Technology & Sciences, Allahabad 211007, India 2. Computer Science and Engineering, Motilal Nehru National Institute of Technology, Allahabad 211004, India
(Received 25 March 2010) C Tianjin University of Technology and Springer-Verlag Berlin Heidelberg 2010 ƻ
p-cycle is one of the most promising technique of span protection in optical transport networks with mesh-like efficiency and ring-like speed. Longer p-cycle provides better efficiency in term of spare capacity, but longer restored path increases end-to-end propagation delay, which reduces the reliability of the restored network. Hence, minimization of restoration path is a critical issue in p-cycle based protection network. In this paper, two new dynamic reconfiguration approaches namely inter-cycles switching (ICS) and local restoration paths (LRP) are discussed to reduce the length of restored paths in existing optimal spare capacity design of p-cycle. Both proposed approaches are meant to utilize the idle p-cycles thus significantly reducing the path length. This reduction in restored path length also releases the redundant spare capacity. Document code: A Article ID:1673-1905(2010)04-0291-4 DOI 10.1007/s11801-010-0020-9
p-cycle is an efficient approach for protecting working capacities in optical transport networks (WDM networks), because it has short restoration time like the ring protection (50-150 ms) and a highly efficient capacity like the mesh protection. p-cycle is a ring-like pre-configured structure constructed from the spare capacity available in the network, and it occupies a unit of spare capacity on each of the cycle spans[1-3]. In recent years, various advanced issues in the p-cycle have been reported in literatures. For the optimal solutions of p-cycles based protection, the major issue is the path length of the p-cycle restoring the failed span. To achieve the optimal restoration path by means of p-cycle, many methods have been developed[4-7]. Further “hop limit” and “circumference limit” designs[8,9] have been proposed to limit the restored path length of the p-cycle. However, these two constraints increase the redundancy of spare capacity during the design of p-cycle for the single failure survivability of the network. Hence, it is necessary to identify some other techniques to reduce the length of restoration path in optimal spare capacity design of p-cycle. Based on the existing p-cycle network design theory, including the inter-cycles switching (ICS) of p-cycles and the local restoration paths (LRP), a solution is proposed to find a trade-off between two competing goals: realizing efficient *
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use of spare capacity and reducing the restored path length. The proposed solution is an enhanced trade-off between resource utilization and reducing the restored path length compared with existing solutions. This study explores p-cycle network design by reducing the length of end-to-end light paths in the restored network state, when a span failure occurs. The two approaches encompass the tracing out and the utilization of those p-cycles which do not pass over both end nodes of a failed span, i.e. not play any role in the survivability of current failed span. Hence, such p-cycles are termed as idle. From analysis it is found that only 50 to 60 % p-cycles are involved in the survivability of the working paths passing over the current failed span and the left over p-cycles are idle. In this approach the strategy is located for a p-cycle, which is of shorter length and presently idle. Such an idle p-cycle can switch the longer p-cycle crossing the failed span with the shorter protection path-segment in order to reduce the length of the restoration path-segment within the p-cycles available to protect that span. , Let s illustrate the ICS approach with the hypothetical network shown in Fig.1(a). In order to protect all the working traffic of the network, each must be looked out by a number of p-cycles constructed from the spare capacity of the network. For simplicity, we consider only three p-cycles la-
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beled as candidate p-cycle and idle p-cycles in Fig.1(a). Let the working path from external nodes X to Y pass the internal nodes 1-4-5-8 and the failed span upstream and downstream nodes are 1 and 4 respectively in Fig.1(b). Traditionally, as shown in Fig.1(b) the failed span can be restored by the candidate p-cycle and the resultant restored path is X-1-0-2-6-7-9-10-11-12-8-5-3-4-5-8-Y. However, this longer p-cycle is in a so-called “on-cycle” relationship to the protected span, and provides one protection path-segment for the span in the event of failure. Let there be any two nodes common between the restoration paths of the candidate pcycle and the other available idle p-cycle. It is assumed that protection segment-A is the part of candidate p-cycle between the two common nodes. Similarly, protection segment-B is the part of idle p-cycle between the same common nodes. If the length of protection segment-B is shorter than that of segment-A, segment-B can be utilized as the part of restoration
path against segment-A. This will result in reduction of overall length of the restoration path. In Fig.1(c), node-6 and node4 are common between candidate p-cycle and idle p-cycle. The length of protection segment-A (6-7-9-10-11-12-8-5-34) is 9 hops and that of protection segment-B (4-6) is 1 hop. As we see, the length of segment-A is longer than that of segment-B. Consequently, the deployment of segment-B in place of segment-A in the restoration path will reduce the length of restored light path by 9-units. This approach optimizes the original restoration path X-1-0-2-6-7-9-10-11-128-5-3-4-5-8-Y to the modified restored light path X-1-0-2-64-5-8-Y. Due to the utilization of idle p-cycles in the restored path, the spare capacity will be efficiently utilized. If these longer restoration paths can be reduced, all the redundant spare capacity will be released. The released capacity can be used for routing of other protection paths. Such dynamic optimization of restoration path reduces propagation delay and
Fig.1(a) Candidate p-cycle and idle p-cycles for failed span between nodes 1-4; (b) Restored path with conventional pcycle; (c) Restored path with ICS; (d) Restored path with LRP
signal degradation. The reliability of the restored light path will be more due to reduction in the number of hops in the restored network state. , Whenever there is a span failure, the network s spare capacity can be utilized to do the protection job. It may be possible that the provided protection segment is of longer length. For the reduction of restored path length, the rest of the spare capacity can be utilized, which is total spare capacity minus the part that has been used for the span failure
restoration. Now this residual spare capacity, which is idle for the time being, can be utilized as local restoration paths (LRP) for reduction of path length in restoration. From the released spare capacity of the idle p-cycles, we set up new LRPs with shorter length for the survivability of failed span. It is found that the restoration path provided by LRP is always smaller than that offered by the traditional p-cycles. LRP, an on-demand local protection method, is used to reduce the length of longer restoration paths (RPs) using idle
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cycles. It re-establishes the LRPs between end-nodes of the failed span over the spare capacity initially allocated to idle p-cycles, by using any shortest path algorithm. The basic concept of LRP is shown in Fig.1(d). The spare capacity of idle p-cycles over the span , and is released. The shortest path algorithm uses this released spare capacity and finds a local path (1-2-6-4), which passes these nodes. Using this LRP in place of original RP will reduce the length of restored path, i.e. the original restoration path X-10-2-6-7-9-10-11-12-8-5-3-4-5-8-Y will be optimized by new restored path X-1-2-6-4-5-8-Y with reduction of 10 hops. The inter-cycles switching algorithm is used to reduce the length of the restoration paths available for the survivability of the traffics passing over the failed span i. ƽLet span i between pair of nodes F1 (upstream) and F2 (downstream). ƽUpstream node F1 executes the optimization of restoration path (ORP) with inter-cycle switching algorithm which is given as follows: 1. Identify the on-span & straddler p-cycles (OSP); 2. Identify those cycles which are not passing through both nodes F1 and F2, referred as idle cycles (IC); 3. for all on-span & straddler p-cycles osp in set OSP do if osp is on-span p-cycle then ORPL = length of the RP provided by p-cycle osp else ORPL=length of right RP of straddler p-cycle end Final length = ORPL for all idle p-cycles ic in set IC do Find the common switching nodes (s1 and s2) in cycle osp and ic if such common nodes exist then Find the length of the restoration path, lrp, with inter -cycle switching if lrp < Final length then Final length = lrp; Candidate idle cycle (cic) = ic Switching nodes fs1 =s1 and fs2 =s2 end end endfor if Final length < ORPL then Set idle cycle as marked Change the restoration length of the cycle osp in the database with final length Send the control signal through cycle osp for performing inter-cycle switching activity at common nodes fs1 and fs2
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endif endfor The LRP algorithm is used to re-establish new restoration path as the alternative to the available longer restoration path. Let failed span i between pair of nodes F1 (upstream) and F2 (downstream). 1. Identify the idle cycles; 2. Unleash capacity at each span initially engaged for idle cycles; 3. while local path exits do find local restoration path, lrp, between nodes F1 and F2 , using Dijkstra s algorithm over unleash capacity Mark unleash capacity of span used for establishing local path switch over the traffic of longer restoration path, rp, over lrp Unleash capacity at each span initially engaged for rp endwhile It is evident from our discussion that the ICS and LRP algorithms reduce the length of longer RPs. The test network in Fig.2 with 19 nodes, 28 spans and 2.95 average node degree (AND), is simulated to find out the reduced restored path.
Fig.2 Test network with working capacity and available number of idle p-cycles on each span
Each span is taken at a time from the set of all the spans of the test network considered as a failed one and available RPs, are determined as shown in Fig.2 along with the number of working paths passing over the span and available idle p-cycles. It is clear that a sufficient number of idle p-cycles are available at each span for ICS and LRP algorithms to reduce the length of longer calculated restored paths. In Fig.3, the graph plots the average hop counts required for restoration of working paths passing over span after failure against its span number for conventional restored path (CRP) p-cycle and proposed ICS approach. From Fig.3, it is observed that the average end-to-end restored light path length with ICS is between 3 and 8 hops, which is initially 4 to 11
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Fig.3 Lengths of the restored paths for conventional pcycle and ICS in test network
hops for traditional p-cycle. The proposed algorithms are simulated on different test networks (Net1, Net2 and Net3) with various properties. Net1 with 19 nodes, 28 spans, 2.95 AND, Net2 with 13 nodes, 23 spans, 3. 5 AND and Net3 with 10 nodes, 22 spans, 4.4 AND are taken for further verification of the algorithms. The reduction in the average restored path lengths is quite significant in all these networks. Fig.4 shows the variation between the working path lengths and end-to-end restored light paths simulated with CRP, LRP and ICS approaches. In all the threetest networks, it is clear that the length of average restored paths provided by conventional p-cycle is much greater than the average length of the working paths provided by LRP and ICS approaches, approximately three times. Both proposed algorithms obviously optimize the average length of the restored paths.
than the length of working path. We develop ICS and LRP approaches to utilize pre-existed p-cycles, which are idle in current network state, at the time of span failure. From the simulation, it is concluded that on any given span failure, there exist enough idle cycles (approximately 45%), which can be utilized to minimize the length of restoration paths offered by conventional p-cycle design. In ICS approach, we dynamically reconfigure the restoration path by switching the longer path over the idle p-cycles with shorter length, whereas in LRP approach the idle p-cycles are identified and the spare capacity used by them is released. From this released spare capacity, we re-establish the shortest local restoration path. In practice, the test results show that both proposed algorithms significantly reduce the restoration path length and the spare capacity requirements of WDM networks. The main contribution of this work is to address the idle p-cycles and estimate the net spare capacity existing in the current network state. Ultimately, this is the dimension in which the p-cycle network design can be dynamically reconfigured and managed to combine fully pre-connected (ring-like) speed along with the ability to match mesh network efficiencies, as long as the reduction of restoration path is potential. References [1] W. D. Grover and D. Stamatelakis, Proceedings of IEEE International Conference on Communications, 537 (1998). [2] W. D. Grover, Mesh-Based Survivable Networks: Options for Optical, MPLS, SONET and ATM Networking, Prentice-Hall, 2003. [3] R. Ramaswami and K. N. Sivarajan, Optical Networks: A Practical Perspective, Second Edition, Morgan Kaufmann, 667 (2002). [4] R. Yadav, R. S. Yadav and H. M. Singh, International Journal of Computer Sciences and Engineering Systems 1, 225 (2007). [5] D. A. Schupke, C. G. Gruber and A. Autenrieth, Proceedings of IEEE International Conference on Communications, 2761 (2002).
Fig.4 Effect on restored path lengths with CRP, LRP and ICS approaches
We have explored issues related to reducing the restoration path lengths in p-cycle network designs. It is shown that conventional method of restoration could lead to significant longer restored path as well as increasing in spare capacity requirements. The simulation accentuates our hypothesis through an initial example that the restoration path length of the p-cycle attained by conventional approach is much longer
[6] P. Cholda and A. Jaiszczyk, IEEE Global Telecommunications Conference 1, 5 (2005). [7] A. Grue, W. D. Grover, M. Clouqueur, D. A. Schupke, J. Doucette, B. Forst, D. Onguetou and D. Baloukov, Proceedings of the 6th International Workshop on Design and Reliable Communication Networks, 2007. [8] A. Kodian, A. Sack and W. D. Grover, Optical Switching Networking 2, 72 (2005). [9] Adil Kodin, Anthony Sack and Wayne D. Grover, IEEE Computer Society, 2004.
