Progress Report No. 13. P-Cycles and Quality of Recovery

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1 NEXT GENERATION NETWORK (NGN) AVAILABILITY & RESILIENCE RESEARCH Progress Report No. 13 P-Cycles and Quality of Recovery The University of Canterbury Team 12 April 2006

2 Abstract Since the p-cycle resilience technique was proposed for the first time in 1998 by Grover, many supportive papers concerning the topic have been published. However, some researchers argue against the efficiency and theoretical reliability properties of this recovery techniques.in this report, we intend to open a discussion on the applicability of p-cycles to this NGN project as well as to introduce a new concept of Quality of Recovery (QoR), which is an indicator helpful in planning of recovery schemes e.g., global path, link, p-cycles recovery. The QoR is a trade-off function, which can be used to balance the cost-effectiveness and reliability performance in network recovery design. 1. Brief Introduction to P-Cycles The technique of p-cycles is a relatively new approach to survivable network architectures, first introduced in [1]. The name has been coined out of preconfigured protection cycle. Basically, a p-cycle is a sequence of capacity units on links that make a cycle in the network topology. The objective of the p-cycle concept is to obtain the restoration speed of self-healing rings (which use 1+1 protection) while simultaneously retaining the total capacity reduction offered by the shared protection paradigm, which is called ring-like speed with mesh-like capacity resilience approach. Figure 1 Example of a p-cycle The idea behind a p-cycle is simple but it ensures potentially cost-effective resilience. A p-cycle consists only of spare resources in some spans forming a closed path in a network, Figure 1(a). There are two types of span whose failures are recovered by a p- cycle: on-cycle and straddling ones. In an on-cycle span, the protected (working) capacity is co-located with spare capacity which forms the p-cycle. This is not the case in a straddling span. If an on-cycle span fails, the working capacity is recovered due to the usage of the spare capacity in a p-cycle, Figure 1(b). In the case of a straddling span failure, the situation is similar, but then two possible recovery paths are present, Figure 1(c). Due to this fact, a p-cycle with the capacity of units in each of its spans, can recover up to 2 units of capacity in the straddling span. This C p is the greatest advantage of the method. Additionally, recovery by p-cycles is fast due to the local character of protection switching. The planning, operation and other C p

3 details related to p-cycles are exhaustively described in [2]. Networks characterized by a high average nodal degrees are the ones in which p-cycles are most effective. Therefore, p-cycles are best suited to mesh topologies. In addition, finding p-cycles is an NP-hard combinatorial problem for which heuristics may approach optimal solutions for small networks Grover obtained optimal solutions for networks in the order of 20 nodes and 30 links. 2 Arguments against p-cycles 2.1 The reliability of p-cycles is contested The advocates of p-cycles eagerly stress their merits over protection rings. For example, [2] enumerates the advantages from the standpoint of: modularity, protection yield, protection flexibility, routing and provisioning of working paths, network redundancy and average length of protection paths. However, some researchers have different opinions and they pay heed to the one feature that is not mentioned above, i.e., reliability. They agree that cost-effectiveness should be one of the objectives of the network design, but they emphasize the fact that in the case of recovery planning the first goal must be reliability because this is the main objective of such an activity. In [3], the authors derived theoretical formulas which describe the reliability functions for traditional rings such as UPSR, BLSR and p-cycles and also present numerical examples which prove that, from the reliability point of view, the superiority of p-cycles can be contested. They mention that this very promising new technique, p-cycles, often compared to traditional rings and considered as better, can be assessed as inferior when the most important factor, i.e., the reliability, is taken into account. It presents the derivation of the formulas describing the all-terminal reliability (which is defined as the probability that all network nodes are connected for a given period of time) and 2- terminal reliability (which is the probability calculated for the selected source node and termination node that are connected) and the numerical examples prove that the superiority of p-cycles over protection rings is doubtful. See Figure 2 below, which is Figure 5 in [3], where N is the number of Figure 2 All-terminal availability of protection rings and p-cycles

4 nodes in a ring/p-cycle, and L is the number of straddling spans in a p-cycle. It can be seen that BLSR yields large resilience, while UPSR and p-cycle are much worse. Additionally, p-cycle is far less reliable than USPR. Obviously, the availability decreases with an increasing number of nodes and straddling spans. Moreover, the simulation results presented in [3] also confirm the theoretical studies which say that it is more reasonable to use p-cycles in smaller networks. For example, a three 9 s level of link availability means that the fiber is approximately 100km long. Such a short length of a span is obviously possible, but in metropolitan rather than in wide-area networks. This suggests that p-cycles could be a good solution for MANs, but in long haul networks, the advantages of traditional shared protection related to availability performance exceed the advantages of p-cycles related to sharing of spare resources. In addition, consideration of node reliability would make the reliability functions for p-cycles far more inferior because this technique is not well suited for the treatment of node failures. The greatest advantage of p-cycles (the existence of straddling spans) appears to be also their greatest disadvantage: secondary failures are more likely to occur, since the more the number of spans, the greater the probability of faults. As p- cycles are designed to be resilient to single failures only, secondary failures incur faults which cannot be recovered. There is also a summary on the disadvantage of p-cycle recovery in [4], which includes: 1) It is hard for a cycle to protect a node failure and multiple link failures; 2) Usually the path length in a cycle increases with the network size and it is expensive to reroute the traffic with a long protection path; 3) It is not easy to implement the distributed algorithm of the cycle-based protection schemes. 2.2 The spare capacity allocation efficiency of p-cycle is doubtful Recently, the report [5] presented some interesting results on the aspect of spare capacity allocation design regarding different protection and restoration schemes, including the p-cycle. They mention that the distinction between path restoration and link restoration is that path restoration uses alternative routes from the origins to the destinations of the demand pairs affected by the failed link rather than simply taking a detour around it. Notice that p-cycles reroute all of the traffic affected by a link failure on one or two paths around the link whereas a path restoration scheme may distribute the rerouted traffic over a larger number of backup paths. Thus, path restoration will generally require less total spare capacity than either link or p-cycle restoration as shown in Table 1 (which is Table 2 in [5]).

5 Table 1: An example network with 6 node and 9 links We can see that when the path restoration version of the spare capacity allocation model is applied to the example problem, the total spare capacity needed was only 95 units compared to 140 and 110 units for p-cycle and link restoration, respectively. Table 2: Another example network with 18 node and 35 links

6 Another larger example network, for which the computational results are given in Table 2, also illustrates this point. The p-cycle model was the most difficult to solve taking approximately 10 seconds of CPU time. More details about this computation scenario can be found on page 18 of [5]. We assert that when dealing with network recovery, the assessment of the method should take into account not only cost-effectiveness but also reliability performance or vice visa. It is regrettable that among factors usually used for the purposes of assessment of the quality of recovery (recovery time, capacity sharing), indicators aiming to evaluate the influence of sharing on the reliability appear so rarely. Therefore, here we introduce a new concept, named Quality of Recovery (QoR), for possible use in this NGN project. 3 Quality of recovery A Trade-off function As an indicator helpful in planning of p-cycles, as well as others recovery schemes, the paper [6] proposes the use of a combined index comprising two subsequent factors. One is Redundancy R as an indicator of the extension of capacity sharing; carrier is interested in the lowest value possible value of R. The other is availability A as an indicator of the resilience to failure; a carrier is interested in the highest possible value of A. As the two factors have inverse desirabilities, a simple Quality of Recovery function can be defined as below: k A QoR = R Generally, the choice of k or QoR itself has many degrees of freedom which should be chosen by an operator, possibly on the basis of some technical and economical analysis. The detailed ideas related to a broader framework related to QoR can be found in [6]. Further study on QoR is needed if Telecom were to accept the applicability of this concept to the NGN project. Conclusions We have opened a discussion on the applicability of p-cycle at the stage of preparation of network recovery procedures for the NGN project. The speed of the method and cost-effectiveness related to the redundancy may be insufficient to make it a satisfactory choice. Although P- Cycles can be very attractive in certain respects, they appear to be sufficiently reliable only for quite large values of fiber availability. This would restrict their usage to metropolitan area networks. Also the efficiency with respect to spare capacity allocation is also doubtful in comparison with other recovery schemes. We also suggested an appropriate function, i.e., QoR, for making decisions on balancing the recovery factors e.g., capacity sharing, recovery time and reliability that could be applied in the planning stage of this NGN project.

7 References [1] W. D. Grover, D. Stamatelakis: Cycle-Oriented Distributed Pre-configuration: Ring-Like Speed with Mesh-Like Capacity for Self-Planning Network Restoration, in Proc. International Conference on Communications (ICC), Vol. 1, Atlanta, June 7-11, 1998, pp [2] W. Grover, Mesh-Based Survivable Networks. Options and Strategies for Optical, MPLS, SONET, and ATM Networks. Upper Saddle River, NJ: Prentice Hall PTR, [3] P. Chołda and A. Jajszczyk, Reliability Assessment of Rings and p-cycles in DWDM Networks, in Proc. NGI2005 1st Conference on Next Generation Internet Networks Traffic Engineering, Rome, Italy, Apr , 2005 [4] Yuna Zhang, Oliver W.W.Yang, Different Implementations of Token Tree Algorithm for DWDM Network Protection/Restoration, Computer Communications and Networks, Proceedings. Eleventh International Conference on Oct Page(s): [5] Basic Mathematical Programming Models for Capacity Allocation in Mesh-based Survivable Networks, Jeffery Kennington, Eli V. Olinick, and Georghe Spiride, Technical Report, July 2005 [6] P. Chołda, A. Jajszczyk, and K. Wajda, A Unified Framework for the Assessment of Recovery Procedures, in Proc. IEEE 2005Workshop on High Performance Switching and Routing (HPSR 2005), Hong Kong, China, May 12-14, 2005.