Quantitative Comparison of End-to-End Availability of Service Paths in Ring and Mesh- Restorable Networks Matthieu Clouqueur, Wayne D. Grover National Fiber Optic Engineers Conference - NFOEC 2003 Orlando, Florida, USA

Matthieu Clouqueur and Wayne D. Grover NFOEC Outline Motivation Goals of Study Survivability Schemes Models Description of Test Cases Sample Results Discussion of Results Conclusion

Matthieu Clouqueur and Wayne D. Grover NFOEC Motivation Rings are often associated with high availability because they provide high restoration speed. Mesh is often thought to provide lower availability associated with low capacity redundancy and slower restoration. We ask: Which really provides higher availability: ring or mesh ? How do ring and mesh compare in service path availability and in the range of availability levels they can offer ? In mesh, what would be the effects of affecting priorities to selected services ?

Matthieu Clouqueur and Wayne D. Grover NFOEC Goals and Methodology Provide a true “apples-to-apples” comparison of end-to-end service path availability in ring and mesh: –Ring and Mesh are compared on identical facilities graphs serving identical demands –Efficient capacity designs are used for both architectures –Exact survivability mechanisms are emulated –Both architectures experience identical failure sequences Comparison is based on: –Average path unavailability (versus path length) –Average number of outages experienced per year per service path (versus path length) –Statistical frequency of total path outage times per year

Matthieu Clouqueur and Wayne D. Grover NFOEC Simulation Numerical Details Statistics of Failures: –Mean time between failures (MTBF): 1 year on each span –Negative exponential distribution (Poisson process) Statistics of Repair: –Mean time to repair (MTTR): 12 hours –Negative exponential distribution

Matthieu Clouqueur and Wayne D. Grover NFOEC Main Assumptions of the Availability Analysis Previous work shows that what dominates service unavailability is: –Reconfiguration times (to single failures) –Single node-failures –Dual span-failures –Triple failures The present analysis is based on the effects of dual span-failures –The contribution of reconfiguration times is not taken into account. The analysis is based on restorability investigations performed on a per path basis for 7each particular failure scenario occurring in the simulation

Matthieu Clouqueur and Wayne D. Grover NFOEC Modelling Ring Survivability The Bi-directional Line Switched Ring (BLSR) model is assumed: Protection channels Working channels Loop-back Note: A dual fibre cut affecting a ring does not cause all service paths on the ring to experience outage  Detailed inspection is performed on a per path basis to identify paths affected by the dual failure. surviving path failed path

Matthieu Clouqueur and Wayne D. Grover NFOEC Modelling Mesh Survivability Adaptive mesh restoration behaviour is assumed: –Restoration paths for a failed span are dynamically searched within remaining spare capacity (not according to pre-plan) Note: Restoration of a failed span includes an effort to restore any spare capacity used on that span  A restoration path affected by a second failure may survive failed backup path is itself restored Restoration to a first failure can be based on a pre-plan. Second failure response is adaptive

Matthieu Clouqueur and Wayne D. Grover NFOEC Mesh Restoration with Priorities Capacity designs used are identical to normal single failure restorable designs (no additional capacity) A certain percentage of demands is given “Priority” status on each origin/destination node pair –The Priority service paths are considered first for restoration and will therefore have a higher dual-failure restorability and therefore a higher availability Three service mixes: 10/90, 30/70, 50/50 (% high P. / % low P.) Questions: –How much is the availability of Priority services improved? –How much is the availability of non-Priority services degraded?

Matthieu Clouqueur and Wayne D. Grover NFOEC Three test cases (32 nodes, 45 edges)(25 nodes, 50 edges) Hubbed Demand Matrix Gravity-based Demand Matrix net32-Anet32-B25n50s1 Topology Type of Demand Test case

Matthieu Clouqueur and Wayne D. Grover NFOEC Study Details Mesh: –Working paths are routed on shortest path –Minimal spare capacity placed by Integer Linear Programming Optimization* –Average working path length: net32-A: 5.5; net32-B: 2.2; 25n50s1: 2.8 Ring: –Ring designs using efficient methods developed in PhD work by D. Morley** –# of rings in design: net32-A & net-32B: 8 OC-48; 25n50s1: 19 OC-48 –Average number of spans/ring: net32-A & net32-B: 11; 25n50s1: 9.4 –Working paths routed by shortest ring-constrained routing * J. Doucette, W. D. Grover, “Influence of modularity and economy-of-scale effects on design of mesh-restorable DWDM networks,” IEEE JSAC, vol. 18, no. 10, October 2000, pp ** G. D. Morley, Analysis and Design of Ring-Based Networks, Ph.D. Thesis, University of Alberta, Spring and G. D. Morley, W. D. Grover, “Tabu search optimization of optical ring transport networks,” in Proceedings of IEEE GLOBECOM 2001, November 2001, vol. 4, pp

Matthieu Clouqueur and Wayne D. Grover NFOEC Statistical Considerations Results for each test case are based on series of 1000 one-year simulations Total of dual (or higher order) failures arising over 1000 trials: –Net32-A & Net32-B: 2619 –25n50s1: 3180 Average number of outage events per path being the basis for availability results: –Ring: 64.7 –Mesh: 45.9 This was shown to give good confidence levels on results: –E.g. for 25n50s1 test case, over 10 separate 1000 one-year trials, standard deviation of average unavailability U ave results was calculated to be 2.7 % of U ave

Matthieu Clouqueur and Wayne D. Grover NFOEC Comparative Results of Path Unavailability Path unavailability is significantly higher in the ring architecture, especially for longer paths (up to a factor 2 in the worst case) ~ 26 % chance of outage in a given year (worst case) ~ 17 % chance of outage in a given year (worst case) net32-B net32-A 25n50s1 Average working path lengths

Matthieu Clouqueur and Wayne D. Grover NFOEC Comparing Distributions of Outage Times Ring: 93.7 % of paths experience no outage in a year Mesh: 95.4 % of paths experience no outage in a year median ~ 6 hours Results for test case 25n50s1 median ~ 6 hours 90 th percentile 15 hours 90 th percentile 13.5 hours

Matthieu Clouqueur and Wayne D. Grover NFOEC Effects of Priorities in Mesh The effect of prioritizing in mesh is a considerable reduction of path unavailability for the Priority class. The non-Priority service class suffers only a small degradation of service availability. The availability of non-Priority services remains comparable or better than in ring Results for test case 25n50s1 A priority path in mesh can have a more than five times lower unavailability than in ring

Matthieu Clouqueur and Wayne D. Grover NFOEC Additional Insights from the Study The effect of priorities is a significant reduction of the probability of paths experiencing outage in a given year: –E.g. for 6-hop paths in the 25n50s1 test case, the probability of experiencing outage in a given year drops from 12% to 2% is the path is included in the 10% priority class (10/90 scheme). Experimental results show that the advantage of mesh over ring is greater in the highly connected topology –This is especially true for results of prioritized mesh and confirms the understanding that mesh benefits greatly from high network diversity Under the 10/90 service mix Priority services achieve the lowest unavailability while the availability of non-Priority services remains virtually unchanged

Matthieu Clouqueur and Wayne D. Grover NFOEC Conclusion Despite its lower capacity requirements, the mesh architecture achieves better availability than ring –The key is mesh’s better ability to withstand dual-failure states even with less redundancy than rings. With prioritization of services, high priority services can be offered very low unavailability (3 to 5 times less than with rings) while non-Priority service still enjoy comparable or better availability than with rings. Concluding comment: What matters most for high availability is not fast restoration to single failures but the ability to provide high restorability to dual failures. This minimizes the probability of MTTR-scale outages on service paths.