1. Capacity Design Studies of Span-Restorable Mesh Transport Networks With Shared-Risk Link Group (SRLG) Effects John Doucette, Wayne D. Grover doucette@trlabs.ca, grover@trlabs.ca TRLabs and University of Alberta Edmonton, AB, Canada OptiComm 2002 Boston, MA, USA 30/July/2002

2. John Doucette and Wayne D. Grover OptiComm 2002 - Boston, MA, July 2002 2 Capacity Design Studies of Span-Restorable Mesh Transport Networks With Shared-Risk Link Group (SRLG) Effects Outline What is a Shared-Risk Link Group (SRLG)? Research Questions Experimental Approach Design Formulation Results –Cost of Protecting Against SLRGs –Co-Incident SRLG Effects on Restorability –Effects of Co-Incident SRLG Location –Identifying Troublesome SRLGs Concluding Remarks

3. John Doucette and Wayne D. Grover OptiComm 2002 - Boston, MA, July 2002 3 Capacity Design Studies of Span-Restorable Mesh Transport Networks With Shared-Risk Link Group (SRLG) Effects What is a Shared-Risk Link Group (SRLG)? Shared-Risk Link GroupHere a Shared-Risk Link Group (SRLG) is a set of spans with a common cause of failure. Co-Incident SRLGA Co-Incident SRLG is an SRLG involving multiple (nominally disjoint) spans, incident upon a common node. –Most common type of multi-span SRLG.

4. John Doucette and Wayne D. Grover OptiComm 2002 - Boston, MA, July 2002 4 Capacity Design Studies of Span-Restorable Mesh Transport Networks With Shared-Risk Link Group (SRLG) Effects Research Questions What impacts do SRLGs have on survivable network capacity requirements? –How costly is it to protect against SRLGs? How do SRLGs affect a network’s restorability? Which types of SRLGs are most costly to protect against? How many SRLGs can be sustained before the capacity penalty becomes too severe? Can we identify which SRLGs are most troublesome? –When is it worthwhile to take physical measures to eliminate an SRLG?

5. John Doucette and Wayne D. Grover OptiComm 2002 - Boston, MA, July 2002 5 Capacity Design Studies of Span-Restorable Mesh Transport Networks With Shared-Risk Link Group (SRLG) Effects Experimental Approach Define new design model: –Spare capacity design for 100% restorability in the face of all single span failures and specified SRLGs. Test with varying numbers and position of random co-incident SRLGs. –What is the cost of spare capacity for full restorability? –Test network has 40 nodes, 70 spans, 780 O-D demands of 1-10 wavelength paths each. Analyze and rank individual co-incident SRLG impact based on nodal degree and location. –Which individual SRLGs cost more to make fully restorable?

6. John Doucette and Wayne D. Grover OptiComm 2002 - Boston, MA, July 2002 6 Capacity Design Studies of Span-Restorable Mesh Transport Networks With Shared-Risk Link Group (SRLG) Effects SRLG-Tolerant Design Formulation Minimize: Total Spare Capacity Subject To: (1) Restoration flow for all single-span failure scenarios (2) Restoration flows for all specified SRLG scenarios (3) No restoration flow on co-failed spans in SRLG scenarios (4) Spare capacity to support (1) and (2)

7. John Doucette and Wayne D. Grover OptiComm 2002 - Boston, MA, July 2002 7 Capacity Design Studies of Span-Restorable Mesh Transport Networks With Shared-Risk Link Group (SRLG) Effects Cost of Protecting Against SLRGs It costs nearly as much to make a given percentage of co-incident SRLGs restorable as it does to make the same percentage of all possible dual span failures restorable.

8. John Doucette and Wayne D. Grover OptiComm 2002 - Boston, MA, July 2002 8 Capacity Design Studies of Span-Restorable Mesh Transport Networks With Shared-Risk Link Group (SRLG) Effects Co-Incident SRLG Effects on Restorability Making a given percentage of all co-incident SRLGs restorable provides restorability/availability nearly as good as making the same percentage of all dual span pairs restorable. Making even a small number of co-incident SRLGs restorable will make a much greater number of dual failures fully restorable. Random Dual Failure SRLGs Co-Incident SRLGs % Fully restorable dual failures

9. John Doucette and Wayne D. Grover OptiComm 2002 - Boston, MA, July 2002 9 Capacity Design Studies of Span-Restorable Mesh Transport Networks With Shared-Risk Link Group (SRLG) Effects Classifying Node Locations “edge” “near-edge” “interior”

10. John Doucette and Wayne D. Grover OptiComm 2002 - Boston, MA, July 2002 10 Capacity Design Studies of Span-Restorable Mesh Transport Networks With Shared-Risk Link Group (SRLG) Effects Effects of Co-Incident SRLG Location interior Co-incident SRLGs at nodes in the interior of the network are more costly than those closer to the edge.

11. John Doucette and Wayne D. Grover OptiComm 2002 - Boston, MA, July 2002 11 Capacity Design Studies of Span-Restorable Mesh Transport Networks With Shared-Risk Link Group (SRLG) Effects Effects of Co-Incident SRLG Location (2) degree-3 Co-incident SRLGs at degree-3 nodes are more costly than those at degree-4 nodes. Degree-3 Degree-4

12. John Doucette and Wayne D. Grover OptiComm 2002 - Boston, MA, July 2002 12 Capacity Design Studies of Span-Restorable Mesh Transport Networks With Shared-Risk Link Group (SRLG) Effects Identifying The Most Troublesome SRLGs A random set of 18 co-incident SRLGs and evaluated their costs as a set and individually. Remove the most costly SRLGs and re-solve the capacity design with remaining SRLGs. Removing the 4 worst ranked SRLGs gives a 55% reduction in capacity penalty.

13. John Doucette and Wayne D. Grover OptiComm 2002 - Boston, MA, July 2002 13 Capacity Design Studies of Span-Restorable Mesh Transport Networks With Shared-Risk Link Group (SRLG) Effects Concluding Remarks Co-incident SRLGs are more troublesome than typical dual span failures. –Believed to be most common type of multi-span SRLG. –Requires more spare capacity on average than other SRLGs or dual failures. –Dual failure restorability (hence availability) responds best to investment for coverage of the co-incident SRLGs. Co-incident SRLG effects are generally more troublesome (require more capacity) if located at degree-3 nodes in the network interior. Identification and removal of the individually most troublesome co-incident SRLGs gets most of the benefit of removal of all SRLGs.