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Short Overview of Dynamic Routing and Wavelength-Assigment in Survivable WDM Networks

Short Overview of Dynamic Routing and Wavelength-Assigment in Survivable WDM Networks. Carlos Simões 1,3 e Teresa Gomes 2,3 1 Escola Superior de Tecnologia de Viseu Instituto Politécnico de Viseu 2 Departamento de Engenharia Electrotécnica e Computadores FCTUC – Universidade de Coimbra

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Short Overview of Dynamic Routing and Wavelength-Assigment in Survivable WDM Networks

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  1. Short Overview of Dynamic Routing and Wavelength-Assigment in Survivable WDM Networks Carlos Simões1,3 e Teresa Gomes2,3 1 Escola Superior de Tecnologia de Viseu Instituto Politécnico de Viseu 2 Departamento de Engenharia Electrotécnica e Computadores FCTUC – Universidade de Coimbra 3 INESC Coimbra - Instituto de Engenharia de Sistemas e Computadores csimoes@ipv.pt, teresa@deec.uc.pt

  2. Plan • Introduction • The RWA problem • Routing • Wavelength assignment • Resilient routing • Conclusion

  3. IntroductionThe RWA Problem • Optical network • A connection oriented network • Lightpaths • The RWA • Without wavelength converters: the wavelength continuity constraint. • With wavelength converters. l1 l2 l3 A A B B Converter l A-B connection blocked A-E connection successful

  4. IntroductionThe RWA Problem • There are two variations of the RWA problem: • Static – the entire set of connections is known beforehand: • static lightpath establishment(SLE) problem. • Dynamic - the connection requests arrive based on some stochastic process, and the lightpath is released after some amount of time: • dynamic lightpath establishment(DLE) problem.

  5. IntroductionThe RWA Problem • SLE can be formulated as a mixed-integer linear program (ILP), which is NP-complete. • Because of the real-time nature of the problem, DLE problem is more difficult to solve. • Strategy – The RWA problem can be partitioned into two sub-problems: • (1) Routing and • (2) Wavelength Assignment and each sub-problem is solved separately

  6. IntroductionRouting • Static Routing: • Fixed routing • Fixed alternative routing • Table of alternative (link disjoint) paths • Dynamic routing • Paths are chosen based on network state • An adequate use of node costs can reduce the need for wavelength conversion

  7. IntroductionWavelength Assignment • Static Lightpath Establishment (SLE) • Objective: Minimizing the number of used λs and satisfying the wavelength continuity constraint • Graph colouring: NP-Complete problem • There are efficient sequential algorithms • Heuristics for Dynamic Lightpath Establishment (DLE) • The most common objective is the minimisation of blocking probability.

  8. IntroductionWavelength Assignment • Heuristics for WA in DLE: • R (Random) • FF (First Fit) • LU (Least Used, spread) • UM (Most Used, pack) • MP (Min-Product) • LL (Least-Loaded) • M∑ (Max-Sum) • RCL (Relative Capacity Loss) • Rsv (Wavelength Reservation) • . . .

  9. Protection Restoration Path Link Subpath Path Link Subpath Resilient Routing • Advantages of recovery mechanisms in the optical layer: • Fast recovery of course grain traffic flows • Protects higher layers protocols without its own recovery mechanisms. • Recovery mechanisms classification: Fault Recovery Schemes

  10. Shared Protection l1 l2 l3 l2 Resilient Routing • Dedicated versus shared protection Dedicated Protection A B fiber Active Path A-B l1 Backup Path A-B l2 Active Path C-D l3 Backup Path C-D l4 C D

  11. A F C C 2 2 B B D D 1 1 1 A F 2 3 E E Resilient Routing • Dedicated path protection • Two Step Approach (TSA) • Difficulty: Trap topology problem • Disjoint Shortest Path Pair • (Suurballe & Tarjan, 84), Min-Sum problem

  12. Resilient Routing • Path protection • The active path cost is made higher than the protection path: • The Min-Sum with ordered dual link costs problem (MSOD) – NP-Complete • Suurballe & Tarjan algorithm is no longer applicable • Shared protection results in MSOD, with the additional difficulty that the cost of the protection path depends on the choice of the active path: C(AP)+C’(BP) .

  13. Resilient Routing • Complexity of underlying problems [1] Dahai Xu, et al., “On Finding Disjoint Paths in Single and Dual Link Cost Networks,” INFOCOM’04, March 2004. [2] J. W. Surballe, et al., “A Quick Method for Finding Shortest Pairs of Disjoint Paths,” Networks, 14:325–336, 1984. [3] Arunabha Sen, et al., “Survivability of Lightwave Networks - Path Lengths in WDM Protection Scheme,” Journal of High Speed Networks, 10(4):303-315, 2001. [4] Li, et al., “The Complexity of Finding Two Disjoint Paths with Min-Max Objective Function,” Discrete Applied Mathematics, 26(1):105–115, Jan. 1990.

  14. A B A B fiber span Active Path A-B Backup Path A-B C D C D Link TopologySRLG Topology A B C D Resilient Routing • Shared Risk Link Group (SRLG) • Finding a SRLG disjoint path pair: NP-Complete • Therefore any optimisation problem… • Routing with the λ-continuity constraint is also NP-Complete!

  15. Resilient RoutingBasicLink Method [Li02] • Basic Link: • A link that transverses only one fiber span. • A link that traverses multiple fiber spans but it is the only link in those spans. • Construct a topology with only basic links. • Find a pair of link disjoint paths (Suurballe). • Select first path as the AP. [Li02] Guangzhi Li, et al., “Fiber Span Failure Protection in Mesh Optical Networks,” Optical Networks Magazine, 3(3):21-31, 2002.

  16. Resilient RoutingBasicLink Methodcont. • Second path is SRLG disjoint and can be the Backup Path, but a better path can be found easily: • Delete links along the first path in the original graph, • find the shortest path  use it as backup path. • Problems • Uses only basic links for the AP selection. • May fail too (like TSA), although two SRLG disjoint paths exists.

  17. Resilient RoutingBypass Method[Li02] The basic idea is to construct a single layer sub-network over the original optical network and find two Link disjoint paths on the constructed sub-network. • Compute shortest path p from s to d: p = (s=a1, a2, ..., ak=d) • If second path fail, construct a directed auxiliary graph H with k nodes, (labeled 1,..., k). • Delete all links along path p in the original graph, and the links that belong to the same SRLGs. [Li02] Guangzhi Li, et al., “Fiber Span Failure Protection in Mesh Optical Networks,” Optical Networks Magazine, 3(3):21-31, 2002.

  18. Resilient RoutingBypass Methodcont. • If there is a path from ai to aj - add a direct edge from i to j in H • Add back edges from i to i -1 (i=2,..., k) • Run Dijkstra’s algorithm on H. • If we find a path in H from 1 to K then its possible to find two link disjoint paths on H, without considering the arc direction.

  19. 1 1 2 2 3 3 4 4 5 5 6 6 Resilient RoutingBypass Methodcont. • Better than BasicLink Method – allows both paths to use non basic links. Problems • The two paths may not be SRLG disjoint (links 1-4 and 3-6 may belong to the same SRLG).

  20. Resilient RoutingGraph Transformation Technique [Datta04] • Add dummy nodes for each SRLG and new edges (graph H) • Apply Edge Disjoint Shortest Cycle Algorithm (Bhandari) to H. • The two paths derived from shortest cycle on H gives two SRLG disjoint routes. 2 2 d1 d2 6 6 3 3 1 1 d3 5 5 4 4 [Datta04] Pallab Datta et al., “Diverse Routing for Shared Risk Resource Groups (SRRG) Failures in WDM Optical Networks,” First International Conference on Broadband Networks, BROADNETS’04, 2004.

  21. Resilient RoutingGraph Transformation Technique • Problems Only applies if: • Each SRLG size is smaller than the degree of the node on which the group is incident. • A link can be shared between almost two SRLG.

  22. Resilient RoutingKSP - K Shortest Paths Natural extension of TSA (Two Step Approach). • Compute K shortest paths as candidate APs. • Test them one by one in increasing order of their costs, until an SRLG disjoint path is found (or all of them have been tested). Problems: • If current candidate AP fails the test, the next candidate is selected only based in cost, without considering which link(s) in current AP caused the trap. • Many candidate APs have to be tested. • The path pair is not optimal.

  23. Resilient RoutingJPS-Joint Path Selection [Xin02] • Compute k shortest paths as candidate APs (APi, i=1,…, k, with cost CAPi). • For each APi compute the shortest SRLG disjoint path BPi (with cost CBPi). • Find h such that CAPh+CBPh=min(CAPi +CBPi),1 ik. • Select APh as service path and BPh as Backup Path. Link Cost Function • Integrated link cost function (additive) based in hop-count and available l’s: [Xin02] Chunsheng Xin, et al., “A Joint Lightpath Routing Approach in Survivable Optical Networks,” Optical Networks Magazine, 3(3):13-20, 2002.

  24. Resilient RoutingJPS-Joint Path Selectioncont. Path Cost Function • Active Path Cost: • Backup Path Cost: • Dedicated Protection: • Shared Protection: a - sharing control weight

  25. Resilient RoutingITSA-Iterative Two-Step Approach [Ho03] Enhancement of the TSA. Repeats TSA iteratively, in which each k-shortest path is taken as a working path in each iteration. • Start with shortest path as the AP in the first iteration. • Repeat until stop condition is met. • Find spare link state capacity in each link, according to AP. • Compute BP, according spare link state capacity. • Update the best path pair. • Compute next AP (next shortest path) and repeat [Ho03] Pin-Han Ho, et al., “Diverse routing for shared protection in survivable optical networks,” IEEE Global Telecommunications Conference, GLOBECOM'03, Vol. 5, pp. 2519-2523, 2003.

  26. Resilient RoutingITSA-Iterative Two-Step Approach • The stop condition uses more than one criteria: • Optimal Path Pair found • Predefined number of iterations • Optimality check • If the cost of candidate AP is greater than the cost of the current best path pair Advantage: • guarantees the derivation of the best working and protection path-pair under the current link-state provided a sufficient amount of time. Disadvantage: • Number of APs that need to be explored grows exponentially with network size.

  27. Conclusion • The RWA in survivable networks is a difficult problem • Many heuristics have been proposed • We intend to develop a multiobjective model, exploring the trade-offs between: • Blocking Probability • Fairness • Impact of accepting a connection in future requests (future cost/shadow price)

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