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Routing and Wavelength Assignment Approaches for Wavelength-Routed Optical WDM Networks

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Agenda Introduction Routing Assignment Wavelength Assignment Distributed Relative Capacity Loss(DRCL) Conclusion

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Introduction Wavelength-division multiplexing(WDM) handle the ever-increasing bandwidth demands of network users A lightpath occupies the same wavelength on all fiber links, known as wavelength-continuity constraint Setting up a connection is routing and wavelength assignment(RWA)

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Routing Assignment Fixed routing Fixed-alternate routing Adaptive routing

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Wavelength Assignment Random Assignment First-Fit Least-Used/SPREAD Most-Used/PACK Min-Product Least Loaded MAX-SUM Relative Capacity Loss(RCL) Wavelength Resevation Protecting Threshold Distributed Relative Capacity Loss(DRCL)

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Fixed Routing Off-line calculation Shortest-path algorithm: Dijkstra’s or Bellman-Ford algorithm Advantage:simple Disadvantage: high blocking probability and unable to handle fault situation

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Fixed-Alternate Routing Routing table contains an ordered list of fixed routes -e.g. shortest-path, followed by second-shortest path route, followed by third-shortest path route and so on Alternate route doesn’t share any link(link-disjoint) Advantage over fixed routing: -better fault tolerant -significantly lower blocking probability

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Adaptive Routing Route chosen dynamically, depending on the network state Adaptive shortest-cost-path - Each unused link has the cost of 1 unit; used link ∞; wavelength converter link c units. Disadvantage: extensive updating routing tables Advantage:lower blocking probability than fixed and fixed-alternate routing Another form:least-congested-path(LCP) Recommended form:shortest paths first, and use LCP for breaking ties

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Static Wavelength Assignment Two lightpaths share the same physical link are assigned different wavelengths Reduced to graph-coloring problem: 1.Construct a graph, such that each lightpath is represented by a node. There is one edge in between if two lightpaths share the same physical link. 2.Color the nodes such that no two adjacent nodes have the same colors. Theorem: Let G be a graph with V(G)=v 1,v 2,…,v n where deg(v i ) =deg(v i+1 ) for i= 1,…,n-1. Then minimum number of colors needed <= max 1<=i<=n min {I, 1+deg(v i )}

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Random Wavelength Assignment Randomly chosen available wavelength Uniform probablity No global information needed

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First-Fit First available wavelength is chosen No global information needed -prefered in practice because of its small overhead and low complexity Perform well in terms of blocking probability and fairness The idea behind is to pack all of the in-use wavelengths towards lower end and continous longer paths towards higher end

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Least-Used(LU) Wavelength Assignment Least used in the network chosen first Balance load through all the wavelength Break the long wavelength path quickly Worse than Random: -global information needed -additional storage and computation cost -not preferred in practice

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Most-Used(MU) Assignment Select the most-used wavelength in the network Advantages: -outperforms FF, doing better job of packing connection into fewer wavelength -Conserving the spare capacity of less-used wavelength Disadvantages: -overhead, storage, computation cost are similar to those in LU

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Min-Product(MP) Used in multi-fiber network The idea is to pack wavelength into fibers, minimizing the number of fibers in the network ∏ D lj l є π(p) for each wavelength j, 1<=j<=W Chose a set of wavelength j minimizing the above value Disadvantage: not better that multi-fiber version of FF -introduce additional computation costs -

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Least-Loaded(LL) Assignment Multi-fiber network Select the wavelength that the largest residual capacity in the most-loaded link along route p. Advantage: outperforms MU and FF in terms of blocking probability

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MAX-SUM Assignment Applied to multi-fiber and single-fiber also Before lightpath establishment, the route is pre-selected; After lightpath establishment, it attemps to maximize the remaining path capacity

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MAX-SUM Assignment (continued) r(ψ, l, j) = M j - D(ψ) lj r(ψ, l, j):link capacity, the number of fibers on which wavelength j is unused on link l r(ψ, p, j) = min r(ψ, l, j) l є π(p) r(ψ, p, j):the number of fibers on which wavelength j is available on the most-congested like along the path p

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MAX-SUM Assignment(continued) w R(ψ,p) = Σ min r(ψ, l, j) j=1 l є π(p) At last, chose the wavelength j that maximizes the quantity: Σ R(ψ’(j),p) pєP ψ’(j) be the next state of the network if j is assigned P is all the potential paths for the connection

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Relative Capacity Loss(RCL) Assignment Chose wavelength j to minimize the relative capacity loss: Σ (r(ψ, p, j) - r(ψ’(j), p, j))/ r(ψ, p, j) pєP Sometimes better than MAX-SUM -MAX-SUM could cause blocking Longer lightpaths have a higher block probability than shorter ones Some schemes to protect longer paths: Wavelength reservation(Rsv) and protesting threshold(Thr)

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Distributed Relative Capacity Loss(DRCL) Using Bellman-Ford algorithm to exchange information between nodes Routing table as well as RCL table to be exchanged Calculate the rcl(w,d) value for all the paths from the source node to every other node, excluding the destination node Choose the wavelength that minimizes the sum of rcl(w,d) over all possible destinations d

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Distributed Relative Capacity Loss(DRCL) (continue) Calculating the value rcl(w,d) at node s: If there is no path from node s to node d on wavelength, then rcl(w,d) =0; otherwise, If there is a direct link from s to d, and path from s to d on wavelength w is routed though this link, rcl(w,d) = 1/k, k is the number of available wavelength; otherwise, rcl(w,d) = max(1/k, rcl(w,d) at node n), n is the second node from s to d

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Path p1: (2,4) Wavelength is used Calculation in DRCL (example)

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wavelength (2,0)(2,1)(2,3)(2,5)(2,6) The total capacity loss on each wavelength λ301/31/41/3011/12 λ201/31/41/31/217/12 λ1001/41/31/213/12 λ011/31/40019/12 Calculation in DRCL (example) ( continue ) λ3 is chosen

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Conclusion Adaptive routing has lower blocking probability in routing assignment Max-Sum and RCL provide lower blocking probability in wavelength assignment, but rely on fixed routing DRCL based on both RCL and adaptive routing, having better performance Routing algorithm play a significant role

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