Routing and Wavelength Assignment Approaches for WavelengthRouted Optical

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

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

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)

Routing Assignment • Fixed routing • Fixed-alternate routing • Adaptive routing

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)

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

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

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

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(vi)}

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

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

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

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

Min-Product(MP) • Used in multi-fiber network • The idea is to pack wavelength into fibers, minimizing the number of fibers in the network ∏ l є π(p) D lj 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 -

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

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

MAX-SUM Assignment (continued) r(ψ, l, j) = Mj - 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

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

Relative Capacity Loss(RCL) Assignment • Chose wavelength j to minimize the relative capacity loss: Σ pєP(r(ψ, p, j) - r(ψ’(j), p, j))/ r(ψ, p, j) • 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)

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

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

Calculation in DRCL (example) Path p 1: (2, 4) Wavelength is used

Calculation in DRCL (example) (continue) wavelength (2, 0) (2, 1) (2, 3) (2, 5) (2, 6) The total capacity loss on each wavelength λ 3 λ 2 λ 1 λ 0 0 1 1/3 0 1/3 1/4 1/4 1/3 1/3 0 11/12 17/12 13/12 19/12 λ 3 is chosen 0 1/2 0

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