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**In the name of God, The Beneficent, The Merciful**

A Review of Routing and Wavelength Assignment Approaches for Wavelength-Routed Optical WDM Networks Mohammad Reza Faghani

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**Outline Introduction Static Lightpath Establishment (SLE) problem**

Routing Wavelength Assignment Simulation Results Wavelength assignment in distributed fashion.

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Introduction

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**Wavelength Routed Network**

Definition A wavelength routed network consists of WXC (wavelength crossconnect) interconnected by point-point fiber links in any arbitrary topology. A lightpath is an all-optical communication path between two nodes, established by allocating the same wavelength throughout the route of the transmitted data. Issues in wavelength routed networks Route and wavelength assignment Centralized Versus Distributed Control

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**Wavelength-continuity constraint**

SONET F J 1 O B IP 2 A 1 3 K E 2 1 N D C 1 L IP SONET M OXC OXC allows the efficient network management of wavelengths at the optical layer. The variety of functions that it provides are signal monitoring, restoration, provisioning and grooming.

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**Wavelength Routed Networks**

Given a set of connections, the problem of setting up lightpaths by routing and assigning a wavelength to each connection is called the Routing and Wavelength-Assignment (RWA) problem. Minimize the number of wavelength needed for certain set of connection or Alternatively, maximize the number of connection for a given fixed number of wavelengths.

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**Connection Requests Static Lightpath Establishment (SLE) problem**

Static : The set of connections is known in advance Dynamic Lightpath Establishment (DLE) problem Incremental : Connection requests arrive sequentially,a lightpath is established for each connection, and a lightpath remains in the network indefinitely Dynamic : A lightpath is set up for each connection request as it arrives, and the lightpath is released after some finite amount of time

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**Static Lightpath Establishment (SLE) problem**

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**Static Lightpath Establishment**

Characteristic Lightpath requests are known in advance RWA operations are performed off-line Objective Min (# of flow each link) Max (# of connections that can be established)

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**Static Lightpath Establishment**

The SLE problem can be formulated as a linear program or ILP while DLE employs heuristic methods. SLE can be partitioned into two subproblems Routing Wavelength assignment Each subproblem can be solved separately

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**ILP of SLE with wavelength-continuity constraint**

integer linear program (ILP) objective function is to minimize the flow in each link, which, in turn, corresponds to minimizing the number of lightpaths passing through a particular link.

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**ILP of SLE with wavelength-continuity constraint**

Number of connection requests. Number of connection request on any s,d,w Number of connection needed.

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**ILP of SLE with wavelength-continuity constraint**

This approach may be used to obtain the minimum number of wavelengths required for a given set of connection requests by performing a search on the minimum number of wavelengths in the network. We can apply the ILP to see if a solution can be found.This procedure is iterated until the minimum number of wavelengths is found. the next ILP is used for maximizing the number of established connections for a fixed number of wavelengths

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**ILP of SLE with wavelength-continuity constraint (cont.)**

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**ILP of SLE with wavelength conversion**

the wavelength-continuity constraint can be eliminated if we use wavelength converters to convert one wavelength on into another at an intermediate node before forwarding it to the next link. wavelength conversion may improve the efficiency by resolving the wavelength conflicts. This method can also be formulated using ILP.

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**ILP of SLE with wavelength conversion**

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**ILP of SLE with wavelength conversion**

full wavelength conversion in the network may not be preferred and may not even be necessary due to high costs and limited performance gains. a subset of the nodes may allows wavelength conversion, or a node employs converters that can only convert to a limited range of wavelenghts. Some Problem may arise due to limited conversions.

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**Limited wavelength conversion**

Sparse location of wavelength converters in network Place few converters in an arbitrary network Where Optimally to place ? Sharing of converters Switch architectures that allow sharing of converters among the various signals. Performance Saturates as no. of converters increases. Routing Dependent Limited-range wavelength conversion Range is limited to k i max(i-k,1) through min(i+k,w)

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Routing

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**Routing Both SLE and DLE use three basic approches for routing.**

Fixed Routing Fixed Alternate Routing Adaptive Routing Fixed Routing is Simplest, Adaptive yields the Best performance. Alternate offers Tradeoff.

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Fixed Routing Always choose the same fixed route for a given source-destination pair Ex: fixed shortest-path routing Dijkstra’s algorithm Bellman-Ford algorithm Disadvantage Hign blocking probability in the dynamic case Unable to handle fault situation (altPath,Dyn)

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Fixed Routing Fixed shortest path route from node 0 to 2.

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**Fixed-Alternate Routing**

Each node is required to maintain a routing table that contains an ordered list of a number of fixed routes to each destination node A primary route between s-d is defined as the first route An alternative route doesn’t share any links with the first route (link disjoint) Advantage Provide some degree of fault tolerance Reduce the blocking probability compared to fixed routing

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**Fixed-Alternate Routing**

Primary (Solid) and Alternate (Dashed) routes form node 0 to 2

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Adaptive Routing The route from a source node to a destination node is chosen dynamically, depending on the network state Ex: Shortest-cost-path routing Least-congestion-path routing Congestion is measured by available wavelengths Advantage Lower connection blocking than fixed and fixed-alternate routing

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**Adaptive Routing shortest-cost-path routing,**

well-suited for use in wavelength-converted networks. Each unused link has a cost of 1 unit, each used link has a cost of ∞, and each wavelength-converter link has a cost of c units. If wavelength conversion is not available, c = ∞. When a connection arrives, the shortest-cost path between the source node and the destination node is determined.

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Adaptive Routing Adaptive shortest cost path route from node 0 to 2.

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**Consider fault-tolerant**

Protection Set up two link/node-disjoint lightpaths Primary lightpath transmit data Backup lightpath must be reserved Fast but need reserve resource Restoration The backup path is determined dynamically after the failure has occurred Slow but doesn't need reserve resource

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**Wavelength Assignment**

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**Static Wavelength-Assignment**

Minimizing the number of wavelengths used in wavelength-continuity constraint, reduced to the graph coloring problem Construct an auxiliary graph G(V,E) Color the nodes of the graph G Largest First Smallest Last

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**Static Wavelength-Assignment (cont.)**

Auxiliary Graph. Network With 8 routed Lightpath

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Largest First

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Smallest Last

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**Dynamic or Incremental Wavelength Assignment Heuristics**

For the case in which lightpaths arrive one at a time (either incremental or dynamic traffic), heuristic methods must be used to assign wavelengths to lightpaths. In dynamic problem, we assume that the number of wavelengths is fixed (as in practical situations), and we attempt to minimize connection blocking.

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**Dynamic or Incremental Wavelength Assignment Heuristics**

Random Wavelength Assignment (R) First-Fit (FF) Least-Used (LU)/SPREAD Most-Used (MU)/PACK Min-Product (MP) Least-Loaded (LL) MAX-SUM (MΣ) Relative Capacity Loss (RCL) Distributed Relative Capacity Loss (DRCL) Wavelength Reservation (Rsv) Protecting Threshold (Thr)

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**Wavelength-usage pattern**

Consider P1(2,4) and three potential paths that share common link P2(1,5) P3(3,6) P4(0,3).

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**Random Wavelength Assignment (R)**

First searches the space of wavelengths to determine the set of all wavelengths that are available on the required route Among the available wavelengths, one is chosen randomly Advantage NO communication overhead

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First-Fit (FF) When searching for available wavelengths, a lower-numbered wavelength is considered before a higher-numbered wavelength The first available wavelength is then selected Advantage Computation cost is lower No communication overhead

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**FF example λ0 will be assigned**

λ0 will also be assigned MP and LL as single fiber.

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**Least-Used (LU)/SPREAD**

LU selects the wavelength that is the least used in the network, thereby attempting to balance the load among all the wavelengths Disadvantage Additional communication overhead

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**LU example λ0 ,λ1 ,λ3 are each used two links λ2 is used only one link**

So LU will choose λ2

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**Most-Used (MU)/PACK MU selects the most-used wavelength in the network**

Packing connections into fewer wavelengths Advantage Overhead is similar to LU but MU outperforms LU and FF Outperforms LU (fewer wavelength used).

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**MU example λ0 ,λ1 ,λ3 are each used two links λ2 is used only one link**

So MU will choose one of λ0 ,λ1 ,λ3 with equal probability.

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**Min-Product (MP) MP is used in multi-fiber network**

In a Single Fiber , MP becomes FF. The goal of MP is to pack wavelengths into fibers Dlj indicates the number of assigned fibers on link l and wavelength j. MP does it for all j. π(p): Set of links comprising path p.

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**MP example λ1 : 2*3*1*3*5=90 λ2 : 3*2*4*1*2=48 λ3 : 1*2*1*2*1=4**

1 2 3 4 5 λ1=2 λ2=3 λ3=1 λ1=3 λ2=2 λ3=2 λ1=1 λ2=4 λ3=1 λ1=3 λ2=1 λ3=2 λ1=5 λ2=2 λ3=1 λ1 : 2*3*1*3*5=90 λ2 : 3*2*4*1*2=48 λ3 : 1*2*1*2*1=4 So choose λ3 for path 0 to 5.

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**Least-Loaded (LL) LL is also used in multi-fiber network**

To select the wavelength that has the largest residual capacity on the most-loaded link along route p Ml: Number of fibers on link l. Sp: Set of available wavelengths along the selected paths p.

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**LL example Set up lightpath from 0 to 2**

1 2 3 4 5 Assume 7 fibers per link λ1=2(5) λ2=3(4) λ3=1(6) λ1=3(4) λ2=2(5) λ3=2(5) λ1=1(6) λ2=4(3) λ3=1(6) λ1=3(4) λ2=1(6) λ3=2(5) λ1=5(2) λ2=2(5) λ3=1(6) Set up lightpath from 0 to 2 Choose λ3 Max(min(residual capacity))=5

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MAX-SUM (MΣ) MΣ considers all possible paths in the network and attempts to maximize the remaining path capacities after lightpath establishment. Applied to both single and multi-fiber Networks

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**MΣexample λ2 has the highest capacity loss What about choosing λ0 ?**

Choosing λ0 will block path P4

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**Relative Capacity Loss (RCL)**

RCL attempts to improve on MΣ by taking into consideration the number of available alternate wavelengths for each potential future connection. MΣ RCL

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RCL example Choosing the Wavelength with smallest Total RCL.

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**Distributed Relative Capacity Loss (DRCL)**

MΣ and RCL are difficult and expensive to implement in a distributed environment. MΣ and RCL both require fixed routing, which makes it difficult to improve network performance. Two Problem of implementation. how is information of network state exchanged? how can we reduce the amount of calculation upon receiving a connection request?

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**Distributed Relative Capacity Loss (DRCL)**

Speed up the wavelength-assignment procedure each node stores information on the capacity loss on each wavelength. only table lookup. (w,d,rcl(w,d)) small amount of calculation are required upon the arrival of a connection request. Routing is implemented using the Bellman-Ford (each node exchange table with its neighboring nodes and updates its table).

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**Distributed Relative Capacity Loss (DRCL) (cont.)**

DRCL considers all of the paths from the source node of the arriving connection request to every other node ,excluding the destination node of the arriving connection request. DRCL choose the wavelength that minimize the sum of rcl(w,d) over all possible destination d.

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**Distributed Relative Capacity Loss (DRCL) (cont.)**

If there is no path from node s to node d on wavelength w, then rcl(w,d) = 0 If there is a direct link from node s to node d, and the path from s to d on wavelength w is routed through this link, then rcl(w,d) = 1/k, where k is the number of available wavelengths on this link through which s can reach.

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DRCL example We have to Calculate RCL for (2,0) (2,1),(2,3),(2,5) and (2,6). (2,0) can only be established on λ0. (2,1) can establish on three wavelengths giving the RCL value of 1/3 and so on. These entry are calculated just using the RCL table of Adjacent nodes.

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**Wavelength Reservation (Rsv)**

λ1 is always reserved on link (1,2) for traffics of node 0 to node 3. So node 1 cannot connect to node 2 using λ1.

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**Wavelength Reservation (Rsv)**

A given wavelength on a specified link is reserved for a traffic stream, usually a multihop stream To protect only the connections that traverse multihop connections. Must be combined with other wavelength-assignment scheme.

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**Protecting Threshold (Thr)**

A single-hop connection is assigned a wavelength only if the number of idle wavelengths on the link is at or above a given threshold.

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Simulation Results

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**Simulation Results Comparison of Random, FF, LU,MU, Max-Sum, and**

RCL for single-fiber network with 16 wavelengths.

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**Simulation Results Comparison of Random, FF, LU,MU, LL, Max-Sum,**

and RCL for two-fiber network with 8 wavelengths.

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Simulation Results In the single-fiber case, MU is found to achieve the best performance under low load while MΣ and RCL work well when the load is high ( ≥ 50 Erlangs), with the other approaches not that far behind. When the number of fibers per link is two (M = 2), MU, MP, and RCL perform well under low load, while LL and MΣ offer better performance under a higher load.

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Simulation Results Comparison of Random, FF, LU, MU, MP, LL,Max-Sum, and RCL for four-fiber network with 4 wavelengths. LL performs better.

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**Simulation Results Comparison of DRCL, FF with adaptive routing, RCL**

(which can only be implemented with fixed routing), and FF with fixed routing.

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Simulation Results Note that RCL cannot be implemented with adaptive routing. DRCL slightly outperforms FF (with adaptive routing) in the reasonable region, which is Erlangs in this network, and they both perform better than RCL and FF with fixed routing.

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Simulation Results Overall, it appears that the routing scheme has much more of an impact on the performance of the system than the wavelength-assignment scheme. It is important to first decide on a good routing mechanism, and then to choose a wavelength-assignment scheme that can easily be implemented in conjunction with the selected routing mechanism.

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References [1] H. Zang et al A Review of Routing and Wavelength Assignment Approaches for Wavelength Routed Optical WDM Networks, Optical Networking Magazine, IEEE Jan 2000

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Thanks to Audiences

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