1. 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

2. Outline Introduction Static Lightpath Establishment (SLE) problem
Routing
Wavelength Assignment
Simulation Results
Wavelength assignment in distributed fashion.

3. Introduction

4. 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

5. 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.

6. 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.

7. 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

8. Static Lightpath Establishment (SLE) problem

9. 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)

10. 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

11. 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.

12. ILP of SLE with wavelength-continuity constraint
Number of connection requests.
Number of connection request on any s,d,w
Number of connection needed.

13. 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

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

15. 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.

16. ILP of SLE with wavelength conversion

17. 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.

18. 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)

19. Routing

20. 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.

21. 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)

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

23. 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

24. Fixed-Alternate Routing
Primary (Solid) and Alternate (Dashed) routes form node 0 to 2

25. 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

26. 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.

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

28. 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

29. Wavelength Assignment

30. 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

31. Static Wavelength-Assignment (cont.)
Auxiliary Graph.
Network With 8 routed Lightpath

32. Largest First

33. Smallest Last

34. 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.

35. 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)

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

37. 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

38. 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

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

40. 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

41. LU example λ0 ,λ1 ,λ3 are each used two links λ2 is used only one link
So LU will choose λ2

42. 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).

43. 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.

44. 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.

45. MP example λ1 : 2*3*1*3*5=90 λ2 : 3*2*4*1*2=48 λ3 : 1*2*1*2*1=41 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.

46. 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.

47. LL example Set up lightpath from 0 to 21 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

48. 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

49. MΣexample λ2 has the highest capacity loss What about choosing λ0 ?
Choosing λ0 will block path P4

50. 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

51. RCL example
Choosing the Wavelength with smallest Total RCL.

52. 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?

53. 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).

54. 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.

55. 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.

56. 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.

57. 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.

58. 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.

59. 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.

60. Simulation Results

61. Simulation Results Comparison of Random, FF, LU,MU, Max-Sum, and
RCL for single-fiber network with 16 wavelengths.

62. Simulation Results Comparison of Random, FF, LU,MU, LL, Max-Sum,
and RCL for two-fiber network with 8 wavelengths.

63. 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.

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

65. Simulation Results Comparison of DRCL, FF with adaptive routing, RCL
(which can only be implemented with fixed routing), and FF with
fixed routing.

66. 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.

67. 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.

68. 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

69. Thanks to Audiences