Optical Networks Routing and wavelength assignment in WDM wide area networks 3 September 2012, Belem, Para, Brazil Dr. Cicek Cavdar, [email protected]Optical Networks Lab (ONLab) Royal Institute of Technology, Stockholm, Sweden Special thanks to Biswanath Mukherjee from UC-Davis, Aysegul Yayimli from ITU, Paolo Monti and Lena Wosinska from KTH for the class material.
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Optical Networks Routing and wavelength assignment in
WDM wide area networks 3 September 2012, Belem, Para, Brazil
n Physical topology: set of WDM links and switching-nodes n Some or all the nodes may be equipped with wavelength
converters
n The capacity of each link is dimensioned in the design phase
Wavelength converter
Optical path termination
Optical Cross Connect (OXC)
WDM optical fiber link
WDM PHYSICAL TOPOLOGY
Optical Networks, RWA, Cicek Cavdar, KTH
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Split the Problem: VTD + RWA n Hard to determine the lightpath
topology jointly with the routing and wavelength assignment
n Split into two sub-problems: (1) Virtual Topology Design (VTD) and (2) Routing and Wavelength Assignment (RWA)
¨ Solve the VTD problem and then map the obtained VT within the physical layer (i.e., for the given lightpaths solve RWA problem)
Optical Networks, RWA, Cicek Cavdar, KTH
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λ 1 λ 2
Optical wavelength"channels
LP1 LP2
LP4
LP3 LP1 LP2 LP3
LP4
LP = LIGHTPATH
Routing and Wavelength Assignment (RWA): Mapping the virtual topology over the physical topology
n Solving the resource allocation problem is equivalent to mapping the virtual topology over the physical topology
λ 3
Mapping (a) No wavelength
converters (b) with
wavelength converters
(a) (b)
CR1
CR2 CR4
CR3
Optical Networks, RWA, Cicek Cavdar, KTH
Optical Networks, RWA, Cicek Cavdar, KTH 10
Routing and Wavelength Assignment (RWA) n The offered traffic is “circuit-oriented”
¨ the offered traffic consists of a set of connections ¨ each connection requires the full bandwidth of a
lightpath to be routed between its corresponding source-destination pair.
n Once a set of lightpaths has been chosen or determined, we need to: ¨ route each lightpath in the network, and ¨ assign a wavelength to it.
n This is referred to as the routing and wavelength assignment (RWA) problem.
11"
RWA Constraints n Resources (fiber and/or wavelength) n Wavelength continuity n Physical impairments n Survivability n ...
Optical Networks, RWA, Cicek Cavdar, KTH
Optical Networks, RWA, Cicek Cavdar, KTH 12
Wavelength Continuity n Normally, a lightpath operates on the same
wavelength across all fiber links that it traverses. ¨ the lightpath is said to satisfy the wavelength-
continuity constraint. n Thus, two lightpaths that share a common fiber
link should not be assigned the same wavelength.
n If a switching/routing node is also equipped with a wavelength converter facility, then the wavelength-continuity constraint disappears. ¨ a lightpath may switch between different wavelengths
on its route from its origin to its termination.
Wavelength Conversion n Three types of wavelength conversion capabilities: • (1) C Full conversion: the node capable of FWC can
change the wavelength of an incoming lightpath to any of the outgoing wavelengths
• (2) FC Fixed conversion: lightpath entering a node at a particular wavelength will always exit the node at another given wavelength. (determine in- and output wavelength channels when network is designed)
• (3) LC Limited conversion: possible conversion from one wavelength to a limited subset of other wavelengths. Will become important when applying all-optical wavelength converters that won’t allow to convert to an arbitrary wavelength but just to a limited set of wavelengths.
Optical Networks, RWA, Cicek Cavdar, KTH 13
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Physical Impairments Constraint n Directly related to the nature of the optical
physical medium and transparent transmission n Optical physical impairments affect the quality of
the lightpath signal n Lightpaths have a reduced reach n Physical impariments can be mitigated by
regenerating the signal ¨ 3R regeneration: Reamplification, Reshaping and
Retiming n Trade-off: cost vs. performance
Optical Networks, RWA, Cicek Cavdar, KTH
Optical Networks, RWA, Cicek Cavdar, KTH 15
Connection Requests n Connection requests may be of three types:
¨ Static: n The entire set of connections is known in advance. n Set up lightpaths for the connections in a global fashion while
minimizing network resources n Known as static RWA or static lightpath establishment (SLE)
problem ¨ Incremental:
n Connection requests arrive sequentially and remains in the network indefinitely.
¨ Dynamic: n A lightpath is set up for each connection request as it arrives,
and it is released after some amount of time. n Known as dynamic RWA or dynamic lightpath establishment
(DLE).
Optical Networks, RWA, Cicek Cavdar, KTH 16
Outline n Introduction to concepts: Lightpath, connection, virtual
topology, physical topology.. n Static RWA
¨ ILP design to solve the routing problem ¨ Graph coloring to solve the wavelength assignment problem
n Dynamic RWA: ¨ Routing:
n Fixed routing, Fixed-alternate routing, Adaptive routing ¨ Wavelength assignment
n Random, First-fit, Least-used, Most-used,...
n Routing and flow assignment algorithms: ¨ Dijkstra, Ford-Fulkerson, Minimum spanning tree: Prim.
Optical Networks, RWA, Cicek Cavdar, KTH 17
RWA Problem Statement n Formally, the static RWA problem can be stated as
follows. n Given:
¨ a set of lightpaths that need to be established on the network, ¨ a constraint on the number of wavelengths and transceivers
n Determine: ¨ the routes over which these lightpaths should be set up ¨ the wavelengths which should be assigned to these lightpaths
n Minimize: number of wavelengths. n In a dynamic traffic scenario, future arrivals are not known in
advance, therefore a new lightpath request may not be set up due to constraints on routes and wavelengths. Called BLOCKING. The corresponding network optimization problem is to minimize this blocking probability.
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n Routing: ¨ Constrained: only some possible paths between source and
destination (e.g. the K shortest paths) are admissible n Great problem simplification
¨ Unconstrained: all the possible paths are admissible n Higher efficiency in network resource utilization
n Cost function to be optimized (optimization objectives) ¨ Route all the lightpaths using the minimum number of
wavelengths ¨ Route all the lightpaths using the minimum number of fibers ¨ Route all the lightpaths minimizing the total network cost,
taking into account also switching systems
Static WDM network optimization
Optical Networks, RWA, Cicek Cavdar, KTH
19"
Objective Realize the lightpath topology, meet all
constraints
¨ Minimize the number of wavelengths used per link
n Offline: For all lightpaths determined by the LTD
¨ Minimize blocking and number of wavelengths used per link
n Online: For demands coming during operation
Optical Networks, RWA, Cicek Cavdar, KTH
Optical Networks, RWA, Cicek Cavdar, KTH 20
Solving the Static RWA n Physical topology and lightpath requests are
known. n Offline RWA n The objective is to minimize the number of
wavelengths. n SLE can be formulated as an integer linear
program (ILP). ¨ Objective: minimize the flow in each link ¨ means minimizing the number of wavelengths used
on each link. n The general problem is NP-complete.
Optical Networks, RWA, Cicek Cavdar, KTH 21
Solution to SLE n Approximation algorithms to solve the RWA problem for
large network sizes. n The RWA problem can be decomposed into different
sub-problems, and each sub-problem can be solved independently with the results of one stage fed in as the input to the next stage. ¨ A linear program (LP) relaxation for routing (using the idea of
multi-commodity flow in a network) A general-purpose LP solver to derive solutions to this problem.
¨ Graph coloring algorithms to assign wavelengths to the lightpaths.
n This method of subdividing the overall problem into smaller sub-problems, allows practical solutions to the RWA problem for networks with a large number of nodes.
Optical Networks, RWA, Cicek Cavdar, KTH 22
Formulation n The RWA problem, without the wavelength-continuity
constraint, can be formulated as a multi-commodity flow problem with integer flows in each link.
n Let λsd denote the traffic (in terms of a lightpath) from any source s to any destination d. ¨ at most one lightpath from any source to any destination; ¨ λsd = 1 if there is a lightpath from s to d ¨ otherwise λsd = 0.
n Let Fsdij denote the traffic (in terms of number of
lightpaths) that is flowing from source s to destination d on link ij.
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RWA: ILP formulation The image cannot be displayed. Your computer may not have enough memory to open the image, or the image may have been corrupted. Restart your computer, and then open the file again. If the red x still appears, you may have to delete the image and then insert it again.Minimize:
Such that: maxF
# of lightpaths between s and d.
# of lightpaths on link i j that are following from s to d.
:
:0
,max
sdij
sd
sd
sd
i k
sdjk
sdij
ds
sdij
F
otherwisejdifjsif
FF
ijFF
λ
λ
λ
=
=
⎪⎩
⎪⎨
⎧
−=−
∀≥
∑ ∑
∑
Optical Networks, RWA, Cicek Cavdar, KTH
Optical Networks, RWA, Cicek Cavdar, KTH 24
Problem Size n If we consider the general multi-commodity formulation,
the number of equations and the number of variables in the formulation grow rapidly with the size of the network.
n For example, assume that there are: ¨ 10 nodes ¨ 30 physical links (ij pairs), ¨ an average of 4 connections per node, 40 connections (sd pairs)
n In the general formulation, ¨ number of λsd variables: 10 × 9 = 90 ¨ number of Fsd
ij variables: 90 sd pairs × 30 ij pairs = 2,700. ¨ number of equations will be 3,721
n Even for a small problem, the number of variables and equations are very large.
n These numbers grow proportionally with the square of the number of nodes.
Optical Networks, RWA, Cicek Cavdar, KTH 25
Problem Size Reduction n A smarter solution can be obtained by only
considering the λsd variables that are 1, ¨ reduces the number of λsd variables from 90 to 40. ¨ eliminates all of the third set of equations. ¨ reduces the number of Fsd
ij variables to 40 × 30 = 1,200.
n This approach is specific to the particular instance of lightpaths that need to be set up.
n It only takes into account the lightpaths that need to be established.
Optical Networks, RWA, Cicek Cavdar, KTH 26
Problem Size Reduction – Routing n Assume that a particular lightpath will not pass through
all of the ij links. n If we can determine the links which have a good
probability of being in the path through which a lightpath may pass, we can only consider those links as the Fsd
ij variables for that particular sd pair.
n Thus, if on an average, a lightpath sd passes through seven links, there will be approximately 40 × 7 = 280 Fsd
ij variables.
n We can find a set of alternate, shortest paths between a given source-destination pair.
n The links constituting these alternate paths can then be used as part of the LP formulation.
Optical Networks, RWA, Cicek Cavdar, KTH 27
Problem Size Reduction - Relaxation n We can relax the integrality constraints, getting rid of all of
fourth set of equations. n Using this approach, we will be left with a total of 351
equations. ¨ 1 objective function ¨ 30 instances of first set ¨ 320 instances of second set.
n Since there are on an average of seven links considered per connection, we need to enumerate equations in the second set for ¨ eight nodes (6 intermediate and 2 end nodes) per connection, ¨ for each of 40 connections.
n Hence, using knowledge which is specific to a particular set of lightpaths, we can drastically reduce the size of the LP problem formulation and make it tractable for large networks.
Optical Networks, RWA, Cicek Cavdar, KTH 28
Randomized Rounding n Used to construct a good solution to the original ILP
using the information derived from the solution of the relaxed problem.
n The randomized rounding technique is applicable to a class of 0-1 ILPs.
n The technique is probabilistic. ¨ With high probability, the algorithm will provide an integer
solution in which the objective function takes on a value close to the optimum of the relaxation.
n The optimal value of the objective function in the relaxed version is better than the optimal value of the objective function in the original 0-1 integer program.
n This technique has been effectively used in multi-commodity flow problems.
Optical Networks, RWA, Cicek Cavdar, KTH 29
Multi-commodity Flow n In a general multi-commodity flow problem, we are given:
¨ an undirected graph G(V,E) ¨ k commodities that need to be routed.
n In an instance of the problem, various vertices are the sites of source and sink for a particular commodity.
n One unit of flow is to be conveyed from each source s to each destination d through the edges in E.
n Each edge e has a capacity c(e) ¨ it is an upper limit on the total amount of flow in E.
n The flow of each commodity in each edge must be 0 or 1. n The objective is to minimize the common capacity in each link. n The general integral problem is known to be NP-complete. n The non-integral version can be solved using linear programming
methods in polynomial time.
Optical Networks, RWA, Cicek Cavdar, KTH 30
Multi-commodity Flow n In the formulation of the problem:
¨ Each commodity corresponds to a lightpath from a source node to a destination node.
¨ The capacity is the number of wavelengths supported in each fiber.
¨ The objective function is to minimize the number of wavelengths needed to accommodate all of the requests.
n The algorithm consists of the following three phases: ¨ solving a non-integral multi-commodity flow problem ¨ path stripping ¨ randomized path selection
Optical Networks, RWA, Cicek Cavdar, KTH 31
Non-integral Multi-commodity Flow n We relax the requirement of the 0-1 flows to allow
fractional flows in the interval [0,1]. n The relaxed capacity minimization problem can be
solved by a linear programming method. n If the flow for each commodity i on edge e is fi(e),
¨ a capacity constraint of the form:
is satisfied for each edge in the network. ¨ C is the optimal solution to the non-integral optimization problem.
n The value of C is a lower bound on the best possible integral solution.
fi (e)i=1
k
∑ ≤C
Optical Networks, RWA, Cicek Cavdar, KTH 32
Path Stripping n The main idea is to convert the edge flows for
each commodity i into a set τi of possible paths which could be used to route the flow of that commodity.
n Initially, τi is empty. n For each commodity i do:
A. Discover a loop-free, depth-first, directed path e1, e2, . . . , ep from the source to the destination.
B. Let fm = min fi(ej ), where 1 ≤ j ≤ p. n For 1 ≤ j ≤ p, replace fi (ej) by fi(ej) − fm. n Add the path e1, e2, . . . , ep to τi along with its weight fm.
Optical Networks, RWA, Cicek Cavdar, KTH 33
Path Stripping C. Remove any edges with zero flow from the set of
edges that carry any flow for commodity i. n If there is nonzero flow leaving si, repeat Step B. n Otherwise, continue for next commodity i.
n Upon termination, the sum of the weights of all the paths in τi is 1.
n Path stripping gives us a set of paths τi that may carry the flow for commodity i in the optimal case.
Optical Networks, RWA, Cicek Cavdar, KTH 34
Randomized Path Selection n For each i, cast a |τi| die with face probabilities
equal to the weights of the paths in τi. n Assign the path whose face comes up, to
commodity i. n As a summary:
¨ The formulation of the problem allows the Fsdij
variables to take on fractional values. ¨ These values are used to find the fractional flow
through each of a set of alternate paths. ¨ A coin-tossing experiment is used to select the path
over which to route the lightpath λsd based on the probability of the individual paths.
Optical Networks, RWA, Cicek Cavdar, KTH 35
Assigning Wavelengths n Once a path has been chosen for each connection, we
need to assign wavelengths to each lightpath. ¨ any two lightpaths that pass through the same physical link are
assigned different wavelengths.
n If the intermediate switches do not have the capability to perform wavelength conversion, a lightpath is constrained to operate on the same wavelength throughout its path.
n Assigning wavelength colors to different lightpaths, to minimize the number of wavelengths under the wavelength-continuity constraint, reduces to the graph coloring problem.
Optical Networks, RWA, Cicek Cavdar, KTH 36
Graph Coloring n Construct a graph G(V,E)
¨ Each lightpath in the system is represented by a node in graph G.
¨ There is an undirected edge between two nodes if: corresponding lightpaths pass through a common physical fiber link.
n Color the nodes of the graph G such that no two adjacent nodes have the same color.
n This problem has been shown to be NP-complete n The minimum number of colors needed to color a graph
G is difficult to determine. n However, there are efficient sequential graph coloring
algorithms.
Optical Networks, RWA, Cicek Cavdar, KTH 37
Physical Topology for Simulations n A randomly-generated physical topology:
¨ 100 nodes ¨ Each node having a physical nodal degree uniformly
distributed between two and five. ¨ All links are unidirectional ¨ 357 directed links ¨ There are enough transceivers at the access nodes to
accommodate all of the lightpath requests that need to be established.
n No lightpath request will be blocked due to lack of transceivers at the access nodes.
Optical Networks, RWA, Cicek Cavdar, KTH 38
Traffic Model n A set of lightpaths to be established between
randomly-chosen source-destination (sd) pairs ¨ An sd pair can have zero or one lightpath, and all sd
pairs are treated equally. n Associated with each node, we identify d:
¨ the average number of lightpath connections the nodes will source.
n Thus, in an N-node network, the probability that a node will have a lightpath with each of the remaining (N − 1) nodes equals d/(N − 1).
n d is the average “logical degree” of a node.
Optical Networks, RWA, Cicek Cavdar, KTH 39
Solving LP n To reduce the size of the LP formulation, consider a set
of K alternate, shortest paths between a given sd pair. n Only the links which constitute these alternate paths are
used as the Fsdij variables.
n The LP formulation result (Fsdij values) is used as input
for the randomized rounding algorithm. n The value of the objective function denotes the lower
bound on congestion that can be achieved by any RWA algorithm.
n The individual flow variables are used in the path-stripping technique and the randomization technique to assign physical routes for the different lightpaths.
n Once this procedure is completed, the congestion on the different links in the network is obtained.
Optical Networks, RWA, Cicek Cavdar, KTH 40
Results
Optical Networks, RWA, Cicek Cavdar, KTH 41
Results n The maximum network congestion gives the
number of wavelengths we would need, if the intermediate switching nodes were equipped with wavelength converters.
n The time taken to solve the LP increases rapidly as the number of connections increase (corresponding to larger problem formulations).
n The table entries which are nil correspond to the case when the LP solver failed to give a solution.
Optical Networks, RWA, Cicek Cavdar, KTH 42
Outline n Introduction to concepts: Lightpath, connection, virtual
topology, physical topology.. n Static RWA
¨ ILP design to solve the routing problem ¨ Graph coloring to solve the wavelength assignment problem
n Dynamic RWA: ¨ Routing:
n Fixed routing, Fixed-alternate routing, Adaptive routing ¨ Wavelength assignment
n Random, First-fit, Least-used, Most-used,...
n Routing and flow assignment algorithms: ¨ Dijkstra, Ford-Fulkerson, Minimum spanning tree: Prim.
Optical Networks, RWA, Cicek Cavdar, KTH 43
Routing Algorithms for dynamic RWA
n In case of dynamic traffic, LP is not an option. n Heuristic methods are used to solve the dynamic
dist[v] = infinity ; // distance vector from the source node
previous[v] = NULL ; // pointer to the previous node in sh-path
endfor}
dist[s] = 0 ; // distance from root to itself
U = V ; // all nodes are set to not visited
while (U is not empty) do {
u = vertex in U with min dist[] ; // node with min dist from the root
if (dist[u] = infinity) do{ // the root is disconnected
break ;
endif}
remove u from U // mark node u as visited
for (each neighbor v in U of u) do { // where v belongs to U,
temp = dist[u] + distance(u, v) ;
if (temp < dist[v]) do { //update the distances
dist[v] = temp ;
previous[v] = u ;
endif}
endfor}
end while}
endDijkstra}
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Dijkstra Example
n One Dijkstra interactive example available at: ¨ http://www-b2.is.tokushima-u.ac.jp/
~ikeda/suuri/dijkstra/DijkstraApp.shtml?demo1
Optical Networks, RWA, Cicek Cavdar, KTH
61"
Dijkstra Algorithm Applications n Link weights in Dijkstra algorithm can
be assigned according to the parameter that needs to be optimized. ¨ Distance (for shortest path) ¨ Delay (for minimum delay) ¨ Power consumption (for minimum power) ¨ Reliability (for maximum reliable path) ¨ Load (for load balancing)
Optical Networks, RWA, Cicek Cavdar, KTH
Minimum spanning tree n There are two solutions:
¨ Prim ¨ Kruskal
n See following for an example and the pseudo-code: ¨ http://www-b2.is.tokushima-u.ac.jp/~ikeda/suuri/
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