Lecture: 4 WDM Networks Design & Operation
Ajmal Muhammad, Robert Forchheimer
Information Coding Group
ISY Department
Outline
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Key Terminology in WDM Optical Network
Different Core Network Topologies
Designing Network Nodes
Categorizations of WDM Networks
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Routing and Wavelength Assignment (RWA)
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Wavelength-routed and broadcast-and-select
Static and dynamic
Static, dynamic
Grooming
Optical core Networks
Key Terminology in WDM Optical
Networks
Optical node/cross-connect/switch/router
Optical node has a number of input (output) fibers, each carrying
one or more incoming (outgoing) optical signals
The purpose of which is to direct each incoming optical signal to
an appropriate outgoing fiber
End nodes: all possible sources or destinations of data
Physical
topology:
graph showing the
components (i.e., fibers, nodes) of the network
major
physical
Key Terminology….
Lightpath: optical connection from one end node to another,
used to carry data in the form of encoded optical signals
Logical/Virtual topology: graph whose nodes indicate the end
nodes and edges as lightpaths
From E1 to E3
From E2 to E4
From E1 to E2
From E3 to E4
From E4 to E1
Physical topology of WDM network with four end
nodes E1,..,E4, and four optical routers R1,..,R4
Lightpaths on physical topology
Logical/Virtual Topology
From E1 to E3
From E2 to E4
From E1 to E2
From E3 to E4
From E4 to E1
Lightpaths on physical topology
Corresponding logical topology
Topologies for core Networks
National scientific foundation (NSF) network
Optical cross-connect
Topologies for core Networks
14 nodes, 21 bidirectional links
European optical network topology
German network topology
Designing Network Node
Example
4 input and output fibers
32 wavelengths on each fiber
Design the node such that
 4 signals can be
dropped/added
 Wavelengths are
added/dropped through
tunable transponders
14 nodes, 21 bidirectional links
Designing Network Node
4 Nos. of 1x32 DMUX
4 Nos. of 32x1 MUX
32 Nos. of 8x8 optical switch
1 144x144 optical switch
16 Nos. of transponder
14 nodes, 21 bidirectional links
Constructing a Large Switch from
Smaller Switches
4 wavelength channels in fiber
Optical add-drop multiplexer (OADM)
constructed from MUX, DEMUX, a 6x6
optical switch, and 2 tunable
transponders
How to construct an OADM with the same functionality by using 4x4 switches ?
First Method
Constructing an OADM using 4x4 switches
4 wavelength channels in fiber
Second Method
Constructing an OADM using 4x4 switches
4 wavelength channels in fiber
Categorizations of WDM Networks
Wavelength-routed and Broadcast-and-select networks
Wavelength-routed
– optical signal is sent along a specified path and not
broadcast to all nodes in the network
Broadcast-and-select
– source end node selects an appropriate
wavelength and broadcasts the data to be transmitted to all end nodes in the
network
Static and Dynamic lightpath allocation
Static
– once the lightpaths are set-up between the ordered pairs of the end
nodes, they will continue to exist for a relatively long period of time (months or
years)
Dynamic
– set-up on demand and, when the communication is over, the
corresponding lightpath is taken down (i.e., no longer remain operational)
Categorizations of WDM….
Single-hop and Multi-hop WDM networks
Single-hop–
all data communication involves a path length of one
logical edge, i.e., one lightpath is involved in each communication
Single-hop networks are also called all-optical networks
Multi-hop
– some data communication involves more than one
lightpath
Multi-hop network
Single-hop network
Static Routing and Wavelength
Assignment (RWA)
Assumption: The amount of traffic for each source-destination pair is in
wavelength units
Traffic Model: Set of lightpaths to be established in the network is
known in advance
Constraint: Any two lightpaths sharing the same physical link are
assigned different wavelengths
Objective: Establish a set of lightpaths in such away to minimize the
number of wavelengths used in the network
Application: Static RWA problem arises naturally in the design and
capacity planning of an optical network
Static RWA
Decompose into two sub-problems
Routing
 Fixed routing
 Alternate routing
 Adaptive routing
Wavelength assignment (WA)
 Random WA
 First-fit
 Least-used/SPREAD
 Most-used/PACK
WA :: Graph Coloring Problem
Problem can be reduced to graph coloring
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Construct a graph G where nodes represents lightpaths, an edge
exists between two nodes if the corresponding lightpaths pass
through a common physical link
Color the nodes in G such that no two adjacent nodes have the
same color
2
3
1
4
6
5
Network with eight routed-lightpaths
Auxiliary graph for the
lightpaths in the network
Static RWA :: a Layered Graph
Approach
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Route and assign wavelength to each connection one by
one
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Use layered graph to deal with wavelength continuity
constraint
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Create W copies of the network graph, W = number of
wavelengths in a fiber
RWA is solved by finding a path in one copy of the network
graph
Limited/fixed conversion: add links between layers
19
Static RWA with Wavelength
Conversion
If each node has full wavelength conversion capability
 Only need solve routing problem
 Minimizing the maximum flow will minimize the
number of wavelengths used
Dynamic RWA
Traffic Model: Service requests arrive to and depart from the network
dynamically in a random manner
Constraint: Any two lightpaths sharing the same physical link are
assigned different wavelengths
Objective: Route and assign wavelengths in such a way as to
minimize the blocking probability of the network
Application: Dynamic RWA problem is encountered during the realtime network operational performance of the optical networks
Dynamic RWA :: Assumptions
Each service request or call needs one wavelength units of
transmission rate
Service requests arrivals for source-destination pair form a Poisson
process
Source-destination pairs are uniformly distributed among all network
nodes
Each service request has the holding-time that is exponentially
distributed
Blocked calls are lost from the network; there is no reattempt
RWA :: In General
Sub-wavelength Traffic:: Traffic Grooming
So far we assume that each source-destination (s-d) pair has its traffic
demand equal to an integer multiple of wavelength unit
What if the traffic of an s-d equal to 0.3 wavelength unit ?
In this scenario, a single lightpath may carry multiple traffic streams
from different s-d pairs
Traffic grooming multiplexing several traffic streams onto a common
lightpath
Necessary for efficient wavelength channel usages
Traffic Grooming Strategies
Aim: Minimize electronic costs by reducing the number of add-drop
multiplexers (ADMs) and make efficient use of wavelengths
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Each ADM can multiplex several lower rate streams to form a higher
rate stream OR demultiplex a higher rate stream to several lower
rate ones
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Employs O-E-O conversion
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Works at a particular wavelength
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ADM works on a single wavelength, if there are W wavelengths,
every node would need N*W ADMs
Example
Network Topology
a) Physical Network
0
1
b) Traffic on the Network
t1
0
t5
1
t6
fiber
t2
t4
3
2
3
t3
2
Traffic Grooming Approach1 (Random)
Total number of ADMs needed = 8
Traffic Grooming Approach 2
Total number of ADMs needed = 7