lecture: 4 wdm networks design & operation
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Lecture: 4 WDM Networks Design & Operation
Ajmal Muhammad, Robert ForchheimerInformation Coding Group
ISY Department
Outline
Key Terminology in WDM Optical Network Different Core Network Topologies Designing Network Nodes Categorizations of WDM Networks
Wavelength-routed and broadcast-and-select Static and dynamic
Routing and Wavelength Assignment (RWA) 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 major physical components (i.e., fibers, nodes) of the network
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
Physical topology of WDM network with four end nodes E1,..,E4, and four optical routers R1,..,R4
Lightpaths on physical topology
From E1 to E3
From E2 to E4
From E1 to E2
From E3 to E4
From E4 to E1
Logical/Virtual Topology
Lightpaths on physical topology
From E1 to E3
From E2 to E4
From E1 to E2
From E3 to E4
From E4 to E1
Corresponding logical topology
Topologies for core Networks
National scientific foundation (NSF) networkOptical cross-connect
Topologies for core Networks
European optical network topology
14 nodes, 21 bidirectional links
German network topology
Designing Network NodeExample
14 nodes, 21 bidirectional links
4 input and output fibers
32 wavelengths on each fiber
Design the node such that4 signals can be dropped/addedWavelengths are added/dropped through tunable transponders
Designing Network Node
14 nodes, 21 bidirectional links
4 Nos. of 1x32 DMUX
4 Nos. of 32x1 MUX
32 Nos. of 8x8 optical switch
1 144x144 optical switch
16 Nos. of transponder
Constructing a Large Switch from Smaller Switches
Optical add-drop multiplexer (OADM)constructed from MUX, DEMUX, a 6x6 optical switch, and 2 tunable transponders
4 wavelength channels in fiber
How to construct an OADM with the same functionality by using 4x4 switches ?
First Method
4 wavelength channels in fiber
Constructing an OADM using 4x4 switches
Second Method
4 wavelength channels in fiber
Constructing an OADM using 4x4 switches
Categorizations of WDM Networks
Wavelength-routed and Broadcast-and-select networksWavelength-routed – optical signal is sent along a specified path and not broadcast to all nodes in the networkBroadcast-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 allocationStatic – 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 networksSingle-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 networksMulti-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-problemsRoutingFixed routingAlternate routingAdaptive routing
Wavelength assignment (WA)Random WAFirst-fitLeast-used/SPREADMost-used/PACK
WA :: Graph Coloring Problem
Problem can be reduced to graph coloring 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
1
2 3
4
56
Network with eight routed-lightpathsAuxiliary graph for thelightpaths in the network
19
Static RWA :: a Layered Graph Approach
Route and assign wavelength to each connection one by one
Use layered graph to deal with wavelength continuity constraint 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
Static RWA with Wavelength Conversion
If each node has full wavelength conversion capabilityOnly need solve routing problemMinimizing 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 real-time 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
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
Employs O-E-O conversion
Works at a particular wavelength
ADM works on a single wavelength, if there are W wavelengths, every
node would need N*W ADMs
ExampleExample
0 1
3 2
0 1
23
fiber
t1
t2
t3
t4
t5 t6
a) Physical Network b) Traffic on the Network
Network TopologyNetwork Topology
Traffic Grooming Approach1 (Random)Traffic Grooming Approach1 (Random)
Total number of ADMs needed = 8
Traffic Grooming Approach 2Traffic Grooming Approach 2
Total number of ADMs needed = 7
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