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Lecture: 4 WDM Networks Design & Operation Ajmal Muhammad, Robert Forchheimer Information Coding Group ISY Department

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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

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Optical core Networks

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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

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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 E 1,..,E 4, and four optical routers R 1,..,R 4 Lightpaths on physical topology From E 1 to E 3 From E 2 to E 4 From E 1 to E 2 From E 3 to E 4 From E 4 to E 1

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Logical/Virtual Topology Lightpaths on physical topology From E 1 to E 3 From E 2 to E 4 From E 1 to E 2 From E 3 to E 4 From E 4 to E 1 Corresponding logical topology

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Topologies for core Networks National scientific foundation (NSF) network Optical cross-connect

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Topologies for core Networks European optical network topology 14 nodes, 21 bidirectional links German network topology

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Designing Network Node Example 14 nodes, 21 bidirectional links 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

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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

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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 ?

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First Method 4 wavelength channels in fiber Constructing an OADM using 4x4 switches

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Second Method 4 wavelength channels in fiber Constructing an OADM using 4x4 switches

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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)

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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 networkSingle-hop network

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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

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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

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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 Network with eight routed-lightpaths Auxiliary graph for the lightpaths in the network

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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

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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

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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

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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

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RWA :: In General

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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

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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

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Example fiber t1 t2 t3 t4 t5t6 a) Physical Networkb) Traffic on the Network Network Topology

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Traffic Grooming Approach1 (Random) Total number of ADMs needed = 8

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Traffic Grooming Approach 2 Total number of ADMs needed = 7

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