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