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Research Issues on Routing and Wavelength Assignment for Wavelength Routed WDM Networks. Qualifying Exam. Hsu-Chen, Cheng PhD. Student Department of Information Management National Taiwan University 10/27/2003. Outline. Introduction of Optical Networks WDM Technology

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Research Issues on Routing and Wavelength Assignment for Wavelength Routed WDM Networks

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Research Issues on Routing and Wavelength Assignment for Wavelength Routed WDM Networks

Qualifying Exam

Hsu-Chen, Cheng

PhD. Student

Department of Information Management

National Taiwan University



  • Introduction of Optical Networks

    • WDM Technology

    • Optical Network Control Plane

    • IP/WDM Traffic Engineering

  • Optical Network Design and Engineering

    • Routing and Wavelength Assignment (RWA)

    • Heuristics

  • Optical Multicasting

  • Multi-granularity Architecture of Optical Network

  • Future Research Direction

The Trend of Network Technology

Introduction of Optical Networks

Optical Networks

  • First Generation:

    • FDDI

    • Gigabit Ethernet

  • Second Generation:

    • WDM Local Area Network

      • Passive Star Network

      • Single-Hop WDM Network

    • WDM Wide Area Network

      • Wavelength routed Network

      • OBS

      • OPS

  • Wavelength-routed optical networks are the most promising candidates for backbone high-speed WAN.

Introduction of Optical Networks

Optical Network Technologies

  • WDM Technology

    • Fixed Point-to-Point  Wavelength routed

    • 300 λ x 40Gbps

  • Optical Components

    • OADM (Optical add/drop multiplexer)

    • OXC (Optical cross-connect)

    • Tunable Laser

    • Amplifier

    • Wavelength Converter

    • Wavelength Splitter

Introduction of Optical Networks

The Architecture of Wavelength Router

Introduction of Optical Networks

Physical Topology and Logical Topology

  • A physical topology is a graph representing the physical interconnection of the wavelength routing nodes by means of fiber-optic cables.

  • A logical topology is a directed graph that results when the lightpaths are setup by suitably configuring the wavelength routing nodes.

Optical Network Design and Engineering

The Architecture of Wavelength-routed Optical Network

Introduction of Optical Networks

Optical Network Control Plane

  • Apparatus

    • IETF (Internet Engineering Task Force)

    • ODSI (Optical Domain Service Interconnection)

    • OIF (Optical Internetworking Forum)

  • Issues

    • Signaling Mechanism (UNI)

    • Signaling and control protocol for dynamic lightpath establishment and traffic engineering

Introduction of Optical Networks

Optical Network Control Plane (cont’d)


  • CR-LDP

  • Neighbor discovery

  • Link Monitoring

  • State distribution

  • LMP [J. P. Lang 2001]

Link-state routing protocol (OSPF)

Introduction of Optical Networks

IP/WDM Traffic Engineering

  • Overlay Model

    • Closer to classical IP and ARP over ATM scheme

    • The IP and optical network are independent of each other

    • Edge IP router interacts with its ingress OXC over a well-defined UNI

  • Peer Model

    • The IP and optical network are treated together as a single network

  • Augmented Model

    • IP and optical networks use separate routing protocol, but information from one routing protocol is passed through the other routing protocol

G. N. Rouskas and H. G. Perros, A Tutorial on Optical Networks, Networking 2002Tutorials, vol. 2497, 2002, pp. 155-193.

Introduction of Optical Networks

TE Model

  • WDM Traffic Engineering Model

    • Minimum average packet delay

    • Maximize scale up capability

  • MPLS Traffic Engineering Model

    • Overlay model

    • Virtual topology (LSPs)

  • IP over WDM Traffic Engineering Model

    • Virtual topology design

    • Routing and wavelength assignment

Introduction of Optical Networks

RWA constraints

  • Wavelength continuity constraint

  • Distinct wavelength constraint

Optical Network Design and Engineering

Wavelength converters translate wavelength fi to fk.

They may be used as components of the wavelength routing nodes.

Wavelength Conversion

Optical Network Design and Engineering

RWA Problem

  • Static RWA

    • Topology Subproblem

    • Lightpath routing subproblem

    • Wavelength assignment subproblem

    • Traffic routing subproblem

  • Dynamic RWA

    • Route Computation

    • Wavelength Assignment

Optical Network Design and Engineering

Physical Topology Design

  • This step includes –

    • Sizing the links (no. of wavelength channel, capability of each channel)

    • Sizing the OXCs

    • Placement of resources (Amplifiers, Converters, Splitters)

    • Dealing with link or OXC failures

  • Placement of converters [J. Iness, 1999]

    • Sparse location of wavelength converters

    • Sharing of converters (Nodal Design)

    • Limited-range wavelength conversion [R. Ramaswami and G. H. Sasaki, 1998]

Static RWA

Virtual Topology Design

  • A solution to the static RWA problem consists of a set of long-lived ligthpaths which create a logical topology among the edges node.

  • It is not possible to implement fully connected virtual topologies.

    • N(N-1) lightpaths

  • Objective

    • Minimize the maximum congestion level

    • Minimize the average weighted number of hops

    • Minimize the average packet delay

Static RWA

Wavelength Assignment

  • Graph-coloring problem

  • NP-complete

  • Sequential graph-coloring algorithms

Static RWA

Route Computation

  • Static algorithm and adaptive algorithm

  • Lightpath routing

    • Constraint algorithm

      • Fixed routing algorithm

      • Fixed alternative algorithm

    • Adaptive unconstrained routing (AUR)

    • Hybrid routing

      • Least to most congested

  • AUR has better improvement on call blocking probability[A. Mokhtar and Murat Azizoğlu, 1998]

Dynamic RWA

Candidate Paths

  • The candidate paths for a request are considered in increasing order of path length (or path cost).

  • Path length is defined as the sum of the weights assigned to each physical link along the path.

    • K-minimum hop paths

    • K-minimum distance paths

  • K-shortest path

    • Pair-wise link disjoint

    • Dijkstra algorithm

  • Physical constraints (attenuation, dispersion et al.)

    • Constraint-based shortest path first algorithm [B. Davie, 2000]

Dynamic RWA

Wavelength Assignment

  • [H. Zanf et al., 2000]

    • Random Wavelength Assignment (R)

    • First-Fit (FF)

    • Least-Used (LU)

    • Most-Used (MU)

    • Min-Product (MP)

    • Least-Loaded (LL)

    • MAX-SUM (MΣ)

    • Relative Capacity Loss (RCL)

    • Wavelength Reservation (Rsv)

    • Protecting Threshold (Thr)



Dynamic RWA

General Model of Virtual Topology Design

  • Mixed Integer Linear Programming

    • [B. Mukherjee et al., 1994]

    • [B. Mukherjee et al., 1996]

    • [R.Ramaswami and K. N. Sivarajan, 1996]

    • [R. Krishnaswamy and K. N. Sivarajan, 1998]

    • [R. Krishnaswamy and K. N. Sivarajan, 2001]

  • Objective

General Model


  • Congestion may be viewed as a function of the various parameters of the network such as

    • the traffic matrix,

    • number of wavelengths the fiber can support,

    • resources at each node (number of transmitters and receivers),

    • the hop lengths of the logical links,

    • the multiplicity restrictions on the logical topology,

    • the multiplicity restrictions on the physical topology,

    • symmetry/asymmetry restrictions,

    • the propagation delay.

General Model


  • The motivation for choosing this objective is that the electronic processing (switching speed) requirement is proportional to the congestion.

  • If the switching speeds at the nodes are limited, then minimizing congestion would be appropriate as it would enable the traffic carried per wavelength to increase.

  • If there is heavy traffic between some source–destination pair, then there is a logical link between them; this is a desirable property.

  • This happens because of the objective function, i.e., if there is heavy traffic between node i and node j then because of the objective there would tend to be an edge in the logical topology.

General Model


s,d source and destination of a packet, when used as superscripts;

i,j originating and terminating node of a logical link (lightpath);

l,m endpoints of a physical link;

k wavelength number, when used as a superscript.

General Model


General Model


General Model

Constraints- Degree Constraints

  • The above constraint ensures that the number of logical links originating (out-degree) and terminating (in-degree) at node is less than or equal to the number of transmitters and receivers at that node.

General Model

Constraints- Traffic Constraints

Traffic routing constraints

Flow Conservation

  • The above first two equations ensure that the load on any logical link is no greater than the maximum load, which is being minimized.

General Model

Constraints- Wavelength Continuity Constraints

Unique wavelength constraints:

  • This ensures that if logical link exists, then only one wavelength is assigned to it, among the possible choices.

  • This equation ensures that only those could be nonzero for which the corresponding variables are nonzero.

General Model

Constraints- Wavelength Continuity Constraints

Wavelength clash constraints

Conservation of wavelength constraints

  • Let logical link bijuse wavelength k. Then by conservation of wavelength constraints there is a path in the physical topology from node i to node j with wavelength assigned to it.

General Model

Constraints- Hop Bound Constraints

Hop bound Constraints

General Model


  • Subproblems:

    • Topology Subproblem: bij

    • Lightpath routing subproblem:

    • Wavelength assignment subproblem:

    • Traffic routing subproblem:

  • Some of above subproblem are NP-hard.

  • Solving the subproblems in sequence and combining the solutions may not result in the optimal solution for the full integrated problem.

RWA Heuristics

Lower Bound on Congestion

  • Physical topology independent bound (p.43)

  • Minimum flow tree bound (MFT) (p.43)

  • Iterative LP-relaxation bound (p.43)

    • Aggregate formulation

    • Cutting Plane

  • Independent topologies bound (p.43)

    • Uniform traffic: 5% - 10% tighter than the bound obtained from MFT

    • Nonuniform traffic: up to 50% higher than MFT bound

RWA Heuristics

Lower Bound on the Number of Wavelength

  • Physical topology degree bound (p.45)

    • Derived from the Simple consideration that he node with the minimum physical degree.

  • Physical topology link bound (p.46)

    • Assuming that each node sources lightpaths to exactly those node it can be reach with the minimum number of hops.

  • Lagrangian relaxation bound

    • 顏宏旭 (2001)

RWA Heuristics

Regular Topologies

  • Regular topologies such as hypercube have several advantages as virtual topologies.

  • They are well understood, and results regarding bounds and averages are comparatively easier to derive.

  • Two subproblem

    • Node Mapping Subproblem

    • Path Mapping Subproblem


Pre-specified Topologies

  • The topology in terms of a list of lightpaths with their source and destination nodes is supposed to be given for each instance of problem.

  • The lightpath routing and wavelength assignment subproblems can be viewd as having goals defined purely in terms of lightpaths.

  • Static Lightpath Establishment problem [I. Chlamtac, 1995]

  • Maximize wavelength utilization [C. Chen, 1995]

  • Randomized rounding and graph coloring [D. Banerjee 1996]


Arbitrary Topologies

  • [R. Ramaswami, 1996]

    • HLDA (Heuristic topology design algorithm)

    • MLDA (Minimum-delay logical topology design algorithm)

    • TILDA (Traffic Independent logical topology design algorithm)

    • LPLDA (LP-relaxation logical topology design algorithm)

    • RLDA (Random logical topology design algorithm)

  • [D. Banerjee, 2000]

    • Maximizing Single-Hop Traffic

    • Maximizing Multihop Traffic


Optical Multicasting

  • Multicasting at IP layer

Optical Multicasting

Optical Multicasting

  • Multicasting by Lightpaths

Optical Multicasting

Optical Multicasting

  • Multicasting at WDM Layer (Light Tree Architecture)

Optical Multicasting

Light Tree Architecture

  • Application

    • Optical multicast

    • Enhanced virtual connectivity

    • Traffic grooming

  • Steiner Tree Problem

    • Shortest path-based heuristic (SPH)

    • Spanning tree-based heuristic (STH)

    • Metaheuristics

Optical Multicasting

WDM Multicast Mode

  • [Y. Yang et al., 2000]

    • Multicast with same wavelength (MSW)

    • Multicast with same destination wavelength model (MSDW)

    • Multicast with any wavelength model (MAW)

Optical Multicasting


  • MC-RWA bears many similarities to the RWA problem.

  • The tight coupling routing and wavelength assignment remains and becomes stronger.

  • Physical topology design problem

    • Resources placement

    • [M. Ali and J. Deogun, 2000]

  • Virtual topology design problem

    • Minimize call blocking probability

    • Minimize the number of transceiver needed

    • Minimize average packet hop distance

Optical Multicasting

MC-RWA Researches (1/2)

  • [L. H. Sahasrabuddhe, 1999]

    • An optimum light tree-based virtual topology has a lower value of average packet hop distance than that of an optimum lightpath-based virtual topology

    • An optimum light tree-based virtual topology requires fewer opto-electronic components

  • Light forest [X. Zhang et al., 2000]

    • Reroute-to-Source

    • Reroute-to-Any

    • Member-First

    • Member-Only

Optical Multicasting

MC-RWA Researches (2/2)

  • The USCH1 algorithm gives the worst network throughput

  • The wavelength continuity constraint limits the performance of the MSCH1 algorithm

  • The best approach is the MSCH2 if the wavelength converters are not available

  • [Y. Sun et al., 2001]

Optical Multicasting

Super-Lightpath RWA

  • WDM + OTDM

Optical Multicasting

Tree Shared Multicast

  • [D. N. Yang and W. Liao, 2003]

    • A light tree can carry data of multiple multicast streams, and data of a multicast stream may traverse multiple light-trees to reach a receiver.

    • Multicast routing and wavelength assignment of light-trees

    • Design of light-tree based logical topology for multicast streams

  • [郭至鈞,民國92]

    • Multicasting group aggregation and MC-RWA

    • Source-based tree aggregation

    • Maximize the total revenue

    • Lagrangian relaxation

Optical Multicasting

Tree-Sharing Strategies

  • Given the set Mi at edge router i, we consider a strategy to decompose Mi into a number of MSCs (Multicast Sharing Class)

  • Perfect Overlap (PO), Super Overlap (SO), and Arbitrary Overlap (AO)

Optical Multicasting

Optical Waveband Switch

  • WBS has attracted attention for its practical importance in reducing port count, associates control complexity, and cost of photonic cross-connect.

  • In WBS networks, several wavelengths are grouped together as a band and switch as single entity using single port.

  • MG-OXC not only switch traffic at multiple granularities such as fiber, band, and wavelength, but also add and drop traffic at multiple granularities .

Multi-granularity Optical Networks

Multilayer MG-OXC

Multi-granularity Optical Networks

Single Layer MG-OXC

Multi-granularity Optical Networks

Lightpath Grouping Strategy

  • End-to-end grouping:

    • Grouping the traffic (lightpaths) with same source-destination only

  • One-end grouping:

    • Grouping the traffic between the same source (or destination) nodes and different destination (or source) nodes

  • Subpath grouping:

    • Grouping traffic with common subpath (from any source to any destination)

Multi-granularity Optical Networks


  • WRN

    • Minimum the number of wavelengths

    • Minimum wavelength hops

  • WBS

    • Minimum the number of ports

  • Waveband conversion

Multi-granularity Optical Networks

WBS Failure Recovery

  • Band Merging

  • Band Swapping

Multi-granularity Optical Networks

Hierarchical Routing Model

  • Network node architecture

  • Sequence of routing and waveband aggregation

  • Route Computation

Multi-granularity Optical Networks

Researches of M. Lee et al.

  • Multi-Layer MG-OXC

  • The waveband is formed by grouping lights with the same destination in a network

  • ILP formulation

  • Maximize the reduction gain of crossconnect size with the minimum number of wavelengths

  • Results

    • The introduction of waveband leads to a very large reduction in crossconnect requirements for large-scale networks.

    • A large reduction of crossconnect requirements can still be expected even at nonoptimal wavelength granularity.

    • The reduction depends on network topology, traffic demand and traffic pattern.

Multi-granularity Optical Networks

Researches of Y.Suemura et al.

  • Propose and analyze two heuristic routing and

    aggregation algorithms (online and offline) to be used for homogeneous networks in separate routing framework.

  • Minimum the routing cost

    • Assume that all the ports (OEO and optical ones) have the same cost.

    • The cost of routing is the total number of used ports.

  • The simulations demonstrate a significant cost reduction by employing hierarchical routing (from 33% in online algorithm to almost 60% in offline one)

Multi-granularity Optical Networks

Researches of X. Cao et al.

  • This research show that WBS is different from traditional wavelength, and thus techniques developed for wavelength-routed networks cannot be directly applied to effectively WBS-related problem.

  • The objective is to route lightpaths and assign appropriate wavelength to them so as the minimum the total number of prots required by the MG-OXCs.

  • Static offline problem (Network Planning)

    • Balanced Path Routing with Heavy Traffic (BPHT)

  • Dynamic real-time problem (Network Servicing)

    • Maximum Overlap Ratio (MOR)

  • Results

    • BPHT: 50% fewer total ports than using ordinary OXCs

    • MOR: 35 % saving in the number of ports

Multi-granularity Optical Networks

Researches of P. Ho et al.

  • Dynamic tunnel allocation (DTA)

    • Use fixed alternative routing with k-shortest paths to inspect networks along each alternative path for dynamically setting up lightpaths.

  • Capacity-balanced static tunnel allocation (CB-STA)

    • Fiber and waveband tunnels are allocated into networks at the planning stage according to weighted network link-state.

  • Simulation Results

    • DTA is outperformed by CB-STA in the same network environment duo to a well-disciplined approach for allocating tunnels with CB-STA.

    • The mix of the two approaches yields the best performance given the same network environment apparatus.

Multi-granularity Optical Networks

Future Research Topics

  • New optical component application

  • Optical Multicasting

    • QoS Multicasting

    • Tree Aggregation Problem

    • Call Admission Control

  • WBS

    • Converter and Splitter Placement

    • Waveband Multicasting

    • Heterogeneous WBS Network

    • Survivability

  • Optical Packet Switch and Optical Burst Switch

  • Passive Optical Network- EPON and GPON

  • After the Optical Bubble?

Q & A

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