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

2. 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

3. Optical core Networks

4. 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

5. 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

6. Logical/Virtual Topology
From E1 to E3
From E2 to E4
From E1 to E2
From E3 to E4
From E4 to E1
Corresponding logical topology
Lightpaths on physical topology

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

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

9. 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

10. 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

11. 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 ?

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

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

14. 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)

15. 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

16. 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

17. 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

18. 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 2 3 1 4 6 5
Network with eight routed-lightpaths
Auxiliary graph for the
lightpaths 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

20. 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

21. 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

22. 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

23. RWA :: In General

24. 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

25. 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

26. b) Traffic on the Network
Example
Network Topology
a) Physical Network
b) Traffic on the Network t1 1 1 t5 t6
fiber t2 t4 hi 3 2 3 2 t3

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

28. Traffic Grooming Approach 2
Total number of ADMs needed = 7