Presented by: Brian V. Jarvis Wright State University: CEG790-01 Winter 2004, 2 February 2004

Protection Mechanisms for Optical WDM Networks based on Wavelength Converter Multiplexing and Backup Path Relocation Techniques

Protection Mechanisms for Optical WDM Networksbased on Wavelength Converter Multiplexing and

Backup Path Relocation Techniques

Presented by: Brian V. JarvisWright State University: CEG790-01

Winter 2004, 2 February [email protected]

Sunil Gowda, Expedia.com, Seattle, WAKrishna M. Sivalingam, Univ. of Maryland, Baltimore County, MD

IEEE INFOCOM 2003

Monday, February 02, 2004 2


• Introduction• Background• Proposed Mechanisms• Performance Analysis• Conclusions



Introduction• Exponential growth in bandwidth requirements• This paper considers routing algorithm designs for optical WDM networks

• Fault-tolerance (survivability) is an important consideration• Survivability is introduced into an optical network

• Three different mechanisms to help improve network performance• Conversion Free Primary Routing (CFPR)• Converter Multiplexing Technique (CMT)• Backup Path Relocation Scheme (BPRS)

• Results indicate that the proposed algorithms:• Reduced the required number of wavelength converters at each node by 75%.• Reduced the overall blocking probability from 60% to nearly 100%.• Reduced the number of primary paths undergoing conversion.• Reduced overall network blocking probability by up to an order of magnitude.• Provided higher improvements for low to moderate loads.• All with negligible additional overhead.




• Present relevant background material on wavelength division multiplexing (WDM) networks, routing and wavelength assignment (RWA) algorithms, protection techniques, and wavelength router architectures that incorporate conversion.



Background• Routing and Wavelength Assignment (RWA) Problem

• A dynamic network model where connection requests arrive randomly.• Determining the end-to-end route and the specific wavelength

• Static or dynamic routing scheme approaches• Static routing scheme• Dynamic routing scheme (this technique is used in this paper)



Background• Wavelength Router Architecture

• Without wavelength conversion, connections are often blocked• With wavelength conversion, the lightpath can use different wavelengths• High cost, lack of availability, signal degradation

• Various optical wavelength conversion techniques• Minimize the usage of wavelength conversion• Reduce the number of converters required

• Location of wavelength converters

• Three architectures for a wavelength convertible switch• Dedicated• Share-per-link• Share-per-node



Dedicated Wavelength Convertible Switch Architecture

WC

WC

WC

WC

WC

WC

WC

WC

De-Multiplexers Multiplexers

Input Links

OptIcal

Switch

Output Links

• A wavelength converter is available at each output port.

• The incoming optical signal is de-multiplexed into separate wavelengths.• The output signal may have its wavelength changed.• The various wavelengths are multiplexed.



Share-per-node Wavelength Convertible Switch Architecture


Input Links

OptIcal

Switch

Output LinksWCB

OSW

• A wavelength converter bank (WCB) is available at the optical switch.

• The incoming optical signal is de-multiplexed into separate wavelengths.• Only the wavelengths that require conversion are directed to the WCB.• Converted wavelengths are switched to the appropriate outbound fiber link.• The various wavelengths are multiplexed.



Share-per-link Wavelength Convertible Switch Architecture

WCB


Input Links

OptIcal

Switch

Output Links

WCB

• A wavelength converter bank (WCB) is available at each outgoing fiber link.

• The incoming optical signal is de-multiplexed into separate wavelengths.• The optical switch is configured to direct wavelengths toward a particular link.• Only the wavelengths that require conversion are directed to the WCB.• The various wavelengths are multiplexed.



Protection in Mesh-Topology WDM Networks• Link Failures• Node Failures

Recovery Mechanisms• Protection (proactive)

• Backup lightpaths are identified and resources are reserved along the backup lightpaths at the time of establishing the primary lightpath itself.

• Faster recovery• 100 percent restoration guarantee

• Restoration (reactive)• When an existing lightpath fails, a search is initiated to find a new lightpath which does not use

the failed components. (After the failure happens)• Longer restoration time• It can not guarantee successful recovery,



Lightpath Migration• Migration of lightpath onto new paths to accommodate other connections.• A virtual topology reconfiguration scheme to adapt to the changing traffic patterns

– modeled as an integrated linear programming (ILP) formulation

• Lightpaths are torn down and re-established, providing better paths for the primary.• During reconfiguration, transmission on that path is terminated.• Other migration schemes studied. (Not presented here.)




• Presents the network architecture studied and the details of three proposed mechanisms.



Network Architecture• Dynamic routing; shortest path is computed between nodes based on current situation.• Path level protection; with dedicated and shared protection schemes for backup paths.• Wavelength router architecture is based on share-per-node wavelength converter configuration.

– Offers the best cost to performance ratio.• Connections are blocked only if free wavelength or wavelength converters are unavailable.

The design goals of the proposed mechanisms are to:• Improve performance• Increase cost effectiveness

Proposed mechanisms:• Conversion-Free Primary Routing (CFPR)• Converter Multiplexing Technique (CMT)• Backup-Path Relocation (BPR)



Conversion Free Primary Routing (CFPR)• The goal is to avoid wavelength conversions while routing primary connections.

• Multi-layered graph is used, the layers representing individual wavelength planes.

• CFPR algorithm models such a graph.• For each wavelength plane, the nodes are the physical nodes.

1

2 4

3 5

Wavelength 0

1

2 4

3 5

Wavelength 1

1

2 4

3 5

Wavelength 2

Backup path b

Blocked route

Unoccupied wavelengths//Wavelengths occupied by

Backup paths

Wavelengths occupied byPrimary paths



Potential primary path with some component linksoverlapping existing backup paths

• Advantages of CFPR:• Reduces conversion delays and

degradation due to converters• Reduces costs• Lower computational complexity• Computes path on each wavelength

separately; alternate paths are available if shorted paths are blocked

Conversion Free Primary Routing (CFPR)• An overlapping segment is defined as the part

of the backup path occupying the wavelength assigned to the requested connection.

• Relocation schemes may be used to relocate these overlapping segments.



Converter Multiplexing: • Based on backup path multiplexing.• Allows wavelength converters to be shared among multiple backup paths• The converters are reserved during the establishment of the backup paths.• Objective is to:

– Reduce the number of connections blocked– Reduce the numbers of converters in use.



Converter Multiplexing Between Paths b1 and b2

Converter Multiplexing: • The node returns a CONV-RESV-ACK message if the request is accepted.• The node returns a CONV-RESV-NACK message if the request is not accepted.• A wavelength conversion status table (WCST) is maintained.• When a network fails, a CONV-SETUP message is sent to the node.



Wavelength Converter Status Table

Converter Multiplexing:• Node checks the WCST to select a wavelength converter.• The appropriate entry is inserted into the WCST

• Wavelength converter identifier• Connection (session) identifier• Incoming port & associated wavelength• Outgoing port & associated wavelength• Path status (free, primary, reserved)

Conv. ID Conn. ID Incoming Port Outgoing Port Status

1 1 1 - λ0 2 – λ1 Reserved

1 2 3 - λ2 5 – λ0 Reserved

2 - - - Free

3 4 1 - λ0 4 - λ1 Primary



Backup Path Relocation (BPR)• Used when it becomes necessary for primary paths to accommodate certain routes which are occupied by the backup path.

• Objective is to:• Help in providing primary paths with fewer hops.• Reduce blocking.• Improve network utilization.

• Two relocation schemes are used to migrate an overlapping backup segment.• Wavelength Relocation (WR).• Segment Relocation (SR).

• Backup paths are relocated only if all overlapping segments can be relocated.

• If relocation fails, none of the overlapping segments are relocated.



Examples of backup path relocation mechanisms.

(Network states before relocation) (Network states after relocation)




• Presents the performance analysis of the proposed techniques, based on a discrete-event simulation model.



Simulation Model• A dynamic network traffic model• Connection requests arrive at a node• Each connection request is assignment a wavelength• Traffic load is defined in Erlangs.• Share-per-node architecture is used; all nodes allocate an equal number of converters.• Both the dedicated and shared protection schemes are studied.

Simulations are performed for two networks:• A 24-node ARPANET-like network with 16 and 32 wavelength on each link. The results for

this network is discussed.• Also, a random 50-node network with 32 wavelength per link.



Comparison of mechanisms• Basic hop-count (HC) based shortest path routing algorithm,• CFPR routing algorithm with wavelength relocation, and• CFPR routing algorithm with segment relocation.

• The notation X-Y-Z is used to specify an algorithm, where– X {HC,CFPR} denotes the routing algorithm,– Y {NR,WR,SR} denotes no relocation, wavelength relocation and segment relocation

respectively, and– Z {DP,SP} denotes dedicated and shared protection, respectively.

• The performance metrics presented are the:– blocking probability (Pb),– link and converter utilization, – average hop count, and – backup path relocation statistics



Blocking Probability• Network blocking probability;

attempt to minimize this metric• Architectures employing converter

multiplexing and backup path relocation schemes perform significantly better than the basic scheme.

• CFPR with WR/SR has lower Pb than that of the basic scheme.



• Graph demonstrates the reduction in the number of converters required per node.

• For an offered load of 3.0 Erlangs, the basic architecture needs a minimum of 16 converters to offer a blocking probability of less than10−1.

• The new techniques offer the same performance for as few as 4 converters.

• We can also observe that having more than 8 converters, does not lower the blocking probability any further.

Reduction in Number of Converters.



Percentages of Connections Blocked Due toWavelength Unavailability and Converter Unavailability.

Scheme Blocking Probability %WU %CU

HC-NR-DP 0.319 1% 99%

CFPR-WR-DP 0.114 98% 2%

CFPR-SR-DP 0.106 97% 3%

HC-NR-SP 0.262 0% 100%

CFPR-WR-SP 0.051 80% 20%

CFPR-SR-SP 0.053 80% 20%

Reasons for blocked connections• Converter unavailability accounts for:• 2% - 3% for the CFPR-based techniques w/dedicated protection• 20% for the CFPR-based techniques w/shared protection• 99% for the basic scheme w/dedicated protection• 100% for the basic scheme w/shared protection



Average Hop Count• Graph shows the average hop

count of the primary paths for accepted connections.

• Basic scheme exhausts all the converters as the network load increases.

• In comparison, the converter multiplexing based algorithms have a steady hop count.

* hop = the number of links between a pair of nodes.



Revenue Metric• Metric based on the number of hops routed.• Defined as the shortest–hop count based on the static topology.• For the basic scheme the revenue drops when load increases• The proposed algorithms, show only a marginal drop in revenue.



Conversion Statistics• One objective was to provide

wavelength-conversion free paths for the primary paths.

• Around 30% of the basic routing scheme connections need at least one wavelength converter.

• The proposed algorithm eliminates the need for wavelength conversion.



Relocation Statistics• Performance of systems with and

without backup path relocation schemes.

• When relocation was deployed, the blocking probability was lower.

• More significant benefits were seen in the context of wavelength conversion.

• Results indicate that the backup relocation overhead is manageable, and only a reasonable number of relocations are necessary.



• Introduction• Background• Proposed Mechanisms• Performance Analysis• Conclusions • Wraps up and concludes the paper.



Conclusion• Three different mechanisms were proposed and analyzed.

– The proposed converter multiplexing scheme reduces the number of connections blocked.– The CFPR routing algorithm significantly reduced the number of primary connections undergoing

wavelength conversion.

• Two different backup path relocation mechanisms were presented– Analysis showed that the combination of the mechanisms results in substantial reduction in blocking

probability.– Also, lower number of converters were required per node to achieve a target blocking probability.

• The additional overhead of using segment relocation compared to the wavelength relocation scheme did not result in much improvement.

Presented by: Brian V. Jarvis Wright State University: CEG790-01 Winter 2004, 2 February 2004

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