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

1
Protection Mechanisms for Optical WDM Networks
based on Wavelength Converter Multiplexing and
Backup Path Relocation Techniques

• Sunil Gowda and Krishna M.Sivalingam
• University of Maryland Baltimore Country(UMBC)
• Dept. of CSEE,Baltimore

Presented by Priyanka Das
2
Focus

• This paper studies the problem of designing a
  survivable optical WDM network.
• Focus here is on efficient use of optical
  converters.
• Mechanism for improving network performance for
  survivable WDM mesh networks.
• An enhancement of dynamic route computation
  mechanism.
• Goal
• To minimize the number of converters
  per node used in the optical WDM network

3
Primary and Backup Route Computation Mechanisms

• Conversion free primary routing (CFPR).
• Converter multiplexing.
• Backup path relocation.
• All these mechanisms attempt to improve the
  overall performance of the network.

4
Conversion Free Primary Routing (CFPR)

• This routing scheme is proposed to compute
  wavelength conversion free primary paths as far
  as possible.
• The basic objective here is to
• Reduce the number of converters used in the
  network.
• Reduce cost.
• Eliminate conversion delay.
• Avoid signal degradations.

5
Converter Multiplexing

• Converter Multiplexing Technique which allows
  wavelength converters to be shared
• among
  multiple backup paths.
• Objective
• To reduces the number of connections blocked due
  to the unavailability of wavelength converters
  and reduces numbers of converters in use,
  thereby limiting the expenditure.

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Backup Path Relocation (BPR)

• BPR is used when it becomes necessary for
  primary paths to accommodate certain routes which
  are occupied by the backup path, at such a
  situation the backup paths are migrated so some
  other wavelength or segment.
• Objective
• This helps in providing primary paths with fewer
  hops.
• Reduces blocking.
• Improves network utilization.

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Improving Network Performance

• Reducing the number of converters used per node
  in the network.
• Making it cost effective.
• Providing protection against failure of primary
  path.
• Reducing blockage in the network with the help of
  shared converters.

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Routing and Wavelength Assignment Problem

• Here dynamic network model is used where requests
  arrive dynamically.
• Each request specifies source, destination and
  bandwidth required.
• Each request is then assigned a lightpath for a
  path/wavelength combination for its entire
  duration.
• The problem of determining end-end route and
  wavelength is referred to as RWA problem.

9
Wavelength Router Architecture

• Wavelength constraint is removed by using
  wavelength converters.
• Lightpath can thereby use different wavelengths
  on different links of the path.
• But converters are
• Expensive
• Produce signal degradation and delay
• So here the focus is on Minimizing the usage of
  wavelength conversion for primary path and
• thus reduce the number of converters used.

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Wavelength Converter Switch Architecture

• There are three different architectures proposed
  for a wavelength convertible switch.
• Dedicated wavelength converter switch
  architecture.
• Share -per-node architecture.
• Share-per-link architecture.
• The performance of share-per-node is better than
  dedicated in terms of cost and high utilization
  but it is complex due to higher switching
  complexity and blocking due to unavailability of
  converters.
• The performance of share-per-link in terms of
  cost lies in between the other two.

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Share-Per-Node Architecture

• WBC is the wavelength converter bank and it is
  provided for the entire router.
• It provides best cost to performance ratio

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Protection in Mesh Topology in WDM Networks

• Types of failures
• Link failure This needs rerouting of lightpath
  on the affected link
• Node failure The affected lightpath is handled
  by other nodes.

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Protection vs Restoration

• Works in advance
• Lower recovery time
• Needs redundant spare capacity
• Offers guarantee
• Also called as PROACTIVE

• Functions after failure
• More recovery time
• More resource utilization
• Cannot offer 100 guarantee.
• Also called as REACTIVE

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Protection
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Backup Path Multiplexing

• 1M protection,i.e.one wavelenght can be shared
  by many backup paths provided they are both never
  activated simultaneously.
• This provides 100 restoration guarantee in case
  of single ling failure.

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Lightpath Migration

• Migration of lightpath onto new paths, to
  accommodate other connections is the basic
  concept used.
• A virtual topology reconfiguration scheme to
  adapt to the changing traffic pattern s has been
  modeled as an Integrated linear programming (ILP)
  formulation
• Light paths are however torn down and
  re-established on the new paths.
• During the reconfiguration the transmission on
  that path is terminated.
• This helps to provide better paths for the
  primary.

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Network Architecture

• Dynamic routing is used, where the shortest path
  is computed between nodes based on current
  situation.
• Path level protection is used with both dedicated
  and shared protection schemes for backup paths.
• Wavelength route architecture is based on
  share-per-node wavelength converter
  configuration, as it offers best cost to
  performance ratio.
• Connections are blocked only due to
  unavailability of free wavelength or wavelength
  converters.

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How does things happen?

• Step 1 Request arrives with all the
  specification.
• Step 2 Conversion free primary routing.
• Step 3 If step 2 is not possible then use
  hop-count based shortest path algorithm.
• Step 4 Working on step 3 needs wavelength
  conversions and hence blocking due to less number
  of converters available.
• Step 5 Here converter multiplexing is proposed.
• Step 6 Backup path relocation comes to picture
  when needed.

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Conversion Free Primary Routing (CFPR) Technique.

• Aim To avoid wavelength conversions while
  routing primary connections.
• Multi-layered graph is used, these layers
  represent individual wavelength planes.
• CFPR algorithm models such a graph although the
  network has conversions capabilities.
• For each wavelength plane, the nodes are the
  physical nodes.

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Notations

• Existence of edges
• if wavelength is
  either not allocated or is reserved for some
  backup path(s).
• if wavelength w on
  link (i,j) is assigned to primary
• Computation of routes is done by Dijkstras
  shortest path.
• denotes the shortest path
  from node s to node d wavelength w.
• if no path is available on
  this wavelength
• The routing scheme here calculates up to W paths,
  one on each wavelength.

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Conversion Free Primary Routing and Overlapping

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Advantages of CFPR Mechanism

• Reduces conversion delays and degradation due to
  converters.
• Lower computational complexibility.
• CFPR computes path on each wavelength separately
  and hence alternate paths are available if
  shorted paths are blocked

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Converter Multiplexing

• Based on backup path multiplexing.
• Converters are shared only among backup paths
  that have physically disjoint primary paths.
• The converters are reserved during the
  establishment of the backup paths and are tuned
  to required wavelength during recovery.
• Source sends CONV-RESV message to the node at
  which conversion is needed.

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Converter Multiplexing

• The node responds with CONV-RESV-ACKS if
  accepted.
• The node responds with CONV-RESV-NACK if not
  accepted.
• Backup path is completed by the source node only
  if it receives all such acknowledges.
• A wavelength conversion status table (WCST) is
  maintained at each node.
• When network fails, for path recovery
  initialization CONV-SETUP message is sent to the
  node to configure the converter.

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Example of Converter Multiplexing

• Path p1(1-6-7-8) and p2 (4-8) are the primary
  paths.
• The corresponding backup paths are b1(1-2-5-8)
  and b2(4-5-8).
• Since the primary paths are link disjoint, the
  backup paths can share a wavelength converter at
  node 5.
• Due do converter multiplexing the number of
  converters is reduced from 2 to 1 at node 5.

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Backup Path Relocation

• Two relocation schemes are proposed to migrate an
  overlapping backup segment.
• The wavelength relocation (WR) New wavelength
  is used for the overlapping segment.
• The segment relocation (SR) Overlapping
  segment is relocated on a completely
• different path.

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Understanding the difference between WR and SR.
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Wavelength Relocation vs. Segment Relocation

• WR is simpler since the overlap segments links
  are unchanged
• Control messages have to be sent only to the
  nodes of the overlapping segment about the
  configuration.
• However, free wavelengths may always be not
  available on the same set of links, resulting in
  relocation failure
• SR considers a large set of paths and offers
  higher success probability relocation.
• However, such relocation incurs large overhead as
  overlapping segments are released and
  re-established.
• And consumes more resources due to potentially
  longer paths.

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Performance Analysis

• Simulation model
• Dynamic network traffic
• Request arrive at the node according to Poisson
  process with rate ?
• There is uniform node destination distribution
• Each request is assignment a wavelength
• Traffic load is L,and it is defined as ?/ µ in
  Erlangs.
• session duration is exponentially distributed
  with a mean of 1/µ.
• Share-per-node architecture is used. And C
  denotes of converters.
• Dedicated and shared protection schemes are
  studied.
• Single link failure model is assumed.

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Simulation Models

• Simulation 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 here.
• A random 50-node network with 32 wavelength per
  link.

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Different Mechanisms

• The basic hop count (HC) based shortest path
  routing algorithm.
• The CFPR routing algorithm with wavelength
  relocation.
• The CFPR routing algorithm with segment
  relocation.
• The notations X-Y-Z is used to specify an
  algorithm, where
• X HC,CFPR denotes the routing algorithm
• Y NR,WR,SR denoted no relocation, wavelength
  relocation and segment relocation respectively
• Z DP,SP denotes dedicated and shared
  protection.
• The performance metrics presented are the
  blocking probability (),link and converter
  utilization, average hop count, and statistics on
  backup path relocation

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Blocking Probability.

• Networking blocking probability is defined as the
  fraction of the total connection requests that
  are rejected.
• Converter multiplexing and backup path relocation
  schemes perform better than the basic scheme.
• Result
• CFPR with WR/SR, with dedicated or shared show
  lower blocking probability than the basic scheme.

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Reduction in Number of Converters.

• This graph shows the reduction in the number of
  converters required per node.
• It is evident that wavelength relocation is
  better than segment relocation when combined with
  CFPR and converter multiplexing.
• Although SR-DP and SR-SP works marginally better
  but increases the complexity and overhead.

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Average Hop Count

• This shows the accepted connections for primary
  paths.
• In the basic scheme, the primary path exhausts
  all the converters with increasing load and there
  is a decrease in the average hop count.
• Whereas the average hop count with the converter
  multiplexing based algorithms are steady

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Revenue metric

• This is based on the number of hops routed.
• Revenue metrics is defined as shortest hop count
  based on the static topology.
• Result
• The proposed algorithm shows marginal drop in
  revenue while for the basic scheme the revenue
  drops when load increases

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Conversion Statistics

• One of the primary aim was to provide wavelength
  conversion free paths for the primary paths.
• In the basic scheme 30 of the connections need
  at least one converter.
• While the proposed algorithm eliminates the need
  of conversion for the primary paths.

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Relocation Statistics
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Conclusion

• The CFPR routing algorithm significantly reduced
  the number of primary connections undergoing
  wavelength conversion.
• The proposed converter multiplexing scheme
  reduces the number of connections blocked due to
  unavailability of wavelength converter.
• Two different backup path relocation mechanisms
  were also presented ,results show that the
  combination of the two results in substantial
  reduction in blocking probability.
• Lower number of converters were used per node.
• Between both the relocation schemes ,the
  additional overhead of using segment relocation
  compared wavelength scheme did not result in much
  improvement, however segment relocation can be
  used to allow primary connections to be routed on
  links offering better transmission quality.