Protection & Restoration of
Optical Networks
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CSE 8344
Terminology
• Protection
– Uses pre-assigned capacity to ensure
survivability
• Restoration
– Reroutes the affected traffic after failure
occurrence by using available capacity
• Survivability
– Property of a network to be resilient to failures
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Classification of Schemes
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Reactive / Proactive
• 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)
– It cannot guarantee successful recovery,
– Longer restoration time
• Proactive
– Backup lightpaths are identified and resources are reserved along
the backup lightpaths at the time of establishing the primary
lightpath itself.
– 100% restoration guarantee
– Faster recovery
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Link Based vs. Path Based
• Link-based
– Shorter restoration time
– Less efficient.
– Can only fix link failures
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• Path-based
– longer restoration time
– More efficient.
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Dedicated vs. Multiplexed
Backup
• Dedicated backup
– More robust
– Less efficient.
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• Backup multiplexing
– Less robust
– More efficient.
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Primary Backup MUX
• Wavelength channel to be shared by a
primary and one or more backup paths
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Resilience in Optical Networks
• Linear Systems
– 1+1 protection
– 1:1 protection
– 1:N protection
• Ring-based
– UPSR: Uni-directional Path Switched Rings
– BLSR: Bi-directional Line Switched Rings
• Mesh-based
– Optical mesh networks connected by optical crossconnects (OXCs) or optical add/drop multiplexers
(OADMs)
– Link-based/path-based protection/restoration
• Hybrid Mesh Rings
– Physical: mesh
– Logical: ring
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Unidirectional WDM Path
Protected Rings
• 1+1 wavelength path selection
• Signal bridged on both protection and
working fiber.
• Receiver chooses the better signal.
• Failure:
– Destination switches to the operational link.
– Revertive /Non revertive switching
– No signaling required.
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Bidirectional Line switched
Ring
• Shares protection capacity among all the
spans on the ring
• Link failure
– Working traffic from 1 fiber looped back onto
opposite direction.
– Signaling protocol required
• Node failure
– Line switching performed at both sides of the
failed node.
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2-Fiber WDM Ring
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BLSR - 4 Fiber
• Fibers
– 2 working
– 2 protection
• Protection fiber: no traffic unless
failure.
• Link Failure.
– APS channel required to coordinate the
switching at both ends of a failure.
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4-Fiber WDM Ring.
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4-Fiber WDM Ring
After a Link Failure
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4-Fiber WDM Ring
After a Node Failure
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Path Layer Mesh Protection
• Protect Mesh as a single unit
• Pre-computed routes
– 1+1 path protection
– Protection route per light path
– Protection route per failure.
• On the fly route computation.
– Centralized route computation and coordination
– Route computation and coordination at end nodes.
– Distributed route computation at path ends.
• Decompose into protection domains.
• Pure rings
• P cycles
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Mesh Topologies
• Fibers organized in protection cycles.
– Computed offline
• 4 fibers of each link is terminated by 4
2X2 protection switches
• Before link failure, switches in normal
position.
• After failure, switches moved to
protection state and traffic looped back
into the protection cycles.
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2X2 Switch
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Protection Cycles (cont’d)
• Criterion for protection cycles.
– Recovery from a single link failure in any
optical network with arbitrary topology
and bi-directional fiber links
• All protection fibers are used exactly once.
• In any directed cycle both protection fibers
in a pair are not used unless they are in a
bridge
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Protection Cycles
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Protection Cycles (cont’d)
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Network With Default
Protection Switching
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Network After a Link Failure
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P –cycles
• Ring like restoration needed for some
client signals.
• Mesh topologies: bandwidth efficient.
• P –cycles:Ring like speeds, Mesh like
capacity.
• Addresses the speed limitation of
mesh restoration.
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P –cycles (cont’d)
• Cycle oriented pre configuration of spare
capacity.
• Can offer up to 2 restoration paths for a
failure scenario.
• Span Failure
– On cycle: similar to BLSR
– Off the cycle: 2 paths.
• Time needed for calculating and connecting
restoration path is needed in non-real time.
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P - cycles
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WDM Recovery
• Fiber based restoration
– Entire traffic carried by a fiber is backed by
another fiber.
– Bi-directional connection - 4 fibers.
• WDM based recovery
– Protection for each wavelength.
– Bi-directional connection - 2 fibers
– Allows flexibility in planning the configuration
of the network.
– Recovery procedure similar to BLSR.
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Resilience in Multilayer Networks
• Why resilience in multilayer
networks?
– Avoid contention between different
single-layer recovery schemes.
– Promote cooperation and sharing of
spare capacity
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PANEL: Protection Across
Network Layers
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PANEL Guidelines
• Recovery in the highest layer is recommended when:
– Multiple reliability grades need to be provided with fine
granularity
– Recovery inter-working cannot be implemented
– Survivability schemes in the highest layer are more mature
than in the lowest layer
• Recovery in the lowest layer is recommended when:
– The number of entities to recover has to be limited/reduced
– The lowest layer supports multiple client layers and it is
appropriate to provide survivability to all services in a
homogeneous way
– Survivability schemes in the lowest layer are more mature than
in the highest layer
– It is difficult to ensure the physical diversity of working and
backup paths in the higher layer
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