SMU CSE 8344

Protection & Restoration of Optical Networks

SMU 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

SMU CSE 8344

Classification of Schemes

SMU CSE 8344

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

SMU CSE 8344

Link Based vs. Path Based

• Link-based– Shorter restoration time– Less efficient.– Can only fix link failures

• Path-based– longer restoration time– More efficient.

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Dedicated vs. Multiplexed Backup

• Dedicated backup– More robust– Less efficient.

• Backup multiplexing– Less robust– More efficient.

SMU CSE 8344

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 cross-

connects (OXCs) or optical add/drop multiplexers (OADMs)

– Link-based/path-based protection/restoration

• Hybrid Mesh Rings– Physical: mesh– Logical: ring

SMU CSE 8344

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.

SMU CSE 8344

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.

SMU CSE 8344

2-Fiber WDM Ring

SMU CSE 8344

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.

SMU CSE 8344

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

SMU CSE 8344

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

SMU CSE 8344

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

SMU CSE 8344

Protection Cycles

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Protection Cycles (cont’d)

SMU CSE 8344

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.

SMU CSE 8344

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.

SMU CSE 8344

P - cycles

SMU CSE 8344

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.

SMU CSE 8344

Resilience in Multilayer Networks

• Why resilience in multilayer networks?– Avoid contention between different

single-layer recovery schemes.– Promote cooperation and sharing of

spare capacity

SMU CSE 8344

PANEL: Protection Across Network Layers

SMU CSE 8344

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