Protection and Restoration in Optical Network

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Ling Huang Hling@cs.berkeley.edu

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Outline

Introduction to Network Survivability Optics in Internet Protection and Restoration in Internet Optical Layer Survivability

Protection in Ring Network Protection in Mesh Network

Multi-Layer Resilience Conclusion.

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Network Survivability

A very important aspect of modern networks

The ever-increasing bit rate makes an unrecovered failure a significant loss for network operators. Cable cuts (especially terrestrial) are very frequent. No network-operator is willing to accept unprotected networks anymore.

Restoration = function of rerouting failed connections Survivability = property of a network to be resilient to failure

Requires physical redundancy and restoration protocols.

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Optics in the Internet

Data Center

SONET SONET

DWD M

DWD M

SONET SONET

Access

Metro

Long Haul

Metro

Access

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Optical Network: a Layered vision

Layer
IP ATM IP SONET Optics
MPLS Thin SONET

Layer 3
2 1 0

Opti cs

Interworking

Packet Optical

Packet IP/MPLS Smart Optical

2/3

0/1

Multi-physical layers multi & legacy services robustness, QOS

1999 2001

Fewer physical layers IP service dominance lower cost

2002

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Protection and Restoration in Internet

A well defined set of restoration techniques already exists in the upper electronic layers:

ATM/MPLS IP TCP BGP-4: 15 30 minutes OSPF: 10 seconds to minutes SONET: 50 milliseconds Optical Mesh: currently hundred milliseconds to minutes

Restoration speeds in different layers:

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Why Optical Layer Protection

Restoration in the upper layers is slow and require intensive signaling

On contrary 50-ms range when automatic protection schemes are implement in the optical transport layer.

Purpose of performing restoration in the optical layer:

To decrease the outage time by exploiting fast rerouting of the failed connection.

Main problem in adding protection function in a new layer:

Instability due to duplication of functions. Need the merging of DWDM and electronic transport layer control and management.

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Why Optical Layer Protection?

Advantages.
Speed. Efficiency. Detection of all faults not possible.(3R). Protects traffic in units of light paths. Race conditions when optical and client layer both try to protect against same failure.

Limitation

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Protection Technique Classification

Restoration techniques can protect the network against:

Link failures

Fiber-cables cuts and line devices failures (amplifers)

Equipment failures

OXCs, OADMs, eclectro-optical interface.

Protection can be implemented

In the optical channel sublayer (path protection) In the optical multiplex sublayer (line protection) Ring networks Mesh networks

Different protection techniques are used for

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Protection in Ring Network

1+1 Path Protection

Used in access rings for traffic aggregation into central office

1:1 Span and Line Protection 1:1 Line Protection

Used in metropolitan or longhaul rings Used for interoffice rings

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Protection in Mesh Networks

Network planning and survivability design

Disjoint path idea: service working route and its backup route are topologically diverse. Lightpaths of a logical topology can withstand physical link failures.
Working Path

Backup Path

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Reactive / Proactive

Reactive

A search is initiated to find a new lightpath which does not use the failed components after the failure happens. It can not guarantee successful recovery, Longer restoration time Backup lightpaths are identified and resources are reserved at the time of

Proactive

establishing the primary lightpath itself.

Taxonomy

100 percent restoration Faster recovery

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Path Protection / Line Protection

Normal Operation Path Switching: restoration is handled by the source and the destination.

Line Switching: restoration is handled by is restoration the handled nodes by adjacent the nodes to adjacent the failure. to the Span Protection: if additional failure. fiber is available. Line Protection.

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1+1 Protection

Traffic is sent over two parallel paths, and the destination selects a better one. In case of failure, the destination switch onto the other path. Pros: simple for implementation and fast restoration Cons: waste of bandwidth

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1:1 Protection

During normal operation, no traffic or low priority traffic is sent across the backup path. In case failure both the source and destination switch onto the protection path. Pros: better network utilization. Cons: required signaling overhead, slower restoration.

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Shared Protection
Normal Operation

1:N Protection

In Case of Failure

Backup fibers are used for protection of multiple links Assume independent failure and handle single failure. The capacity reserved for protection is greatly reduced.

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Multiplexing Techniques

Primary Backup Multiplexing Used in a dynamic traffic scenario, to further improve resource utilization. Allows a wavelength channel to be shared by a primary and one or more backup paths. By doing so, the blocking probability of demands decreases at the expense of reduced restoration guarantee. (An increased number of lightpaths can be established)

A lightpath loses its recoverability when a channel on its backup lightpath is used by some other primary lightpath. It regains its recoverability when the other primary lightpath terminates.

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Survivability Design: Joint Optimization Problem

Problem Description Given a network in terms of nodes (WXCs) and links, and a set of point-to-point demands, find both the primary lightpath and the backup lightpath for each demand so that the total required network capacity is minimized. Notation

N: the set of nodes; L: the set of links; D: the set of demands Cij: the capacity weight for link (ij) Wij: the capacity requirement on link (ij) in terms of # of

wavelength

Objective

Minimize

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Integer Programming Formulation

1) Objective function

2) and 3) the flow conservation constraints for demand ds primary path and backup path, respectively.

4) Logical relationship: the backup path consumes link capacity iff the primary path is affected by the fault. 5): Restoration route independent of the failure. 6): Link capacity requirement

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Multi-Layer Resilience

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Multi-Layer Resilience

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Multi-Layer Counter-Productive Behavior

Routing table Revision (no link)
Routing table Revision (with link) Link in Traffic

ALARM

Link Rediscovered Link recovered through optical protection

Link Down 10s ms

10s seconds

10s seconds

Instant response to Level 1 alarms in high layer causes unnecessary routing activity, routing instability, and traffic congestion
Source: RHK

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Multi-Layer Interaction

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Multi-Layer Interaction

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Conclusion

Different resilience schemes applicable in optical network have been discussed. Network planning and topology design for survivability is computationally intractable and faster heuristic solutions are needed. Multi-layer restoration is a hot point in current optical survivability research. Joint IP/optical restoration mechanism is the trend in next generation optical network.

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Unidirectional Path Switched Ring (UPSR)

Signal sent on both working and protected path Best quality signal selected Receiving Traffic

Sending Traffic N1

N2

Outside Ring = Working Inside Ring = Protection

N3 N4 N1 send data to N2

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Unidirectional Path Switched Ring (UPSR)

Best quality signal selected Signal sent on both working and protected path Reply Traffic

Receiving Traffic N1

N2

Outside Ring = Working Inside Ring = Protection

N3 N4 N2 replies back to N1

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Line Switched Ring (2-Fiber BLSRs)

Sending/Receiving Traffic N1

N2

Sending/Receiving Traffic

Both Rings = Working & Protection

N3 N4 N1 send data to N2 & N2 replies to N1

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Bidirectional Line Switched Ring (4-Fiber BLSRs)

Sending/Receiving Traffic N1
OC-48

N2

Sending/Receiving Traffic

2 Outside Rings = Working 2 Inside Rings = Protection

N3 N4 N1 send data to N2 & N2 replies to N1