Protection and Restoration in
Optical Network
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Ling Huang
Hling@cs.berkeley.edu
Outline
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Introduction to Network Survivability
Optics in Internet
Protection and Restoration in Internet
Optical Layer Survivability
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Protection in Ring Network
Protection in Mesh Network
Multi-Layer Resilience
Conclusion.
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A very important aspect of modern networks
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Network Survivability
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
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Requires physical redundancy and restoration protocols.
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Data
Center
Optics in the Internet
SONET
SONET
DWD
M
DWD
M
SONET
SONET
Access
Metro
Long Haul
Metro
Access
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Optical Network: a Layered vision
Layer
3
2
1
0
Layer
IP
ATM
IP
SONET
Optics
MPLS
Thin SONET
Opti
cs
Multi-physical layers
• multi & legacy services
• robustness, QOS
1999
Interworking
Packet
Optical
Packet
IP/MPLS
Smart
Optical
2/3
0/1
Fewer physical layers
• IP service dominance
• lower cost
2001
2002
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A well defined set of restoration techniques already
exists in the upper electronic layers:
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Protection and Restoration in Internet
ATM/MPLS
IP
TCP
Restoration speeds in different layers:
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BGP-4: 15 – 30 minutes
OSPF: 10 seconds to minutes
SONET: 50 milliseconds
Optical Mesh: currently hundred milliseconds to minutes
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Restoration in the upper layers is slow and require
intensive signaling
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On contrary 50-ms range when automatic protection
schemes are implement in the optical transport layer.
Purpose of performing restoration in the optical
layer:
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Why Optical Layer Protection
To decrease the outage time by exploiting fast rerouting
of the failed connection.
Main problem in adding protection function in a
new layer:
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Instability due to duplication of functions.
Need the merging of DWDM and electronic transport
layer control and management.
Why Optical Layer Protection?
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Advantages.
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Speed.
Efficiency.
Limitation
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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.
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Restoration techniques can protect the network
against:
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Link failures
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OXCs, OADMs, eclectro-optical interface.
Protection can be implemented
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Fiber-cables cuts and line devices failures (amplifers)
Equipment failures
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Protection Technique Classification
In the optical channel sublayer (path protection)
In the optical multiplex sublayer (line protection)
Different protection techniques are used for
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Ring networks
Mesh networks
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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
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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
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Reactive
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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
Proactive
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Backup lightpaths are
identified and resources are
reserved at the time of
establishing the primary
lightpath itself.
Taxonomy
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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
thehandled
nodes by
adjacent
the
nodestoadjacent
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
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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
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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
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Objective
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Minimize
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Integer Programming Formulation
1) Objective function
2) and 3) the flow conservation
constraints for demand d’s
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
Link Rediscovered
ALARM
Link recovered through optical protection
Link Down
10s ms
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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
N2
N1
Outside Ring = Working
Inside Ring = Protection
N3
N4
N1 send data to N2
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Unidirectional Path Switched Ring (UPSR)
Signal sent on
both working and
protected path
Best quality
signal selected
Reply Traffic
Receiving Traffic
N2
N1
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
N2
Sending/Receiving
Traffic
N1
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
N2
Sending/Receiving
Traffic
N1
OC-48
2 Outside Rings = Working
2 Inside Rings = Protection
N3
N4
N1 send data to N2 & N2 replies to N1