Protection and Restoration in Optical Network Ling Huang [email_address]
Introduction to Network Survivability
Optics in Internet
Protection and Restoration in Internet
Optical Layer Survivability
Protection in Ring Network
Protection in Mesh Network
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.
Optics in the Internet SONET Data Center SONET SONET SONET DWDM DWDM Access Long Haul Access Metro Metro
Optical Network: a Layered vision 1999 2002 2001
multi & legacy services
Thin SONET IP Optics MPLS
Fewer physical layers
IP service dominance
SONET IP Optics ATM Layer 3 2 1 0 Packet Optical Inter- working Smart Optical Packet IP/MPLS Layer 2/3 0/1
Protection and Restoration in Internet
A well defined set of restoration techniques already exists in the upper electronic layers:
Restoration speeds in different layers:
BGP-4: 15 – 30 minutes
OSPF: 10 seconds to minutes
SONET: 50 milliseconds
Optical Mesh: currently hundred milliseconds to minutes
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.
Why Optical Layer Protection?
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.
Protection Technique Classification
Restoration techniques can protect the network against:
Fiber-cables cuts and line devices failures (amplifers)
OXCs, OADMs, eclectro-optical interface.
Protection can be implemented
In the optical channel sublayer (path protection)
In the optical multiplex sublayer (line protection)
Different protection techniques are used for
Protection in Ring Network 1+1 Path Protection Used in access rings for traffic aggregation into central office 1:1 Line Protection Used for interoffice rings 1:1 Span and Line Protection Used in metropolitan or long- haul rings
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
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 establishing the primary lightpath itself .
100 percent restoration
Reactive / Proactive Taxonomy
Path Protection / Line Protection Path Switching: restoration is handled by the source and the destination. Normal Operation Line Switching: restoration is handled by the nodes adjacent to the failure. Span Protection: if additional fiber is available. Line Switching: restoration is handled by the nodes adjacent to the failure. Line 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
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.
Backup fibers are used for protection of multiple links
Assume independent failure and handle single failure.
The capacity reserved for protection is greatly reduced.
1:N Protection Normal Operation In Case of Failure
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.
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.
N: the set of nodes;
L: the set of links;
D: the set of demands
C ij : the capacity weight for link (ij)
W ij : the capacity requirement on link (ij) in terms of # of wavelength
Survivability Design: Joint Optimization Problem
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
Integer Programming Formulation
Multi-Layer Counter-Productive Behavior
Instant response to Level 1 alarms in high layer causes unnecessary routing activity, routing instability, and traffic congestion
Link Down Link recovered through optical protection Routing table Revision (no link) Routing table Revision (with link) Link Rediscovered 10s ms 10s seconds 10s seconds ALARM Link in Traffic Source: RHK
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.
Unidirectional Path Switched Ring (UPSR) Signal sent on both working and protected path Best quality signal selected Receiving Traffic N1 send data to N2 N1 N2 Outside Ring = Working Inside Ring = Protection Sending Traffic N4 N3
Unidirectional Path Switched Ring (UPSR) Reply Traffic N2 replies back to N1 Receiving Traffic N1 N2 Outside Ring = Working Inside Ring = Protection N4 N3 Signal sent on both working and protected path Best quality signal selected
Bidirectional Line Switched Ring (2-Fiber BLSRs) Sending/Receiving Traffic Sending/Receiving Traffic N1 send data to N2 & N2 replies to N1 Both Rings = Working & Protection N1 N2 N4 N3
Bidirectional Line Switched Ring (4-Fiber BLSRs) Sending/Receiving Traffic Sending/Receiving Traffic OC-48 N1 send data to N2 & N2 replies to N1 2 Outside Rings = Working 2 Inside Rings = Protection N1 N2 N4 N3