This teaching material is a part of e-Photon/ONe Master study in Optical Communications and Networks Course and module: Author(s): This tutorial is licensed under the Creative Commons Optical Network Resilience Network Survivability Achille Pattavina, Politecnico di Milano, Guido Maier, Politecnico di Milano,

Authors: Achille Pattavina, Guido Maier Course: Optical Network Resilience Module: Network Survivability Revision: 2/20082 (61) WDM network survivability Introduction A very important aspect of modern networks  The ever-increasing bit rate makes an unrecovered failure a significant loss for network operators More than 2,000,000 customers of the public network have been affected by link and equipment failures from 92 to 94 (cable failures: customer-minutes) In 1987 a cut of a 12-fiber cable caused the loss of 100,000 connections, only partially rerouted after 2 hours, and millions of dollars  Cable cuts (especially terrestrial) are very frequent Fiber cables share the same ducts of other utility transport media (gas and water pipes, etc.)  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

Authors: Achille Pattavina, Guido Maier Course: Optical Network Resilience Module: Network Survivability Revision: 2/20083 (61) Outline WDM networks: a layered vision WDM survivability: taxonomy Survivability in ring networks Survivability in mesh networks

Authors: Achille Pattavina, Guido Maier Course: Optical Network Resilience Module: Network Survivability Revision: 2/20084 (61) WDM optical networks A client-server approach WDM layer fundamentals  Wavelength Division Multiplexing: information is carried on high-capacity channels of different wavelengths on the same fiber  Switching: WDM systems transparently switch optical flows in the space (fiber) and wavelength domains WDM layer basic functions  Optical circuit (LIGTHPATH) provisioning for the electronic layers  Common transport platform for a multi-protocol electronic-switching environment Optical transmission WDM Layer SDHATMIP... Electronic layers: client Optical layers: servers Lightpath connection request Lightpath connection provisioning

Authors: Achille Pattavina, Guido Maier Course: Optical Network Resilience Module: Network Survivability Revision: 2/20085 (61) Network protection A layered approach Basic assumption: only one failure at a time is handled A well defined set of restoration techniques already exists in the upper electronic layers  SDH/SONET  ATM/MPLS  IP  TCP Restoration speeds in different layers:  BGP-4: 15 – 30 min  OSPF: 10 s to minutes  SONET: 50 ms  Optical mesh: hundred ms to minutes  Restoration in the upper layers is relatively slow and may require intensive signaling

Authors: Achille Pattavina, Guido Maier Course: Optical Network Resilience Module: Network Survivability Revision: 2/20086 (61) Network protection SDH/SONET layer Network protection at SDH/SONET layer  Very well defined and widely used  Fast failure recovery (in the order of 50 ms)  Limited to point-to-point and mesh topologies

Authors: Achille Pattavina, Guido Maier Course: Optical Network Resilience Module: Network Survivability Revision: 2/20087 (61) Network protection IP layer Network protection at IP layer  Intra-domain routing protocols of IP networks in charge of detecting failures RIP, OSPF, IS-IS protocols Distributed approach due to next-hop routing Routing loops can occur upon local changes of routing tables It could take up to 30 s to detect a failure, due to periodic exchange of Hello messages  Procedure enhancement Having more frequent updates does not help a lot: a few seconds are always needed SDH/SONET fast detection could be communicated to the IP layer  Attractive but non-implemented approach Using link layer protocol to make restoration more efficient, e.g. by MPLS  Protection schemes can be implemented to protect LSP entities

Authors: Achille Pattavina, Guido Maier Course: Optical Network Resilience Module: Network Survivability Revision: 2/20088 (61) Network protection WDM layer Reasons for performing restoration in the WDM layer  Restoration in the upper layers is relatively slow and may require intensive signaling  The outage time can be minimized by exploiting fast rerouting of the failed connections 50-ms range when automatic protection schemes are implemented  Significant cost savings can be attained by limiting restoration to the optical layer Protection bandwidth can be minimized acting on wavelengths rather than on connections (sharing of protection bandwidth in optical rings)  Only one protection procedure is started rather than as many as the traffic channels supported by a cut fiber  Due to OXC and ADM, mesh networks can be handled, not only rings

Authors: Achille Pattavina, Guido Maier Course: Optical Network Resilience Module: Network Survivability Revision: 2/20089 (61) Network protection WDM layer Limits to adopt protection at WDM layer  Failure in a client terminal cannot be recovered  Critical conditions of optical links not easy to detect due to poor monitoring functions  Protection granularity: lightpath  Protection routes could be limited by the power link budget, as they are usually longer than working routes Main problem in adopting protection functions in more than one layer  Instability due to duplication of functions (failure recovery race)  This is one of the main drivers that pushes towards the merging of WDM and electronic transport layer control and management

Authors: Achille Pattavina, Guido Maier Course: Optical Network Resilience Module: Network Survivability Revision: 2/ (61) Outline WDM networks: a layered vision WDM survivability: taxonomy Survivability in ring networks Survivability in mesh networks

Authors: Achille Pattavina, Guido Maier Course: Optical Network Resilience Module: Network Survivability Revision: 2/ (61) W D M OCh - Optical Channel OMS - Optical Multiplex Section OTS - Optical Transmission Section Physical media (optical fiber) Electronic Layers The WDM layer-stack is composed of four layers  Optical channel sublayer (OCh)  Optical multiplex section (OMS)  Optical transmission section (OTS)  Physical media layer Fiber-type specification, developed in other Recommendations WDM protocol layers WDM sublayers

Authors: Achille Pattavina, Guido Maier Course: Optical Network Resilience Module: Network Survivability Revision: 2/ (61) WDM Transport Layer Optical channel section  Provides end-to-end networking allowing electrical connections to be routed in the optical layer. Optical multiplexing section  Acts on multiwavelength optical signal, providing multi(demulti)plexing functionality Optical transport section  Allows to accomplish optical connection on optical media Survivability at the optical path layer provides a survivable common platform for the variety of transport signals

Authors: Achille Pattavina, Guido Maier Course: Optical Network Resilience Module: Network Survivability Revision: 2/ (61) WDM protocol sublayers Simplified scheme of a WDM connection DXC OXC OADM cs muxdemux mux OA OCh OMS OTS optical fiber OXC optical Cross Connect DXC digital Cross Connect OADM optical Add-Drop Multiplexer CS optical Channel Switching mux WDM multiplexer demux WDM demultiplexer OA Optical Amplifier

Authors: Achille Pattavina, Guido Maier Course: Optical Network Resilience Module: Network Survivability Revision: 2/ (61) WDM network survivability WDM protection techniques classification Restoration techniques can protect the network against  Link failures Fiber-cables cuts and line devices failures (amplifiers)  Equipment failures OXCs, OADMs, electro-optical interfaces Protection can be implemented  In the optical channel sublayer (OCh protection)  In the optical multiplex section sublayer (OMS protection) Different protection schemes are used for  Ring networks  Mesh networks

Authors: Achille Pattavina, Guido Maier Course: Optical Network Resilience Module: Network Survivability Revision: 2/ (61) WDM network survivability Protection at optical channel sublayer (OCh) Also called path protection  Each lightpath is protected by end-to-end rerouting when a failure occurs  A routing-diversity path must be ready for the protected lightpath Pros  The exact failure localization is not important Cons  Problems in restoration control and managing may rise when more lightpaths are involved in the failure

Authors: Achille Pattavina, Guido Maier Course: Optical Network Resilience Module: Network Survivability Revision: 2/ (61) WDM network survivability Protection at optical multiplex sublayer(OMS) Also called line protection or link protection  All the WDM channels crossing a failed fiber are rerouted together  An idle alternative fiber path must be available Switching features  Protection switching is performed by spatial switches operating on the entire WDM multiplex (fiber switches)  Switching time may be very short (tens of milliseconds to tens of microseconds), if signaling is not considered

Authors: Achille Pattavina, Guido Maier Course: Optical Network Resilience Module: Network Survivability Revision: 2/ (61) WDM network survivability Basic concepts Basic protection schemes in a point-to-point configuration  1+1: same signal sent onto two separate paths and receiver selects the best  1:1: protection path reserved but lower-priority traffic can be sent onto protection path Granularity of protection  OCh: single channels are protected with high cost incurred for protecting small optical traffic flows  OMS: a bundle of channels is protected by making more economical optical protection

Authors: Achille Pattavina, Guido Maier Course: Optical Network Resilience Module: Network Survivability Revision: 2/ (61) WDM network survivability OMS vs OCh protection 1+1 point-to-point configuration  OMS  OCh

Authors: Achille Pattavina, Guido Maier Course: Optical Network Resilience Module: Network Survivability Revision: 2/ (61) WDM network survivability Classification of resilience schemes

Authors: Achille Pattavina, Guido Maier Course: Optical Network Resilience Module: Network Survivability Revision: 2/ (61) Outline WDM networks: a layered vision WDM survivability: taxonomy Survivability in ring networks Survivability in mesh networks

Authors: Achille Pattavina, Guido Maier Course: Optical Network Resilience Module: Network Survivability Revision: 2/ (61) WDM path protection Ring protection: OCh-DPRing Also called Optical Unidirectional Path Switched Ring (OUPSR)  Optical channel dedicated protection Two-fiber bidirectional ring  One fiber for working and the other counter-propagating for protection  The optical signal is physically split at the transmitter and routed onto two different paths (as in 1+1)  The receiver receives both signals and selects the best one  Dedicated protection (additional 100% of the working capacity needed for protection)  No signaling required Working Protection ADM A D B C

Authors: Achille Pattavina, Guido Maier Course: Optical Network Resilience Module: Network Survivability Revision: 2/ (61) WDM path protection Ring protection: OCh-SPRing Also called Optical Bidirectional Path Switched Ring (OBPSR)  Optical channel shared protection OCh-SPRing features  2 fibers with 50% w. and 50% p. each  Similar to SDH/SONET BLSR, but now operated at OCh layer  Shortest path routing Bidirectional connections on the same side Different wavelengths for two directions to avoid need for wavelength conversion  Non-overlapping lightpaths can share a single wavelength along the ring for protection  More efficient than OCh-DPRing Working Protection ADM A D B C

Authors: Achille Pattavina, Guido Maier Course: Optical Network Resilience Module: Network Survivability Revision: 2/ (61) WDM OMS protection Ring protection: OMS-2 DPRing Also called Optical Unidirectional Line Switched Ring (OULSR)  Similar to SDH/SONET UPSR, but now operated at OMS layer  Optical link dedicated protection Two-fiber bidirectional ring  Fibers are counter-propagating  Each node transmits in both directions Different wavelengths used by different nodes to avoid collisions Handling failures  Normal operation: ring is disconnected turning off amplifiers at an interface of one node  Failure: amplifiers close to failure are turned off, turning on the others previously off A D B C

Authors: Achille Pattavina, Guido Maier Course: Optical Network Resilience Module: Network Survivability Revision: 2/ (61) WDM OMS protection Ring protection: OMS-2 SPRing Also called Optical Bidirectional Line Switched Ring (OBLSR)  Optical link shared protection Two-fiber bidirectional ring  Fibers are counter-propagating Fiber 1: working wavelengths 1 to W/2, the others are for protection Fiber 2 working wavelengths W/2+1 to W, the others are for protection  Shortest path routing Different wavelengths in two directions  Protection bandwidth can be shared  Recovery is performed by 2x2 fast switches terminating the fibers  No wavelength conversion is necessary Upon failure, traffic is looped back without wavelength collision A D B C

Authors: Achille Pattavina, Guido Maier Course: Optical Network Resilience Module: Network Survivability Revision: 2/ (61) WDM OMS protection OMS-2 SPRING – Link failure Example  After link failure working paths are routed on protection fibers with the same wavelengths Working path at 1 on fiber 1 Working path at W/2+1 on fiber 2 A D B C

Authors: Achille Pattavina, Guido Maier Course: Optical Network Resilience Module: Network Survivability Revision: 2/ (61) WDM OMS protection OMS-2 SPRing – Node failure Node failure recovery is also possible  Recovery is performed by 2x2 fast switches  Working path is routed in the outer fiber ring  After node failure the path is switched in the inner ring at the same wavelength A D B C

Authors: Achille Pattavina, Guido Maier Course: Optical Network Resilience Module: Network Survivability Revision: 2/ (61) A D B C WDM OMS protection Ring protection: OMS-4 SPRing Also called Optical Bidirectional Line Switched Ring (OBLSR)  Optical link shared protection Four-fiber bidirectional ring  A fiber-pair is used for working and the other for protection  Protection-switching is performed by 2x2 fast switches Optomechanical Other technology  Additional 100% of the working capacity needed for protection Protection fiber

Authors: Achille Pattavina, Guido Maier Course: Optical Network Resilience Module: Network Survivability Revision: 2/ (61) A D B C WDM OMS protection OMS-4 SPRing – Link failure In case of failure of a link the ring connectivity is restored by employing the protection fibers Signaling is required to coordinate switching at both ends of the failed node  An open issue in standardization A D B C

Authors: Achille Pattavina, Guido Maier Course: Optical Network Resilience Module: Network Survivability Revision: 2/ (61) WDM OMS protection OMS-4 SPRing – Node failure Node failure recovery is also possible A D B C

Authors: Achille Pattavina, Guido Maier Course: Optical Network Resilience Module: Network Survivability Revision: 2/ (61) Outline WDM networks: a layered vision WDM survivability: taxonomy Survivability in ring networks Survivability in mesh networks  Protection  Restoration

Authors: Achille Pattavina, Guido Maier Course: Optical Network Resilience Module: Network Survivability Revision: 2/ (61) WDM network survivability WDM protection in mesh networks Various options available  OCh protection Path protection  OMS protection Link protection Span protection (backup resources are reserved in the same link)  Restoration by provisioning Largely an open and pre-standard issue Much more difficult to plan and control than in rings

Authors: Achille Pattavina, Guido Maier Course: Optical Network Resilience Module: Network Survivability Revision: 2/ (61) WDM path protection Dedicated Path protection schemes 1+1 protection  The optical signal is physically split at the transmitter and routed onto two different paths  The receiver receives both the signals and selects the best one  Dedicated protection  No signaling required  The protection path is always in-use TX RX Working Protection Cloud network Splitter

Authors: Achille Pattavina, Guido Maier Course: Optical Network Resilience Module: Network Survivability Revision: 2/ (61) WDM path protection Dedicated Path protection schemes (cont.) 1:1 protection  The working lightpath is switched on the protection path only when a failure occurs  Dedicated protection  Signaling is required to advertise the source that a failure occurred  Protection resources can be used by low-priority traffic in normal conditions TX RX Working Protection Switch APS signalling channel

Authors: Achille Pattavina, Guido Maier Course: Optical Network Resilience Module: Network Survivability Revision: 2/ (61) WDM mesh protection Dedicated OCh protection 1+1 and 1:1 dedicated prot. (>100% of working capacity needed)  Each connection-request is satisfied by setting-up a lightpath pair of a working + a protection lightpath RFWA must be done so that working and protection lightpaths are link disjoint If the connection must be protected against OXC failures, the two lightpaths must be also node disjoint  Additional constraints must be considered in network planning and optimization or in dynamic RFWA  1:1 requires signaling to activate protection-switching in the source node Transit OXCs must not be reconfigured in case of failure

Authors: Achille Pattavina, Guido Maier Course: Optical Network Resilience Module: Network Survivability Revision: 2/ (61) WDM path protection Shared Path protection scheme 1:N protection  A single protection path is used to protect a set of N paths  All the working paths involved must be in routing diversity  The working lightpath is switched on the protection path upon a failure  Shared protection: the amount of resources reserved for protection is decreased  Signaling is required to advertise the source that a failure occurred  Protection resources can be used by low-priority traffic in normal conditions  This type of protection is reverting TX 1 RX 1 Working Protection TX N RX N

Authors: Achille Pattavina, Guido Maier Course: Optical Network Resilience Module: Network Survivability Revision: 2/ (61) WDM mesh protection Shared OCh protection Sharing is a way to decrease the capacity redundancy and the number of lightpaths that must be managed 1:N shared protection  A single protection lightpath between two nodes is setup to protect N working lightpaths between the same nodes  The connection-requests must be processed all-together RFWA must be performed in such a way that the N working lightpaths and the protection lightpath are mutually link (node) disjoint  Additional constraints must be considered in network planning and optimization or in dynamic RFWA: network control can become enormously complicated Severely limited by physical topology: a node degree greater than N+1 and a highly interconnected topology is needed  In many cases there is no solution

Authors: Achille Pattavina, Guido Maier Course: Optical Network Resilience Module: Network Survivability Revision: 2/ (61) WDM mesh protection Shared OCh protection (II) OCh protection-resources sharing  Protection lightpaths of different channels share some wavelength channels  Based on the assumption of single point of failure  Working lightpaths must be link (node) disjoint Very complex control issues  Also transit OXCs reconfiguration is needed in case of failure Signaling involves also transit OXCs Lightpath identification and tracing becomes fundamental

Authors: Achille Pattavina, Guido Maier Course: Optical Network Resilience Module: Network Survivability Revision: 2/ (61) WDM mesh protection Link (OMS) protection Link protection (≥100% of working capacity required)  Each link is protected by providing an alternative routing for all the WDM channels in all the fibers  Protection switching performed by fiber switches (fiber cross-connects)  Signaling is local; transit OXCs of the protection route can be pre-configured  Fast reaction to faults  Some network fibers are reserved for protection Span protection (100% of working capacity required for protection)  Similar to link protection, but exploiting protection-reserved wavelengths on the same link (as in the 2-fiber OMS SPRing) Connection Normal OMS protection (link protection)

Authors: Achille Pattavina, Guido Maier Course: Optical Network Resilience Module: Network Survivability Revision: 2/ (61) WDM mesh protection Link (OMS) protection (II) Link-shared protection  Protection fibers may be used for protection of more than one link (assuming single-point of failure)  The capacity reserved for protection is greatly reduced  An efficient way of planning a mesh network supporting shared-link protection is ring covering with SPRing-like rings

Authors: Achille Pattavina, Guido Maier Course: Optical Network Resilience Module: Network Survivability Revision: 2/ (61) WDM mesh protection Ring cover: solutions and drawbacks Direct approach: designing the network in term of rings  More expensive than mesh  Limited scalability  A single ring is not a practical solution Typically a set of rings are chosen such that:  Every link falls in at least one ring (traditionally ring/cycle cover)  Each node falls in at least one ring (node cover) Does not cover necessarily all the links Open issue: traffic recovery on uncovered links  Minimization of the fiber amount requires to solve NP-hard problem Protection cycle cover solution Other solutions  Cycle double cover  P-cycle

Authors: Achille Pattavina, Guido Maier Course: Optical Network Resilience Module: Network Survivability Revision: 2/ (61) Outline WDM networks: a layered vision WDM survivability: taxonomy Survivability in ring networks Survivability in mesh networks  Protection Self-healing rings (SHR) Protection cycles Cycle double cover (CDC) p-cycles  Restoration

Authors: Achille Pattavina, Guido Maier Course: Optical Network Resilience Module: Network Survivability Revision: 2/ (61) WDM mesh protection Ring cover: Self Healing Ring (SHR) Self Healing Ring  Ring topologies allow distributed recovery  Restoration time bounded by ring size (not provided by path protection)  Fault management is confined in the faulty ring A working lightpath must be protected against single link failure by SHR line protection Sharing is allowed  Rings covering the same link can share fibers, not protection wavelengths  Spare wavelengths are shared within the same ring  The number of wavelengths in each protection fiber is determined by the ring load (=largest number of working lightpaths)  Maximum number of rings covering a link: constrained by network management complexity crossing a node: constrained by node complexity Fiber number used by SHR is not predefined as in 2/4 SPRing

Authors: Achille Pattavina, Guido Maier Course: Optical Network Resilience Module: Network Survivability Revision: 2/ (61) Outline WDM networks: a layered vision WDM survivability: taxonomy Survivability in ring networks Survivability in mesh networks  Protection Self-healing rings (SHR) Protection cycles Cycle double cover (CDC) p-cycles  Restoration

Authors: Achille Pattavina, Guido Maier Course: Optical Network Resilience Module: Network Survivability Revision: 2/ (61) WDM mesh protection Protection cycle cover Protection cycle  Four-fiber protection technique Each network link comprises 2 working and 2 protection fibers (counterpropagating)  Directed cycles must be identified so that each protection fiber is used exactly once  Planar topologies: finding cycles is a simple task Features  Automatic protection switching using cycle cover is a distributed and autonomous recovery process  Faults are detected by end nodes of the failed link  Four 2x2 switches (“protection switches”) terminate the fibers  Fault protection against link failure is possible if the graph modeling the network is at least two-edge connected

Authors: Achille Pattavina, Guido Maier Course: Optical Network Resilience Module: Network Survivability Revision: 2/ (61) WDM mesh protection Constructing the cycles Planar network with e edges and n nodes: f = 2 + e - n faces (f - 1 inner faces and an outer face) [Euler’s number] The family of directed cycles is given by the f faces, each one properly oriented Non-planar networks require more complex results from the graph theory to solve the cycle double cover problem

Authors: Achille Pattavina, Guido Maier Course: Optical Network Resilience Module: Network Survivability Revision: 2/ (61) WDM mesh protection Protection cycle cover In fault-free state, the network supports a large number of source- destination requests The “protection switches” provide connections from the working fiber to the optical switches in the network nodes Faullt-free-network: only working fibers are shown 2x2 protection switches optical switch Protection fiber Working fiber

Authors: Achille Pattavina, Guido Maier Course: Optical Network Resilience Module: Network Survivability Revision: 2/ (61) WDM mesh protection Protection cycle cover 2 x 2 protection switches are considered reliable and fault-free In a fault-free state the switches are connected as follows  Working input port p i w is connected to working output port p o w  Protection input port p i p is connected to protection output port p o p Protection ports of different protection switches are connected together creating the protection cycles Before the failure WPWP After the failure

Authors: Achille Pattavina, Guido Maier Course: Optical Network Resilience Module: Network Survivability Revision: 2/ (61) WDM mesh protection A planar network example A network with f faces can restore only failures using automatic protection switching mechanism source destination Planar graph with its faces protection cycles created with the protection fiber

Authors: Achille Pattavina, Guido Maier Course: Optical Network Resilience Module: Network Survivability Revision: 2/ (61) Outline WDM networks: a layered vision WDM survivability: taxonomy Survivability in ring networks Survivability in mesh networks  Protection Self-healing rings (SHR) Protection cycles Cycle double cover (CDC) p-cycles  Restoration

Authors: Achille Pattavina, Guido Maier Course: Optical Network Resilience Module: Network Survivability Revision: 2/ (61) WDM mesh protection Cycle double cover (CDC) Link failure recovery can be realized in arbitrary mesh networks using oriented cycle double cover A cycle double cover consists of covering the graph with cycles in such a way that each edge is covered by exactly two cycles  Each cycle is a ring that can be used to perform restoration  Primary and secondary rings are defined Two edge-connected networks  Planar network: a polynomial time algorithm exists to discover cycle double cover  Non-planar network: only exhaustive search and heuristics can be used

Authors: Achille Pattavina, Guido Maier Course: Optical Network Resilience Module: Network Survivability Revision: 2/ (61) WDM mesh protection Cycle double cover Bidirectional cycle double cover (B-CDC)  Rings overlap with odd-degree nodes Oriented cycle double cover (O-CDC)  Enables fiber saving

Authors: Achille Pattavina, Guido Maier Course: Optical Network Resilience Module: Network Survivability Revision: 2/ (61) WDM mesh protection Cycle double cover Two different implementations  Fiber recovery: fibers are backed up by fibers  WDM recovery: each ring is both primary for certain wavelengths and secondary for the remaining Cycle double cover for WDM recovery can be expensive  It requires wavelength converters in 2- fiber networks Oriented cycle doble cover for a 6-node network

Authors: Achille Pattavina, Guido Maier Course: Optical Network Resilience Module: Network Survivability Revision: 2/ (61) WDM mesh protection Cycle double cover: example In 2-fiber WDM-based recovery  Primary or secondary wavelength assignment over the whole ring is a difficult task … From a practical point of view, cycle double cover requires 4-fiber link  This overcomes the problem of wavelength interdependence in 2-fiber systems 2-fiber WDM links 1 st solution 2 nd solution ? ?

Authors: Achille Pattavina, Guido Maier Course: Optical Network Resilience Module: Network Survivability Revision: 2/ (61) Outline WDM networks: a layered vision WDM survivability: taxonomy Survivability in ring networks Survivability in mesh networks  Protection Self-healing rings (SHR) Protection cycles Cycle double cover (CDC) p-cycles  Restoration

Authors: Achille Pattavina, Guido Maier Course: Optical Network Resilience Module: Network Survivability Revision: 2/ (61) WDM mesh protection p-cycle (I) A p-cycle (“preconfigured protection cycle”) is the most efficient structure for preconfigured restoration in terms of capacity efficiency p-cycles go beyond the behavior of a ring  Emulate the ring behavior for a class of failures (faults of the peripheral segments of the cycle)  Protect all its chord links like a mesh network, by providing two restoration paths  Recovery action is the same as in bidirectional-line switching rings (2- fiber SPRings)  No signaling is needed to activate restoration Optimal p-cycle selecting (NP-hard problem): a trade off between minimizing path length and spare capacity

Authors: Achille Pattavina, Guido Maier Course: Optical Network Resilience Module: Network Survivability Revision: 2/ (61) WDM mesh protection p-cycle (II) The failure of a peripheral link of the p-cycle is bypassed using the only restoration path available Like a self-healing ring peripheral link chord link P-cycle

Authors: Achille Pattavina, Guido Maier Course: Optical Network Resilience Module: Network Survivability Revision: 2/ (61) WDM mesh protection p-cycle (III) For the failure of a chord link of the p-cycle two restoration paths are available Protection bandwidth less than 100% of working bandwidth  In the example 9/(9+9x2)  33%

Authors: Achille Pattavina, Guido Maier Course: Optical Network Resilience Module: Network Survivability Revision: 2/ (61) Outline WDM networks: a layered vision WDM survivability: taxonomy Survivability in ring networks Survivability in mesh networks  Protection  Restoration

Authors: Achille Pattavina, Guido Maier Course: Optical Network Resilience Module: Network Survivability Revision: 2/ (61) WDM mesh Protection by lightpath provisioning No capacity is permanently dedicated to protection In case of failure  Failed lightpaths are rerouted as new connections by dynamic RFWA  There is a probability that the restoration cannot be performed (blocking probability)  The network must be provided with redundant resources to reduce blocking probability (however much less than 100% of working capacity) Features  Very robust against multiple failures (a realistic situation in case of natural disaster)  Very slow recovery time and complex signaling  It does not require any special protection equipment  It matches very well with a dynamic network environment An unexplored possibility for future development

Authors: Achille Pattavina, Guido Maier Course: Optical Network Resilience Module: Network Survivability Revision: 2/ (61) WDM mesh Lightpath provisioning: SLB, MC approaches Lightpath provisioning  Single link basis (SLB) Only interrupted connections are subject to rerouting  Minimal cost (MC) It rearranges all optical connections It is very cost efficient but it is not suitable for practical use due to the reconfigurations of the entire optical nodes Normal state SLB approach: 5 wavelengths MC approach: 4 wavelengths

Authors: Achille Pattavina, Guido Maier Course: Optical Network Resilience Module: Network Survivability Revision: 2/ (61) WDM mesh Distributed dynamic restoration Distributed dynamic approaches  They find a backup paths after the failure of a network component  Most of distributed algorithms uses a three-phase restoration process A two-phase process has been proposed  Broadcast & Reserve step: the source node of the failed link broadcasts messages on its outgoing links and reserves wavelengths. Since all connections traversing the failed link are disrupted, the source node tries to set up a restoration path in parallel on different wavelength  Confirm & Configure step: after receiving the broadcast message, the destination node sends a confirm message along the reserved path to the source node. Intermediate nodes forward the message and configure their OXCs, establishing a unidirectional path between the source and the destination node of the failed link  Unused reserved capacity is released