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Protection switching

Protection switching. TTM1: Optical transport and access networks By Steinar Bjørnstad 2013. IP restoration v.s. WDM protection..... Fumagalli et.al. Components in a network may fail Equipment ports and connections between client equipment and optical equipment

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Protection switching

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  1. Protection switching TTM1: Optical transport and access networks By Steinar Bjørnstad 2013

  2. IP restoration v.s. WDM protection..... Fumagalli et.al. • Components in a network may fail • Equipment ports and connections between client equipment and optical equipment • Optical layer hardware, transponders etc. • Fibre connections is known as the least reliable component. • Operator faults (Fingertrøbbel) at nodes, power failures with long duration Telenor

  3. Protection/Restoration • Several definitions may be found in the literature • Definition in this paper: • Restoration • Dynamically looks for backup paths of spare capacity in the network • Efficient from a capacity viewpoint • Protection • Reserves in advance dedicated backup-paths and wavelengths in the network • Faster than restoration Telenor

  4. Proposals for protection service classes • Lightpaths with guaranteed protection within a specific restoration time. • Lightpaths being protected at a best effort basis. • Lightpath not being protected. • Low priority lightpaths employing spare paths when these paths are not applied. Telenor

  5. Detection methods • IP layer (MPLS) Restoration • Periodic Internet Control Message Protocol (ICMP) messages • Pinging Echo • Timers and Keepalive/Hello messages • SDH (protection) • Loss of sync. • Optical layer (WDM) • Loss of optical power (typical detection method) • Abnormal polarization variation (pre-detection) (curiosity). Telenor

  6. Optical protection switching questions • Is protection at the optical layer sufficient? • How about failures in an Ethernet switch or interface? • Do we need protection at the optical layer? • What benefits does protection at the optical layer have?

  7. Protection schemes I • Individual lightpath – path • Protection of a single wavelength • Path layer, Optical Channel protection (OCh) • Require demultiplexing/multiplexing at every node for individual processing of wavelengths • Aggregated WDM signal - line • Line layer, Optical Multiplex Section (OMS) • Recover all lightpaths (wavelengths) on the failed line Telenor

  8. Protection schemes II • Dedicating resources • Dedicated protection (e.g. separate dedicated wavelength) • 1+1 duplicating data, or 1:1, pre-empting low-pri. data • In context of rings these are called DPRings • Optical unidirectional path switched rings (OUPSRs), path • Optical unidirectional line-switched rings (OULSRs), line • Shared protection • Protection resources shared between several lightpaths • 1:N, requires signalling • In context of rings these are called SPRrings • Optical bidirectional path switched rings (OBPSRs), path • Optical bidirectional line-switched rings (OBLSRs), line Telenor

  9. Dedicated versus shared protection • Dedicated Path Protection (DPP) • Reserved wavelength between end nodes for each path: Fast, simple, low resource utilization. • Dedicated Line Protection (DLP) • A spare line is standing-by Fast, simple, low resource utilization. • Shared path protection (SPP) • Protection wavelengths are shared, slower, more complex, efficient • Shared line protection (SLP) • 1:N Protection, local line protection of faulty line • Better resource utilization than DLP • Faster than SPP because of local signaling Telenor

  10. Dedicated protection approaches in ring networks • Optical Unidirectional Path switched ring, OUPSR (DP-WSHR) • Two counter propagating paths (wavelengths) in a ring, one for working and one dedicated for protection • Optical Unidirectional Line-switched ring, OULSR • Two counter propagating lines (fibres) in a ring, one for working and one dedicated for protection, joint switching of wavelengths Telenor

  11. Ring protection approaches, shared • Shared-path WSHR Optical Bidirectional path-switched ring (OBPSR) or OCh shared protection ring (OCh/SPRing) • Non loop-back • Each working lightpath switched to protection lightpath at its source node • Bidirectional shared line-switched WSHR (SL-WSHR) OMS/SPRing • Two or four fibres. Working and protection in both directions, simple & fast • Employs loopback switching: Working lightpaths switched to counterpropagating protection lightpaths at failure end. Loopback at the other failure end Telenor

  12. Protection in mesh network (Also used in other networks) • 1+1 protection • Continuous signal on two alternative paths, choose the best. • Hitless protection switching possible (switching without loss) • 1:N protection • Several parties share a single common protection path. • Enables the path to be employed by low priority traffic when not in use. • Implies information loss because of switching. Data in the fibre is lost. Not hitless. Telenor

  13. Classification of resilience schemes WDM • Restoration • Dynamic lookup for backup paths, spare capacity in the network. Typical IP-layer. • Protection • Reserving dedicated backup paths in advance. Typical WDM layer. Telenor

  14. Fast protection • SONET/SDH • 60/50 milliseconds for establishing a connection. • May avoid interruptions in a phone call. • Optical layer • 2 micro – 60 milli seconds. • SONET detects errors within 2.3 – 100 micro seconds. Protection at higher layers may be initiated. • IP-layer • Slow detection, calculation and signalling, typically seconds. • MPLS • Relatively fast detection possible with HELLO messages, BUT the higher frequency of the messages, the higher the overhead. • Fast switching if pre-planned path, LSP. Telenor

  15. Overview protection times • Protection at different layers must be coordinated • Else, protection at multiple layers may result. • Power detection at optical layer may be applied by e.g. MPLS • Lower layer interface issues an alarm Telenor

  16. Protection tables • Precomputed • Several alternatives pr. lightpath is calculated and stored in advance. • Recomputed • Centralized unit calculates and distributes new protection path for every lightpath after each failure. • Working path Restoration (In IP context) • Heals failed path by using dynamic switching in OXCs • Single fault per protection domain • Decompose mesh network into smaller logical networks (ring-structures) • Allows multiple error handling, even if the mechanism do not handle multiple errors. • OXCs necessary only at borders between logical rings, OADMs sufficient elsewhere. Telenor

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