Why Optical Layer Protection?

1. Why Optical Layer Protection? • Optical layer provides lightpath services to its client layers (e.g., SONET, IP, ATM) • Protection mechanisms exist in the client layers, so why need protection in optical layer? • IP and ATM networks don’t have extensive protection functions as SONET • Capacity efficient due to protection capacity sharing across multiple pairs of client layer equipments • Significant savings in equipment cost

2. Why Optical Layer Protection? • Handle fiber cuts more efficiently than the client layers • Provide an additional degree of resilience (e.g., protect against multiple failures) • Can use mesh-based protection schemes that require significantly less protection capacity than ring-based schemes

3. Limitations of Optical Layer Protection • Can’t handle all failures: client equipment failures must be dealt with by the client layer • Protect traffic in units of lightpaths: can’t protect only part of the traffic within a lightpath • Need pay careful attention to interworking of protection schemes between different layers

4. Service Classes Based on Protection • Optical layer can provide multiple classes of service based on the type of protection provided • The classes differ in the level of connection availability provided and the connection restoration time • Different classes are supported using different protection schemes • Possible service classes • Platinum: provide the highest level of availability and the fastest restoration times (comparable to SONET protection schemes) • Can use dedicated 1+1 protection scheme

5. Service Classes Based on Protection • Gold: provide high availability and fast restoration times (hundreds of ms) • Can use shared mesh protection scheme • Silver: below gold in terms of availability and restoration time • Best-effort restoration • Bronze: provide unprotected lightpaths • Lead: supported using protection bandwidth reserved for other classes of service; connections are preempted when the bandwidth is needed to protect other high-priority traffic

6. Optical Layer Protection Schemes • Optical channel (OCh) layer (or path layer) protection schemes • Restore one lightpath at a time • Need demultiplex all wavelengths • Optical multiplex section (OMS) layer (or line layer) protection schemes • Restore the entire group of lightpaths on a link • Require less equipment

7. Optical Layer Protection Schemes • OLTs and OADMs can provide both OCh and OMS layer protection in linear or ring configurations • OXCs can provide OCh layer protection in linear, ring, and mesh configurations • Backbone networks: use unprotected WDM point-to-point systems and rely on OXCs to perform the protection functions • Metropolitan networks: use WDM line terminals and OADMs to perform protection functions

8. Optical Layer Protection Schemes • OMS layer protection schemes • 1+1 • 1:1 • OMS-DPRing • OMS-SPRing • OCh layer protection schemes • 1+1 • OCh-SPRing • OCh-Mesh

9. 1+1 OMS Protection • Dedicated protection in point-to-point links • At one end, the composite WDM signal is bridged onto both the working fiber and the protection fiber using an optical splitter • At the other end, an optical switch selects the better among the two signals

10. 1:1 OMS Protection • Shared protection in point-to-point links • The composite WDM signal is transmitted over only the working fiber • Use a switch at the transmitter, instead of a splitter • If the working fiber is cut, both ends switch over to the protection fiber • Need an APS protocol • Support low priority traffic on the protection fiber • Allow N working systems to share a single protection system

11. OMS-DPRing • Dedicated protection ring • Two fibers operate in opposite directions • Each node transmits on both directions of the ring • Different nodes must transmit at different wavelengths • Normal operation: the ring functions as a bus, with one pair of amplifiers turned off and all the others turned on • When a link fails: an amplifier pair next to the failed link are turned off and the ones that were originally inactive are turned on

12. OMS-SPRing • Shared protection ring • Four fibers, analogous to a SONET BLSR/4 • The two protection fibers do not have attached WDM equipment • Use either span switch or ring switch • Two-fiber version • Dedicate half the wavelengths on each fiber for protection purposes • Make the protection wavelengths on one fiber correspond to the working wavelengths on the other fibersignals can be rerouted w/o requiring wavelength conversion

13. 1+1 OCh Dedicated Protection • Works in point-to-point, ring (OCh-DPRing), and mesh configurations • Two lightpaths on disjoint routes are setup for each client connection • The client signal is split at the input, the destination selects the better of the two lightpaths • No signaling requiredfast restoration • No protection bandwidth sharingbandwidth inefficient

14. OCh-SPRing • Shared protection ring • Similar to SONET BLSR/4, but operate at the optical channel layer • Working lightpaths are set up on the shortest path along the ring • When a working lightpath fails, it’s restored using span switch or ring switch • Non-overlapping lightpaths in the ring can share a single wavelength around the ring for protectionmore efficient than OCh-DPRing

15. OCh-Mesh Protection • Backbone networks are meshed • For mesh networks, OCh-mesh protection schemes are more bandwidth-efficient than rings • Efficiency improvements range from 20% to 60%

16. Path-Based v.s Link-Based Protection • Path-based: the connection is rerouted end to end on an alternate path • Need notify the source node upon a failure • Link-based: the connection is rerouted on an alternate path around the failed link • Need not notify the source node upon a failure • Enable faster restoration than path-based schemes

17. Offline v.s Online Protection • Offline protection • Protection path and wavelengths are reserved at the time of connection setup • In path-based scheme, a link-disjoint protection path is reserved • In link-based scheme, protection paths are reserved around each link of the working lightpath • Fast and guaranteed restoration • Online protection • Search for protection paths using the spare capacity in the network upon a failure • Capacity efficient • Slow and no guarantee of restoration

18. Dedicated v.s Shared Protection • An offline scheme can use either dedicated protection or shared protection • Dedicated protection: each working lightpath is assigned its own dedicated protection bandwidth • Shared protection: if two working lightpaths are link-disjoint, they can share protection bandwidth • More capacity efficient than dedicated protection • Protection lightpaths are set up after a failure occurs

19. Classification of OCh-Mesh Protection Schemes OCh-mesh protection schemes offline online Dedicated Shared Link-based Path-based Link-based Path-based Link-based Path-based

20. Internetworking between Layers • Need coordination between protection mechanisms in different layers • Bottom-up sequential approach: start at the layer where the failure occurs, let the layer try to restore service first, then let the higher layer try • Option 1: have the restoration in the lower layer happen so quickly that the upper layer doesn’t detect the failure • Option 2: impose an additional hold-off time in the higher layer before it attempts restoration