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

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 Path-basedLink-basedSharedDedicated 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