In order to design a survivable optical network, one must lay .. WDM layer, into the layered architecture. . For example, if the BSHR/2 architecture is used in.
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Half the wavelengths in each fiber are reserved for protection.
Optical Networks
If a link failure occurs, the OADM adjacent to the link failure bridges its outgoing channels in a direction opposite to that of the failure and selects its incoming working channels from the incoming protection channels in the direction away from the failure. Note that the transmission from A has been switched from the inner ring to outer ring.
This is called ring switching. The problem with this is the long length of the protection paths, which may limit its use to short haul metropolitan networks. Figure 5 shows the 4-fiber SPRings case. Two fibers each are allocated for working and protection. The operation is similar to that of the 2-fiber SPRing. However, this system can allow span switching in addition to ring switching. Span switching means that if only the working fiber in a link fails, the traffic can use the protection fiber in the same span.
In case of 2-fiber systems, it will have to take the longer path around the ring. Sometimes the need arises to protect against isolated optoelectronic failures that will affect only a single optical channel at a time. Thus, a protection architecture that performs channel level switching based on channel level indications is required.
ITU-T draft recommendation G. This protocol requires that after a span switching a path should not traverse any span more than once. When ring switching occurs, this may not be true. This protocol is essential in long-distance undersea transmissions to avoid unnecessary delay. A Summary of point-to-point and ring architectures is shown in Table 1. Along a single fiber, any two connections cannot use the same wavelength. The whole problem of routing in a WDM network with proper allocation of a minimum number of wavelengths is called the routing and wavelength assignment RWA problem.
It is found that in arbitrary mesh architectures, where the connectivity of each node is high, the number of wavelengths required greatly decreases. This is the advantage of having a mesh architecture. Moreover addition of new nodes and removing existing nodes becomes very easy. There are 3 broad methods for using mesh architectures. These are as follows:. This itself can be brought about in the following ways: Having a centralized route computer would mean route flexibility but a single point of failure.
Distributed processing can be brought about by performing rote computation at the ends of failed line or the end of the path, but route flexibility is compromised. There is lot of signaling complexity associated with this method. The latter is bandwidth efficient but comes with additional size and complexity of the protection tables to be maintained. Both the above methods are time consuming. Hence, an automatic protection switching mechanism, like that for the rings, is required.
Three alternatives are briefly discussed here:. The whole mesh configuration is divided into smaller cycles in such a way that each edge comes under atleast one cycle. Along each cycle, a protection fiber is laid. It may so happen that certain edges come under more than one cycle. In these edges, more than one protection fiber will have to be laid. Hence, the idea is to divide the graph into cycles in such a way that this redundancy is minimized [16]. The networks considered have a pair of bi-directional working and protection fibers.
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Fault protection against link failures is possible in all networks that are modeled by 2-edge connected digraphs. The idea is to find a family of directed cycles so that all protection fibers are used exactly once and in any directed cycle a pair of protection fibers is not used in both directions unless they belong to a bridge[4]. For planar graphs, such directed cycles are along the faces of the graph. For non-planar graphs, the directed cycles are taken along the orientable cycle double covers, which are conjectured to exist for every digraph.
Heuristic algorithms exist for obtaining cyclic double covers for every non-planar graph. Figure 6 shows five protection cycles formed along the faces of a planar graph. The two directions of link 'a' are protected by cycles 1 and 2. Thus both directions of traffic are protected. Similarly, the 2 directions of link 'b' are protected by cycles 3 and 5.
A double-cycle ring cover covers a graph such that each edge is covered by two cycles. Cycles can then be used as rings to perform restoration. For a two edge connected planar graphs, a polynomial time algorithm exists which can create double-cycle covers. For two-edge connected non-planar graph, the existence of a double-cycle cover is a conjecture and no algorithm other than an exhaustive search exists[11]. Primary and secondary wavelengths cannot be assigned in such a way that a wavelength is secondary or primary over a whole ring.
Therefore, the cycle double cover requires wavelength changers which are costly or infeasible. With each edge [x,y] of an undirected graph, arcs x, y and y, x are associated. In the event of a failure, connections on one wavelength in B are looped back on the same wavelength around the failure using R. In figure 7, two spanning tress B and R have been shown for the graph G. Consider the example shown in Figure 7. Then z will use graph B itself to route the traffic as it would do if the traffic would have arrived from x. Edge [y, z] can be successfully bypassed if there exists a path with sufficient capacity from y to z in R and a path with sufficient capacity exists from z to y in B.
Details of determining B, given a graph G can be found in [6]. The optical layer forms the lowest layer of the network architecture. It provides the service of transport to higher layers. The issues are choice of the higher layers and the presence of survivability mechanisms in one or more of the higher layers. Following problems are encountered if only the optical layer provides survivability:. Multi-layer recovery can combine the merits of optical layer and the higher layer schemes.
More specifically, the protection mechanism of optical layer can be combined with the restoration mechanism of the higher layers. A good strategy would be the nesting of survivability mechanisms and avoidance of unnecessary interworking. The rerouting capability of the optical layer can be expanded and newer bandwidth efficient protection can be facilitated if there is some controlled coordination between the optical layer and a higher layer that has a signaling mechanism. Similarly, the optical layer which cannot detect faults in the router or switching node, could learn of the faults if the higher layer communicated this to it.
Then, the optical layer can initiate protection at the lower layer. Fast signaling is the main advantage of the MPLS layer in protection. In fact, a protection priority could be used as a differentiating mechanism for premium services that require high reliability. The MPLS layer has visibility into the lower layer, which alerts about faults by a liveness signaling message.
This restoration mechanism and how it can operate with optical layer protection are discussed in the next section. The model consists of IP routers attached to an optical core network. Each OXC is capable of switching a data stream using a switching function, controlled by appropriately configuring a cross- connect table [9].
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The switching within the OXC can be accomplished either in the electrical domain, or in the optical domain. In this network model, a switched optical path is established between IP routers[3]. FECs associate IP traffic with what is commonly called a next hop: A device that can group traffic into an FEC is called a Label edge router. An arbitrary mesh connection of optical routers OXCs is shown in Figure 8. Protection switching relies on rapid notification of failures.
Once a failure is detected, the node that detected the failure must send out a notification of the failure by transmitting a Failure Indication Signal FIS to those of its upstream LSRs that were sending traffic on the working path that is affected by the failure. Depending on whether or not the PML is a destination, it may either pass the traffic on to the higher layers or may merge the incoming traffic on to a single outgoing LSR.
Since the LSPs are unidirectional entities and protection requires the notification of failures, the failure indication and the failure recovery notification both need to travel along a reverse path of the working path from the point of failure back to the PSL s. When label merging occurs, the working paths converge to form a multipoint-to-point tree, with the PSLs as the leaves and the PML as the root. The reverse notification tree is a point-multipoint tree rooted at the PML along which the FIS and the FRS travel, and which is an exact mirror image of the converged working paths.
The establishment of the protection path requires identification of the Working path, and hence the protection domain. In most cases, the working path and its corresponding protection path would be specified via administrative configuration, and would be established between the two nodes at the boundaries of the protection domain the PSL and PML via explicit or source routing using LDP, RSVP, signaling alternatively, using manual configuration.
The Reverse Notification Tree RNT is used for propagating the failure indication and restoration signals and can be created very easily by a simple extension to the LSP setup process as shown in Figure 9. During the establishment of the working path, the signaling message carries with it the identity address of the upstream node that sent it. Each LSR along the path simply remembers the identity of its immediately prior upstream neighbor on each incoming link.
The node then creates an inverse cross-connect table that for each protected outgoing LSP maintains a list of the incoming LSPs that merge into that outgoing LSP, together with the identity of the upstream node that each incoming LSP comes from. Upon receiving an failure indication signal the LSR does the following:. The reverse notification tree arising from node 8 is shown in the figure 9. When the link between 5 and 8 fails, the failure indication signal starts from 5.
At node 4, it will branch to 6 and 3. So far, we have seen the different existing methods to provide survivability in an optical network. Each method has its own advantages and complexities. This section hints at a method of combining the good features of all. Consider the overlay network configuration shown in Figure Light paths need to be established through the various optical switches in the optical subnet for end-to-end transfers between the edge switches.
The edge switches are those where the optical signal is first introduced. The optical subnet is assumed to be a mesh architecture considering its inherent advantages. Each light path connection may have different setup, hold, and restore priorities, say from 1 to 3. At the time of connection setup, the requests with high setup priorities are allocated resources first. Hold priority determines the eligibility of a connection to be preempted if a shortage of resources arises.
Connections with low hold priority are preempted if a high priority request needs resources for initial setup or restoration. The restoration priority determines the order in which the connections are restored upon a failure. Connections with high restoration priority may have protection paths setup while those with lowest restoration priority may not be restored at all. Although connections with high setup priority may also have high hold priority and a high restoration priority, these three priorities could be different.
In case of a failure along a high restoration priority light path, it would just mean a mechanical switch to the already configured alternate path, when there is available bandwidth along the alternate path. Cross-Layer Design in Optical Networks. Group Cell Architecture for Cooperative Communications. Optical Fiber Sensor Technology.
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