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-38 -36 -34 -32 -30 -28 -26 -24 -22 -20 Average Received Power : Pr(dBm) Tho2 Fig. 3. Bit-error-ratecharacteristics.
the transmission path between station 1 and station 4 through station 2 and station 3 with each input power of - 18 dBm/ch. Allowable transmission loss of 23 dB was confirmed for all transmission paths. *Opto-Electronics Research Laboratories, NEC corporation, 4-1 -1, Miyazaki, Miyamae-ku, Kasawaski, 21 6, Japan ““2nd Transmission Division, NEC Corporation 1. R.E. Wagner et al., IEEE J. Lightwave Technol. 14, 1349-1355 (1996). 2. P.A. Perrier et al., in Optical Fiber Communication Conference, Vol. 2, 1996 OSA Technical Digest Series (Optical Society of America, Washington, D.C., 1996), paper ThD3. 3. R.E. Wagner etal., presented at OEC’94,1994, Japan, paper 14C3-1. 4. T. Hosoi etal., in Optical Fiber Communication Conference, Vol. 2 of 1996 OSA Technical Digest Series (Optical Society of America, Washington, D.C., 1996), paper ThL2. 5. W. Nakabayashi et al., presented at ECOC’96,Oslo, Norway, paper Thd2-4.
Automatic protection switching for multicasting in optical mesh networks
OFC ’97 Technical Digest tection) (see Fig. 1). Restoration times are roughly 50 ms (e.g., SONET) for low-speed rotary switches and a few ps’ for high-speed switches. APS requires a priori knowledge of the full network to preplan routes and sufficient spare capacity to accommodate rerouted traffic. Another common approach, based on digital crossconnect switches (DCS), performs dynamic local rerouting around a link and generally leads to better bandwidth utilization.2 However, the restoration times are of the order of seconds. The most common approach to APS relies on self-healing rings (SHRs), as in SONET, interconnected with diversity protection (DP), where a logical link may be routed along more than one physical link. Limiting ourselves to such building blocks, however, affects the cost of the network. For instance, if the cost of laying fiber to be proportional to length, then for certain node configurations a physical ring connection may not be the cheapest redundant option. In order to extend path protection to arbitrary redundant networks, schemes have been developed for finding a pair of link or node-disjoint paths for every pair of nodes.3 In a network with multicasting, such an approach may be very inefficient in bandwidth. Trees are desirable for multicasting, particularly in optical networks, where multicasting may be performed by signal splitting. Schemes for finding spanning trees which are edge-di~joint~ impose more topology constraints than simple redundancy (see Fig. 2), because an edge cannot be used in both the primary and the secondary tree. Schemes that look at arcs (directed edges)5 do not require stricter requirements than redundancy, but fail to take into account that failure of communications in one direction usually implies failure in the reverse direction also (see Fig. 2, where failure of the circled arcs disconnects the destination). Our novel approach finds redundant directed trees in arbitrary redundant mesh networks where failure of one edge entails failure of communications in both directions on that edge. Thus, the problem of finding the minimum cost redundant topology and the problem of the APS scheme can be handled separately. For every source node s, we find two spanning trees, which we shall denote by Blue and Red, rooted at s. Blue is the tree used when there are no failures. The trees are selected so that, if a failure of a link or a node occurs, the traffic upstream of that failure on Blue need not be re-routed, while the traffic downstream of the failure will use Red. Figure 3 gives an example of our construction. We see, in contrast with Fig. 2, that several edges are used by both the primary and the secondary tree but that failure of two arcs on the same edge does not disconnect any node from the source. We can view our method as a
Muriel Medard, Steve Finn, Rick Barry, MIT Lincoln Laboratoiy, 244 Wood Street, Lexington, Massachusetts 021 73; E-mail: medard@l\.miL edu Reliability in optical networks is a growing area of concern as highbandwidth networks are expanded and interconnected. Even if robust operating conditions are established, redundancy is necessary to preclude catastrophic loss of data in case of failure of a link or of a node in a network. We shall give a brief overview of the common approaches to failure recovery, discuss the related topological issues and present a new approach to multicasting, which reduces topological constraints to a provable minimum. In very high bandwidth networks, service restoration time is important because many bits may be lost while service is disrupted. Therefore, we look at automatic protection switching (APS) schemes, which rely on pre-planned alternative end-to-end routes (path pro-
Tho3 Fig. 1. Example of path rerouting.
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OFC ’97 Technical Digest
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Wavelength-selective cross-connect architecture interconnecting multiwavelength self-healing rings Georgios Ellinas, Gee-Kung Chang, M.Z. Iqbal, John Gamelin, Mamun ur Rashid Khandker, Bellcore, Red Bank, New Jersey 07701; E-mail:
[email protected] -4p d m r r y a c
--woondpry.sc
Tho3 Fig. 2. trees.
(a)
Edge-disjoint spanning trees and (b) arc-disjoint spanning
destination
source
-+ primarytree ---)
secondary tree
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unused links
Tho3 Fig. 3. Spanning trees built with our algorithm.
generalization of rings in whid rings may share an arbitrary number of nodes and links. The time complexity of our algorithm is polynomial in the number of nodes. Our algorithm can yield more than one pair of redundant trees. The ability to select among several possible redundant trees according to some criteria, such as cost or bandwidth considerations, is an interesting area of future research. 1. T.-H. Wu, FiberNetworkServiceSuwivabiZity(ArtechHouse, 1992). 2. W.D. Grover, in Proceedings of GLOBE-COM87, Vol. 2, pp. 28.2.128.2.6. 3. R. Bhandari, in IEEE INF(ICOM’94,Vol. 3, pp. 1 lc.3.1-1 l.c.3.11. 4. B. Yener, Y. Ofek, M. Yung, in Proceedings of GLOBECOM’94, Vol. 1, pp. 169-175. 5. Y. Shiloah, Information Processing Lett. 8, (1979).
Networks using ring architectures provide increased reliability and survivability to the network. The need to connect multiple rings together increases as the network expands. This report focuses on wavelengthselective cross-connect (WSXC) architecures used to interconnect multiwavelength self-healing rings (SHRS).’ Multiwavelength SHRs use most of the protection concepts incorporated in the SONET SHRs.’?’ Cross-connect (XC) architectures for the interconnection of multiwavelength SHRs should be able to preserve the survivability properties of the SHRs, and they should be transparent to growth accommodation in terms of interconnecting capacity and ring variations. It is desirable for these XC switches to have add/drop capabilities for interconnecting multiwavelength ring networks so as to increase the network functionality. Even though this usually requires optical switches with large dimensions for each wavelength “plane,” they must be kept relatively small in size for performance and integration concerns. We have developed a compact XC switch design that preserves the survivability characteristics independent of ring network architectures adopting various protection schemes. Two new approaches are presented for two different kinds of SHRs [Path Protection (PP) and Automatic Protection Switching (APS) SHRs]: (I). For two-fiber U-SHR/PP (Unidirectional) ring interconnection, one of the fibers in each ring is used for protection and carries data identical (in every wavelength) to the working fibers. Thus, we never have to switch wavelengths between working and protection fibers. This simplifies the design of the required 6 X 6 XC switch (4 ports corresponding to the two-fiber ring interconnection and two ports corresponding to the addldrop [Fig. 1(a)] into a configuration of two 3 X 3 XC switches [Fig. l(b)]. One 3 X 3 XC switch can switch wavelengths between the working fibers and the other can switch wavelengths between the protection fibers. Associated with each 3 X 3 XC switch is a single add/drop fiber “cluster” (N fibers corresponding to the N wavelengths in the network). In order to preserve the survivability features, both 3 X 3 XC switches need to perform identical functions. This way, if a failure occurs, the receiver at the destination can switch from the working to the protection fiber signal in order to restore the service. Using this “decoupling” approach we also eliminate the single point of failure in the network. The same analysis applies for the interconnection of two two-fiber WDM B-SHR/2 (bi-directional) rings, which use wavelength interchange for failure restoration. (11). In the case of two-fiber U-SHWAPS ring interconnection, there is never a need to switch wavelengths between the working and protection fibers or between protection fibers in the two rings. This simplifies the design of the required6 X 6 XC switch into a configuration of one 3 X 3 XC switch and two permanently connected loops [Fig. l(c)]. Similar analysis applies for the interconnection of two four-fiber WDM B-SHR/APS rings. The protection fiber loops help us recover from any failure and the XC switch does not change states because of the failure. In addition to the two-ring interconnections we have showed that this concept is scalable to K-ring interconnections using a single XC
