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TOPICS IN LIGHTWAVE

Architectural Issues for Robust Optical Access Muriel Médard, Massachusetts Institute of Technology Steven Lumetta, University of Illinois Urbana-Champaign

ABSTRACT Optical access networks are beginning to be deployed at the edge of the optical backbone network to support access by the high-end users that drive increased bandwidth demands. This development in the applications of optical networking poses new challenges in the areas of medium access, topology design, and network management. In particular, since optical access networks carry high volumes of critical traffic, the level of reliability and robustness traditionally reserved for core applications must be implemented in access networks. In this article we survey access network architectures and outline the issues associated with providing reliability for these architectures.

INTRODUCTION

This work was supported by grant MDA972-99-10005 from the Defense Advanced Research Projects Agency.

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Optical networking challenges are increasingly geared not just at obtaining higher rates, unrepeatered over longer distances, but at provisioning services effectively and reliably to end users. It is the end users and their applications that fuel the demand for bandwidth. The rates currently found in high-end enterprise networks, storage area networks (SANs), application service providers (ASPs), and Web hosting sites are in the tens of gigabits per second, rivaling the rates carried in core systems, prompting the use of optical networking in such high-rate local area networks (LANs). Simultaneously, applications such as site mirroring and corporate intranets require these high rates to be supported over new or existing metropolitan area networks (MANs). This dramatic increase of data traffic has prompted deployment, at the edge of the network, of systems operating at rates heretofore reserved for core transport. Such networks may be solely devoted to access, or may simultaneously perform access and backbone functions. The purpose of such edge deployment is to allow several users to share large portions of bandwidth in a flexible fashion rather than provide wholesale high-bandwidth channels, as in back-

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bone applications. These channels are generally wavelengths in wavelength-division multiplexed (WDM) systems. An overview of system architectures for access networks is given in [1]. We define optical access networks as networks in which several users share a single wavelength. While backbone network nodes are switches with roughly similar capabilities, optical access networks may have different types of nodes. In particular, the purpose of access nodes is to enable users to access shared wavelengths. These nodes may, for purposes of affordability, have significantly fewer features than backbone switches. Switches in access nodes may be the same switches as in backbones, or may have a more limited specialized set of capabilities. Two major issues emerge: the design of effective medium access control (MAC) methods; and the design of architectures, encompassing node features, topologies, and routing, to implement optical access networks. The first area relates to determining how to access the large bandwidth available in the optical domain in an efficient and cost-effective way [2]. In the area of architecture design, two main approaches emerge. The first considers dedicated optical access networks, such as stars or folded buses, to implement optical access LANs and MANs. The second is overlay architectures, which use existing network infrastructure. Overlay architectures seek to replicate, on a smaller scale, logical topologies akin to those of backbone networks, or may instead create architectures specifically designed for access purposes. The purpose of this article is to investigate the architectural issues of optical access networks as they relate to reliability and robustness. In particular, we explore the impact of reliability and robustness considerations on the design of optical access network architecture. Studies of survivability in optical networks are frequently focused on backbone networks [3], but the techniques employed in the backbone context are not always directly applicable and equally effective for access networks. We provide an overview of standalone architectures for optical access networks, in which a

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network is devoted to optical access, and discuss the reliability features of these architectures. We present optical access architectures overlaid on existing MANs or wide-area networks (WANs). These architectures seek to create logical networks for access. We present the reliability implications of creating such logical networks using methods directly borrowed from optical backbone networks, as well as an architecture designed specifically for optical access networks. Finally, we offer our conclusions.

(a) Automatic path switching

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DEDICATED ACCESS ARCHITECTURES In this section we describe the four most common designs for dedicated optical access architecture. The first design alternative attempts to replicate the packet-switching techniques developed for electronic routers in the optical domain. The second alternative uses rings to create robust architectures. The last two alternatives, based on star and bus topologies, suggest simple architectures, but pose challenges for reliability.

PACKET-SWITCHED APPROACHES Packet-switched approaches to optical access seek to develop in optics a subset of the functionality provided by traditional optoelectronic packet-based networks [4, 5]. Attractive functionality includes header recognition, buffering, packet insertion, packet reading, packet retrieval, and rate conversion. Supporting these operations in the optical domain is challenging; even for those operations that have been successfully demonstrated in laboratories, no consensus has emerged as to the best implementation. Nevertheless, generalizations are possible. For example, operations that involve timing issues, or introduce loss and distortion on the data stream, are among the most challenging. Replicating a stream, for instance, is a relatively straightforward operation requiring only passive optical splitters. Merging streams is very challenging: in sharp contrast to the electronic domain, no practical method exists of delaying an optical signal for a dynamically chosen interval. Packet-level operations are among the most challenging, but can also be grouped by difficulty. Reading signals from an optical data stream is relatively easy, since a small portion of an optical signal, bled off of the main signal, suffices for such operations. Removing a packet (perhaps after reading it) is very difficult since it involves performing an operation on the whole stream, as well as timing, phase, and polarization issues. Operations such as packet switching are also challenging because of issues of timing and optical switch speeds. Thus, fully optical packet-switched systems replicating the entire operations of electronic systems are still distant. The overhead required for recovery in packet-based systems depends critically on which functionalities are implemented in the optical domain. Approaches that redirect traffic around a failure on a packet-by-packet basis are challenging in the optical domain, since they require operating on the whole data stream and possibly separating packets from a stream. To avoid packet-level operations, flow switching on a

IEEE Communications Magazine • July 2001

■ Figure 1. Path protection and node recovery in a ring: a) UPSR; b) BLSR.

stream-by-stream basis, say using multiprotocol label switching (MPLS), is a promising alternative. Stream-based operations are more amenable to optical processing, and streambased processing reduces roughly to circuitbased recovery, to which many of the techniques discussed later in this article can be applied.

RING-BASED APPROACHES Ring topologies are attractive for robustness and are the building block behind many LAN and MAN architectures for reliable communications. Rings can recover from a failure using path protection or link/node recovery. Figure 1 shows both types of operation. Figure 1a illustrates path protection: a connection in the clockwise direction, severed by failure of the failed node, shown exploded in the figure, is replaced by a connection in the counterclockwise direction. Path protection is similar to the operation of unidirectional path-switched rings (UPSRs) in synchronous optical network (SONET). Link/node recovery, shown in Fig. 1b, is similar to the operation of SONET bidirectional line-switched rings (BLSRs). In this case, the clockwise connection is redirected to the counterclockwise direction by the node immediately upstream of the failure rather than the source node. This redirection is termed loopback. Loopback recovery can be done in different ways. In fiber-based recovery, the entire traffic carried by a fiber is backed by another fiber, as shown in Fig. 2a. The number of wavelengths carried by the fiber is in this case irrelevant — all light is transferred from one fiber to a second one. In WDM-based recovery, restoration is performed on a wavelength-by-wavelength basis, and recovery capacity may or may not be available for all wavelengths. WDM-based recovery requires only two fibers, although it can be applied to any number. Figure 2b illustrates WDM-based recovery. If primary fibers operate at half of total capacity or less, only two fibers rather than four are needed to provide recovery. Moreover, certain wavelengths may be selectively given restoration capability. Many schemes besides SONET exist for ring-based optical access. Fiber distributed data interface (FDDI), for example, passes a token cyclicly among nodes in a ring to control trans-

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Loopback l1 l2

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Fiber 4 l1 l2 Primary traffic is carried by fiber 1 and fiber 2. Backup is provided by fiber 4 for fiber 1 and by fiber 3 for fiber 2. a. Traditional fiber-based loopback

Fiber 1 l1 l2 l1 l2 Fiber 2 Primary traffic is carried by fiber 1 on l1 and by fiber 2 on l2. Backup is provided by on l1 on fiber 2 for l1 on fiber 1 and by l2 on fiber 2. b. WDM-based loopback

■ Figure 2. a) A four-fiber system with fiber-based loopback; b) a two-fiber system with WDM-based loopback.

mission. IEEE 802.5 defines such token rings. Multiple ring topologies may be interconnected through a hub, or rings may coexist in a logically interconnected fashion over a single physical ring, or rings may be arranged hierarchically. The IEEE 802.17 Resilient Packet Ring (RPR) Working Group was recently set up to investigate the use, mainly at the MAN, of an optical ring architecture coupled with a packet-based MAC. The purpose of this project is to combine the robustness of rings with a flexible MAC that is well-suited to current optical access applications.

STAR TOPOLOGIES There has been significant work in the area of optical LANs and MANs using WDM. The vast majority of the proposed architectures consider star topologies, where some type of switch, router, or other type of hub is placed in the center of a topology, and each node is directly connected to the hub [6]. The emergent 10 Gb/s standard (IEEE 802.3ae) for LANs and MANs also allows for optical stars and trees. In WDM-based LANs, each wavelength constitutes a single channel. These star architectures usually involve a passive optical broadcast star. The broadcast star architectures may have tunable transmitters and fixed receivers, tunable receivers and fixed transmitters, or tunable transmitters and receivers. Figure 3 shows a tunable receiver/fixed transmitter architecture and two different connection configurations. Note that this

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architecture allows for easy multicasting by having several receivers tuned to the same wavelength. Since the topology is very simple, the flexibility in the operation of WDM-based stars stems entirely from adequate scheduling. The main challenge is to provide efficient use of the star resources without excessive overhead due to changing the connectivity of the star. Stars present many weaknesses in terms of reliability. Failure of the hub can bring down the whole network, but total failure of a passive broadcast element is unlikely. However, many partial failure scenarios exist, including amplifier failures, port connection failures at the access nodes or at the hub, transmitter or receiver failures at access nodes (e.g., because of laser failures or tuning skew), and cabling failures in the fiber. Such failures generally lead to failure of one or more arms of the star. With a star topology, the only means of providing recovery is through 1+1 redundancy (i.e., complete replication of the system), but the operation of such totally redundant systems is difficult. In order to illustrate this fact, consider what occurs currently in other reliable star-based networks. The star is the usual topology for most enterprise networks and SANs. These systems currently use single-channel optical connections rather than WDM. Each fiber carries traffic on single wavelength. The MAC is generally Gigabit Ethernet (GigE) for enterprise networking or Fibre Channel (FC) for SANs. Network interface cards (NICs), housing both a receiver and a transmitter, are optically connected to an electronic switch. Such a switch is closer to a traditional router and is very different from the passive broadcast hubs or wavelength-selective switches discussed in the context of WDM starbased LANs. For such networks, redundancy is obtained by full duplication of all resources. Replication alone is not adequate; a connection between the two switches is also desirable. Indeed, consider the case of failure of the primary NIC in server 1. Server 1 communicates via the secondary NIC connected to the secondary switch. Requiring other servers to also communicate via the secondary switch is undesirable, since such networks typically have many servers connected to them, and reconfiguring so many connections simultaneously is difficult and involves initialization overheads. To avoid reconfiguration at all servers, all servers other than server 1 continue to communicate with the primary switch, and the two switches communicate with each other via the interswitch connection. In the context of optical networks, an interswitch connection translates into a connection between two hubs. In order to manage such an interhub connection, the hub must be capable of more than simple optical broadcasting. Thus, reliable optical access networks based on passive stars will be difficult to deploy, since the traditional means of providing robustness for these topologies does not readily extend to the passive elements that form the core of the optical form.

BUS TOPOLOGIES Bus schemes treat wavelengths as shared media between nodes, allowing nodes to place and retrieve traffic according to the rules of a MAC

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protocol. In a folded bus, a single bus, originating at a head-end node, serves all nodes. Typically, nodes use the bus first as a collection bus, onto which they place traffic, say in the left to right direction. The last node folds back the bus to make it travel in the right-to-left direction. In the right-to-left direction, nodes collect traffic placed onto the bus. The traffic may be read only or read and removed. In a dual bus architecture, two buses are used, each with its own head-end. Folded and dual buses are simple options for LANs and certain types of MANs. In particular, they offer an effective way to share bandwidth among several users. Extensive analysis of folded bus schemes, such as distributed queue dual bus (DQDB), has been carried out. WANs using such buses are defined by IEEE 802.6. Optical bus schemes also exist, including helical LAN (HLAN) and optical reservation multiple access (ORMA). The analysis for folded and dual buses is almost entirely concerned with issues of bandwidth allocation, such as fairness and bandwidth efficiency. Folded and dual buses suffer from reliability drawbacks. Figure 4 shows a folded bus and a dual bus after a failure. Partial recovery can be effected by creating a bus on either side of the failure. For a dual bus architecture, the node immediately upstream of the failure needs to be able to fold the bus. In order to reestablish full connectivity after a failure, the end nodes of the original buses must be able to connect outside of the original buses to transmit traffic that was destined to traverse the cut.

OVERLAY ACCESS ARCHITECTURES Optical MAN and LAN access can also be supported as overlay networks on an existing physical infrastructure, avoiding the need to install a full physical infrastructure. Overlay networks can be designed to be independent of other overlay networks and other services supported by the common physical infrastructure. In this regard, overlays are similar to the virtual private networks supported by backbone networks. Internet Protocol (IP) overlays have been proposed in many contexts, and, to some extent, MPLS and other types of tunneling can be viewed as a type of overlay. For our discussion, we assume the existence of a physical infrastructure to which access nodes can be added. Robustness in an overlay network requires redundancy in both the logical and physical topologies. For robustness to link failures, the topologies must be link-redundant, that is, any single link can be removed without breaking the network into disconnected pieces. Similarly, robustness to node failures requires that removal of any single node (and all links connected to it) leaves all other nodes in the network connected. Several overlay methods have been developed for optical backbone systems, including ring-based overlays, redundant trees, and mesh-based recovery. In this section we survey these approaches, and discuss their benefits and limitations. Extending such methods for use with access networks requires the addition of a MAC protocol. Such access may be packet-oriented, and use time-division multiple access (TDMA) or code-division

IEEE Communications Magazine • July 2001

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■ Figure 3. Broadcast star architecture with tunable receivers and fixed transmitters. multiple access (CDMA). In the next section we consider an architecture that is not an extension of methods developed for backbones, but was developed specifically for the optical access network.

RING-BASED OVERLAYS Robustness in many optical backbone networks relies on the presence of physical or logical rings. An overlay network can be constructed on a physical mesh topology by selecting cycles within the mesh and providing service protection or restoration on each cycle as if it were a physical ring. A simple overlay requires only a single ring overlaid on a physical mesh network. However, such a ring may not always exist, or may be too large. A large ring — one that includes many links — forces long routes for many connections. Long routes have several drawbacks, including reduced wavelength assignment efficiency and increased jitter. A more general approach selects an overlay of several rings to cover a set of access nodes. This approach raises issues, similar to those encountered in SONET networks, regarding management of links included in more than one ring and node management for ring interconnection. Consider first a link included in more than one ring. If resources are unavailable or overly expensive, the overlay network must share available resources between rings. In this case, the overlay network protocols must avoid resource conflicts over the shared span, which

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■ Figure 4. a) Folded and b) dual buses being restored after a failure.

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Trees are very attractive for a variety of applications, particularly multicasting applications. In that case, a single node, called the root of the tree, transmits to several nodes in the networks. In optical applications, multicasting can be performed in a simple passive manner, by splitting light.

can incur significant network management overhead. The second issue relates to node interconnection among rings. Minimizing the number of rings in ring covers is a hard problem. A second approach to ring covers, intended to overcome the difficulties of the first approach, is to cover every link with exactly two rings, each with two fibers [7]. The approach is an application of the double-cycle ring cover. A doublecycle ring cover covers a graph with cycles in such a way that each edge is covered by two cycles. Each cycle corresponds to either a primary or a secondary two-fiber ring. Let us consider a link covered by two rings, rings 1 and 2. If we assign a direction to ring 1 and the opposite direction to ring 2, ring-based recovery using the double-cycle cover uses ring 2 to back up ring 1. In effect, this recovery is similar to recovery in conventional self-healing rings (SHRs), except that the two rings that form four-fiber SHRs are no longer collocated over their entire length. For WDM systems in general, we may not be able to assign primary and secondary wavelengths in such a way that a wavelength is secondary or primary over a whole ring. Another drawback of the backup paths afforded by double-cycle covers is that traffic traversing a failed link in opposite directions is typically recovered on two distinct rings, implying that the two directions will recover at different times and will incur different timing jitters. Cycle covers of different types generally work very well for link failures but suffer from significant drawbacks in the area of node failure when failures occur at nodes that are shared by more than one link. For instance, node recovery can be effected with double-cycle ring covers, but such restoration requires cumbersome hopping among rings. These drawbacks are also found in SONET networks. The usual method in SONET to handle nodes shared between rings is called matched nodes. Two rings are joined by two matched nodes, a primary and a secondary. The primary matched node is responsible, under normal operation, for all inter-ring communications. The secondary matched node has a live backup of all the traffic at the primary matched node, and mirrors the operation of the primary matched node. Under normal operating conditions, the output from the secondary matched node is disregarded. If failure of the primary matched node occurs, intra-ring traffic is recovered within the ring wherein it lies, and interring traffic is handled by the second node. How to extend matched nodes to cases other than simple extensions of the two-ring topology is generally not known.

REDUNDANT TREES Trees are very attractive for a variety of applications, particularly multicasting applications. In this case a single node, called the root of the tree, transmits to several nodes in the network. In optical applications, multicasting can be performed in a simple passive way by splitting light. Redundant trees are such that, for any link or node failure, every node remains connected to at least one of the trees. For any link failure, the manner in which the trees are constructed ensures that, if the failure occurs upstream of a

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■ Figure 5. Shared path protection.

node on the primary tree, the failure is not upstream of that node on the secondary tree. These types of trees can be extended to node failures, as long as the underlying physical topology is node-redundant.

MESH-BASED RECOVERY Recovery in most current backbone mesh networks is based on path protection, which involves selection of a recovery path for every connection. Figure 5 shows two primary paths and their corresponding recovery paths. The primary paths and recovery paths do not overlap on any node except their endpoints; thus, no single link or node failure can affect both a primary path and its backup. Moreover, as long as the backup paths are not actively in use, they may share bandwidth, as shown in the figure. Such sharing has significant advantages, on the order of 15 to 30 percent in terms of bandwidth efficiency with respect to a ring or ring-based methods such as a double-cycle cover, in which half the bandwidth is devoted to recovery. For certain overlay access networks, path protection may be quite burdensome. Path protection relies on establishing, after failure, a new connection between two points for a full circuit, typically mapped to one or more wavelengths dedicated to that circuit. In access networks, a circuit, typically a wavelength, may be shared by several nodes. Path protection, in order to be applicable to access networks, must reestablish a path that visits all the nodes sharing a circuit. A very simple example of this is an access network requiring connection to four consecutive nodes in a ring; it is impossible to establish two link-disjoint routes that connect all access nodes, although link recovery can be used in a straightforward manner to reestablish service in the access network. Thus, path protection for optical access networks is limited in its applicability. Even when path protection can be performed for access networks, the constraint of visiting certain access nodes may obviate or significantly reduce the bandwidth efficiency advantages that make path protection attractive in backbone settings. An extension of ring covers to reduce the backup capacity requirement to approximately that required for mesh restoration is given in [8]. Known as protection cycles (p-cycles), the extension uses each ring to protect its chords as well

IEEE Communications Magazine • July 2001

as the links on the ring itself. A link on a p-cycle is recovered as a conventional BLSR. A chord is recovered by passing traffic from the failed link in both directions around the corresponding pcycle. The p-cycles approach does not solve the problem of node failures, however, which remain difficult, and is particularly important when lowend access nodes are present. Another approach to link restoration on mesh networks, known as generalized loopback, was first presented in [9]. The principle behind generalized loopback is to select a directed graph, called the primary graph, such that another directed graph, called the secondary, can be used to carry backup traffic for any link failure in the primary. Construction of a primary graph involves selection of a single direction for each link in the network. Loopback then occurs along the secondary graph in a manner akin to BLSR. More than one path may exist for backup. The fact that multiple paths may exist for restoration allows us to reclaim some arcs (fibers) from secondary graphs to carry additional traffic. The capacity efficiency obtained in this manner is, for typical networks, on the order of 20 percent over methods (e.g., double-cycle cover) that require half the capacity to be devoted to recovery, but is still more than that required for optimal mesh recovery. The generalized loopback protocol, however, recovers from both link and node failures without any need to differentiate between the two before effecting recovery.

ROBUST ACCESS TO MESH NETWORKS The discussion in the previous section illustrates the need to develop, for overlay mesh networks, methods that are specific to access networks. In this section we describe one such type of access overlay network using a folded bus architecture [10]. Such an architecture is compatible with a variety of MAC protocols for folded buses, such as those described in previous sections. Our goal is to overcome the recovery limitations of traditional folded buses mentioned before. Our model allows for access nodes with limited capability for better affordability. The two main elements of our network management architecture are the establishment of routes and the recovery mechanism in case of link or node failure. The routes consists of two elements: a collection portion and a distribution portion. Nodes place packets on the access wavelength in the collection portion of the route, and the distribution portion delivers all packets to all nodes. The collection portion of the route is constructed as follows. Select a root vertex (any choice suffices) and build a depth-first search (DFS) numbering beginning at the root vertex. The collection route is a walk that traverses nodes as they are considered by the DFS numbering algorithm. The distribution route consists of a directed spanning tree rooted at the DFS tree root. We call this tree the primary tree. Robustness is afforded in the distribution route by constructing redundant trees of the type described earlier.

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There exist links not in the DFS tree connecting branchings to nodes upstream in the DFS

(a)

(b)

■ Figure 6. Collection route a) before and b) after a failure. We may now address how to ensure robustness against link failures. The crux of our algorithm lies in our method of performing link recovery in the collection portion of the route. Figure 6a shows the collection route before a failure, and Fig. 6b the collection route after a failure. The failure occurs in the link between the nodes shown in gray. The link shown as a dashed line in Fig. 6a is not used in the DFS tree. After the failure, the collection route is repaired as shown in Fig. 6b. A similar method can be applied to node failure, with a redundant head-end providing resilience against failure of the head-end. We may now discuss the capabilities required to implement our access network. The collection route requires a single wavelength, used in both directions. A single wavelength used in both directions is also sufficient to construct the primary and secondary trees. The head-end at the root of the primary and secondary trees must therefore be able to perform wavelength conversion, by placing the traffic from the collection route onto the primary tree. We only require one node to perform wavelength conversion. Alternatively, the collection and distribution routes may be placed on two separate fibers. The access nodes must be able, on the wavelength of the collection route, to perform loopback and, on the wavelength of the distribution

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In the design and deployment of

trees, to perform simple add-drop multiplexing to switch from the primary tree to the secondary in case of a failure.

optical access

CONCLUSIONS

networks, issues

In the design and deployment of optical access networks, issues of reliability must be taken explicitly into account, since retrofitting architectures to provide reliability does not yield attractive solutions. Many of the techniques used for backbone reliability are not directly applicable in the context of access, while others suffer significant drawbacks when applied to access networks. The design of optical access networks therefore benefits from considering explicitly and in an integrated manner the type of access required, the features of the access nodes and switching nodes that share wavelengths, and the types of routes allowed.

of reliability must be taken explicitly into account, since retrofitting architectures to provide reliability does not yield attractive solutions.

REFERENCES [1] A. A. M. Saleh and J.M. Simmons, “Architectural Principles of Optical Regional and Metropolitan Access Networks,” IEEE J. Lightwave Commun., vol. 17, Dec. 1999, pp. 2431–48. [2] B. Rajagolapan et al., “IP over Optical Networks: Architectural Aspects,” IEEE Commun. Mag., Sept. 2000, pp. 94–102. [3] O. Gerstel and R. Ramaswami, “Optical Layer Survivability — An Implementation Perspective,” IEEE JSAC, 2000, pp. 1885–99. [4] E. Modiano, “WDM-Based Packet Networks,” IEEE Commun. Mag., vol. 37, Mar. 1999, pp. 130–35. [5] IEEE J. Lightwave Commun., Special Issue on Photonic Packet Switching Technologies, Techniques, and Systems, vol. 16 no. 12, 1998. [6] P. A. Humblet et al., “An Efficient Communication Protocol for High-Speed Packet-Switched Multichannel Networks,” IEEE JSAC, vol. 11, May 1993, pp. 568–78. [7] G. Ellinas and T. E. Stern, “Automatic Protection Switching for Link Failures in Optical Networks with Bidirectional Links,” Proc. IEEE GLOBECOM, 1996.

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[8] D. Stamatelakis and W.D. Grover, “Theoretical Underpinnings for the Efficiency of Restorable Networks Using Preconfigured Cycles (p-cycles),” IEEE Trans. Commun., vol. 18, no. 8, 2000, pp. 1262–65. [9] M. Medard, S. G. Finn, and R. A. Barry, “WDM Loopback Recovery in Mesh Networks,” Proc. IEEE INFOCOM, 1999. [10] M. Medard and S. S. Lumetta, “Robust routing for local area optical access networks,” Proc. LEOS Summer Topical Mtgs., 2000, pp. 39–40.

BIOGRAPHIES MURIEL MÉDARD [[email protected]] is an assistant professor in EECS and a member of the Laboratory for Information and Decision Systems at the Massachusetts Institute of Technology (MIT). Her main research interests are in robust communications, wireless capacity, and optical networking. Of particular interest to her are direct access networks, where users can use low-speed wireline or wireless links to access high-speed networks without the levels of indirection currently found in aggregated networks. Her current projects consider packetized access of wireless media, robust packetized access in high-speed networks, capacityefficient restoration in optical networks, local failure recovery in optical networks, and very wideband wireless communications. She consults to Malachite Technologies, which build network switches. She received her B.S. in EECS and B.S. in mathematics in 1989, B.S. in humanities in 1990, M.S. in EECS 1991, and Sc.D. in EE in 1995, all from MIT. She was previously a staff member at MIT Lincoln Laboratory and an assistant professor at the University of Illinois Urbana-Champaign. STEVEN S. LUMETTA [[email protected]] is an assistant professor of electrical and computer engineering and a research assistant professor in the Coordinated Science Laboratory at the University of Illinois at Urbana-Champaign. He received an A.B. in physics in 1991, an M.S. in computer science in 1994, and a Ph.D. in computer science in 1998, all from the University of California at Berkeley. He has worked on a wide range of problems in scalable parallel computing, including languages (Split-C), tools (Mantis debugger), algorithms, and runtime systems, culminating in his dissertation on multiprotocol user-level communication on clusters of SMPs. His research interests are in optical networking, high-performance networking and computing, hierarchical systems, and parallel runtime software.

IEEE Communications Magazine • July 2001