To support bursty traffic on the Internet (and especially WWW) efficiently, optical burst switching (OBS) is proposed as a way to streamline both protocols and hardware in building the future generation Optical Internet. By leveraging the attractive properties of optical communications and at the same time, taking into account its limitations, OBS combines the best of optical circuit-switching and packet/cell switching. In this paper, the general concept of OBS protocols and in particular, those based on Just-Enough-Time (JET), is described, along with the applicability of OBS protocols to IP over WDM. Specific issues such as the use of fiber delay-lines (FDLs) for accommodating processing delay and/or resolving conflicts are also discussed. In addition, the performance of JET-based OBS protocols which use an offset time along with delayed reservation to achieve efficient utilization of both bandwidth and FDLs as well as to support priority-based routing is evaluated.

Optical burst switching (OBS) - a new paradigm for an optical Internet

Considers routing connections in a reconfigurable optical network using WDM. Each connection between a pair of nodes in the network is assigned a path through the network and a wavelength on that path, such that connections whose paths share a common link in the network are assigned different wavelengths. The authors derive an upper bound on the carried traffic of connections (or equivalently, a lower bound on the blocking probability) for any routing and wavelength assignment (RWA) algorithm in such a network. The bound scales with the number of wavelengths and is achieved asymptotically (when a large number of wavelengths is available) by a fixed RWA algorithm. The bound can be used as a metric against which the performance of different RWA algorithms can be compared for networks of moderate size. The authors illustrate this by comparing the performance of a simple shortest-path RWA (SP-RWA) algorithm via simulation relative to the bound. They also derive a similar bound for optical networks using dynamic wavelength converters, which are equivalent to circuit-switched telephone networks, and compare the two cases. Finally, they quantify the amount of wavelength reuse achievable in large networks using the SP-RWA via simulation as a function of the number of wavelengths, number of edges, and number of nodes for randomly constructed networks as well as de Bruijn networks. They also quantify the difference in wavelength reuse between two different optical node architectures. >

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Routing and wavelength assignment in all-optical networks

The problem of designing a logical topology over a wavelength-routed all-optical network (AON) physical topology is studied. The physical topology consists of the nodes and fiber links in the network. On an AON physical topology, we can set up lightpaths between pairs of nodes, where a lightpath represents a direct optical connection without any intermediate electronics. The set of lightpaths along with the nodes constitutes the logical topology. For a given network physical topology and traffic pattern, our objective is to design the logical topology and the routing algorithm so as to minimize the network congestion while constraining the average delay seen by a source-destination pair and the amount of processing required at the nodes (degree of the logical topology). Ignoring the delay constraints can result in fairly convoluted logical topologies with very long delays. On the other hand, in all our examples, imposing it results in a minimal increase in congestion. While the number of wavelengths required to imbed the resulting logical topology on the physical all optical topology is also a constraint in general, we find that in many cases of interest this number can be quite small. We formulate the combined logical topology design and routing problem described above as a mixed integer linear programming problem which we then solve for a number of cases of a six-node network. This programming problem is split into two subproblems: logical topology design, and routing. We then compare the performance of several heuristic topology design algorithms against that of randomly generated topologies, as well as lower bounds.

Design of logical topologies for wavelength-routed optical networks

An overview of emerging all-optical networks is given. The characteristics and alternative architectures for single-hop systems are discussed. The characteristics of lightwave technology that facilitate the design of wavelength-division-multiplexing (WDM) networks are reviewed, and it is explained how WDM local networks can be built based on the single-hop and multihop approaches. Various categories of single-hop systems are discussed: experimental systems, systems based on no pretransmission coordination, and systems based on pretransmission coordination, which also require a separate control channel. A simple classification for single-hop systems is provided. >

WDM-based local lightwave networks. I. Single-hop systems

For pt.I see ibid., vol.6, no.3, p.12-27, 1992. A survey of wavelength-division-multiplexing (WDM)-based local lightwave networks is presented. The general characteristics of multihop systems are discussed, and various multihop approaches are reviewed. The construction of optimal structures based on minimizing the maximum link flow and optimizations based on minimization of the mean network packet delay are also reviewed. Regular topologies that have been studied as candidates for multihop lightwave networks, including the perfect shuffle, the de Bruijn graph, the toroid, and the hypercube, are discussed. Near-optimal node placement algorithms and shared-channel multihop systems are also discussed. >

WDM-based local lightwave networks. II. Multihop systems

Routing algorithms are proposed for setting up calls on a circuit-switched basis in linear lightwave networks (LLN), i.e., networks composed only of linear components, including controllable power combiners and dividers, and possibly linear (non-regenerative) optical amplifiers. The overall problem is decomposed into three subproblems: (1) physical path allocation, (2) checking for violations of the special optical constraints on the allocated physical path, and (3) channel assignment. Only point to point connections are considered. The physical path allocation technique uses the K-shortest path algorithm and tries to minimize the number of sources potentially interfering with each other, as a result of the incoming call. A channel assignment heuristic that tends to spread out calls evenly among the available channels works better than one that tries to maximize channel reuse. >

Algorithms for routing in a linear lightwave network

A novel type of lightwave network architecture that is based on establishing controllable, optically transparent paths among network users is described. The objective is to provide optical connections on demand. These connections would support a high degree of flexibility, including user-chosen modulation formats and bit rates, for large numbers of user stations (e.g., 100000) using mesh-type topologies that are spread over geographical areas that may extend to diameters of several thousand kilometers. The networks in question perform only linear operations on optical signals, essentially operating as a dynamically controlled 'ether' within which light beams are selectively directed between prescribed source-destination pairs. Because these linear lightwave networks are controllable, they are capable of being reconfigured in response to changing load conditions or component failures. >

Linear lightwave networks: how far can they go?

Dynamic routing algorithms are proposed for setting up multicast connections in a linear lightwave network (LLN). The problem of finding a physical path for the multicast connection so as to satisfy all the constraints in the LLN is shown to be NP-complete, and a heuristic approach is presented. An algorithm is presented that decomposes the LLN into edge disjoint trees with at least one spanning tree. A multicast call is allocated a physical path on one of the trees, using the smallest component tree (SCT) or the minimum interference tree (MIT) criterion. Finally, the call is allocated the least used channel from among channels that can be allocated to it. The best performance (low blocking probability) is obtained when the LLN is decomposed into many spanning trees, each of them having a small diameter. It is also found that the selection of trees for each call using the MIT criterion exhibits better performance than with the SCT criterion. >

Multicasting in a linear lightwave network

A routing algorithm for setting up point to point connection is proposed for a linear lightwave network (LLN). The overall problem of routing is decomposed into the subproblems of physical path allocation, checking for violations of some of the special constraints (MISC (mutually independent sources combining) and color clash) of the LLN on the allocated physical path and the allocation of an appropriate channel. It is shown that the problem of finding a physical path that does not violate the MISC constraint of the LLN is NP-complete. >

A minimum interference routing algorithm for a linear lightwave network

A class of multicast multihop lightwave networks (MLNs) is presented. These networks have a two-layered architecture, with the upper "logical layer" implemented electronically, and supported by an underlying "physical layer" implemented as a purely optical network. The unique feature studied is that the logical topology has a regular multicast structure, which is created through the use of optical multicast in the physical layer. It is shown that this type of logical topology yields networks which have significantly superior performance features, when compared with either purely optical networks, or with multihop networks based on point-to-point connections. Special regular logical topologies based on Kautz hypergraphs are used in designing the networks.

T.E. Stern

Papers

Algorithms for routing in a linear lightwave network

Linear lightwave networks: how far can they go?

Multicasting in a linear lightwave network

A minimum interference routing algorithm for a linear lightwave network

Regular multicast multihop lightwave networks