Optical Switching Networks: From Circuits to Packets

Alan Hill and Fabio Neri

      The use of optical technology in wired telecommunications networks is widespread, and interest in optical networking has abruptly increased in the last few years. While the role of optics in networks is often limited today to the realization of transmission functions, next-generation networks will also perform some or all of the switching and control functions in the optical domain. It is well known that one of the major advantages of optical technology is that it provides a huge amount of bandwidth. Interesting properties of optical networks are great flexibility in network design and configuration, and the fact that the cost of switching optical channels can be largely independent of their data rate.
      However, optical processing and queuing are very difficult to achieve. Thus, it is not necessarily advisable to use in the optical domain the same protocols and architectures defined in the electronic domain for packet communications. Architectures such as IP over ATM over SONET over WDM are often used, but they are far from efficient, and introduce a large amount of replication of functionalities.
      Circuit switching currently appears to be better suited for WDM networks, and fast circuit switching techniques are expected to find much earlier application than optical packet switching, provided that circuit switching can be made rapidly adaptive to traffic fluctuations and connection requests. Fast circuit switching can be obtained in the wavelength domain by using transparent optical paths, usually called lightpaths, across wavelength routed networks, or simultaneously in both the time and wavelength domains by allocating part of the transmission resources available in the fiber via optical time multiplexing. While many issues related to circuit switching and lightpath provisioning have been thoroughly studied in past years, control plane techniques for setting up optical paths are currently under active development.
      Nevertheless, the research community has recently devoted increasing effort to the study and development of prototypes for switching packets in the optical domain. Many open issues exist, due to the fact that both protocols and algorithms used in today's packet switching electronic networks were developed to exploit the capabilities of electronic technology. While some researchers aim to emulate in the optical domain the classical packet switching features available in electronic networks, it may be more efficient instead to devise new solutions specifically designed for the optical domain, to gain cost and performance benefits. Furthermore, from an overall networking perspective, a hybrid solution combining the merits of fast (optical) circuit switching with those of optical packet switching may offer better cost and performance. Indeed, such a solution may reduce the throughput requirements of packet switches.
      Revisiting current packet switching solutions may have a profound impact on both network and switch architectures, and could offer new ways of integrating IP and optical networks. The scientific community has recently started to study this possibility, often starting from the definition of different techniques to solve contentions that may arise in a packet switching environment based on statistical multiplexing.
      The five articles in this Feature Topic elaborate on the above issues.
      The first article, by D. Awduche and Y. Rekhter, refers to fast optical circuit switching, and explores the suitability of the MPLS/MP*S control plane for setting up optical circuits in wavelength-routed networks based on optical cross-connects.
      The next article, by M. Veeraraghavan et al., starting from a discussion on how improvements in components, networking architectures, protocols, and applications advance the value of optical networks, provides a general taxonomy of telecommunication networks, and proposes an example of new architectures and protocols for the end-to-end transfer of long files, which employs a combination of both optical circuit switching and electronic packet switching.
      The article by M. O'Mahony et al., from ilotron, Colchester, United Kingdom, presents a network architecture in which optical circuit switching, which is expected to dominate the first phase of network evolution, is combined with optical packet switching: optical packet switches are used as edge network devices, functioning as an interface between the electronic and optical domains. The authors argue that this approach can provide a scalable and efficient IP traffic aggregator that exploits the optical technology to be less costly than an electronic terabit router.
      The next article, by A. Jourdan et al., expresses the viewpoint of Alcatel Corporate Research Center, presenting the rationale for and technical solutions toward the use of optical packet switching techniques for both backbone and metro networks. It also gives information on state-of-the-art technologies for medium-term product development.
      The final article in the Feature Topic, by S. Yao et al., offers arguments for using optical packet switching, providing a wide range of applications and benefits of optical packet switching, and discusses architectural issues and enabling technologies.
      Our selection of articles reflects the current trends in optical networking: while many in the optical networking field believe that fast optical circuit switching will provide the first exploitation of optical technology, at least in the short to medium term, a number of professionals and researchers claim that optical packet switching will be the ultimate evolution of optical networks, perhaps initiated by pragmatic requirements for interfacing, aggregation, and domain isolation at the edge of the network. We hope that our selection helps the reader to gain a better feeling of how optical networks will grow from infancy to maturity.

Biographies

Alan Hill received his B.Sc. and Ph.D. degrees in electronics from London University in 1973 and 1976, respectively, and was awarded the IEE Prize in 1973. He is a Chartered Physicist, member of the Institute of Physics, and co-founder and Fellow of the Cybernetics Society. In 1973 he joined the Post Office Research Laboratories (now BTexaCT Research). His career has covered a wide range of optical systems activities, including receiver design, laboratory and field testing, development of micro-optic components such as connectors, switches, tunable transmitters/receivers, and wavelength-division-multiplexers, studies of linear and nonlinear optical crosstalk, and the development of advanced optical switching and distribution networks. He has more recently been involved in several EU Framework projects on WDM access, core transport, and optical packet switching. He managed BTL's activities in the RACE II project MUNDI for fiber to the home and to the building, dealing with the broadband upgrade of telephony passive optical networks (TPON/BPON) by WDM, and was technical leader of the ACTS project WOTAN, investigating the use of wavelength-agile techniques across both access and core networks. He also managed the system studies activities within the ACTS project SONATA, which envisaged a WDMA/TDMA optical transport network encompassing all concentration, distribution, transmission, switching, and routing functions within a national network, and BT's activities in IST project DAVID, investigating the potential of optical packet switching in metropolitan and backbone networks. He is currently network strategy manager for ilotron, and is based at the ilotron Engineering Center, Wivenhoe Park, Colchester, United Kingdom.

Fabio Neri is full professor in the Electronics Department of Politecnico di Torino, Italy. He received his Dr.Ing. and Ph.D. degrees in electrical engineering from Politecnico di Torino in 1981 and 1987, respectively. From 1991 to 1992 he was with the Information Engineering Department at the University of Parma, Italy, as an associate professor. From 1982 to 1983 he was a visiting scholar at George Washington University, Washington, D.C. In the summer of 1995 he was visiting researcher at the Computer Science Department of the University of California Los Angeles. In the summer of 1998 he visited Bell Laboratories/Lucent Technologies, Holmdel, New Jersey. His teaching duties include graduate-level courses on computer communication networks and the performance evaluation of telecommunication systems. His research interests are in the fields of performance evaluation of communication networks, high-speed and all-optical networks, packet switching architectures, discrete event simulation, and queuing theory. He leads a research group on optical networks at Politecnico di Torino. He has recently been involved in European projects on WDM networks, including the ACTS project SONATA, in which he contributed to the design of network control and signaling protocols for a single-layer nationwide optical transport network, and the IST project DAVID, in which he is currently contributing to the design of the architecture and control strategy of an optical packet switching metropolitan network based on interconnected rings. He also coordinates the participation of his research group with national Italian research projects on optical networking. He has served several IEEE conferences and journals. He is general co-chair of the next IEEE Local and Metropolitan Area Networks Workshop. He has participated in the technical program committees of several conferences, including IEEE INFOCOM, IEEE GLOBECOM, and the Working Conference on Optical Network Design and Modeling. He has co-authored over 100 papers published in international journals and presented at leading international conferences.