IEEE Communications Magazine • June 2008 138 0163-6804/08/$25.00 © 2008 IEEE INTRODUCTION The current migration of access networks from digital subscriber line (xDSL) technologies toward various fiber to the-x (FTTx) solutions will expose the inner networks not only to a con- tinuous increase in traffic but also to higher traf- fic fluctuations. This will bring the contemporary metro and core network architecture to its limit. The main limitation of this approach is the excessive amount of layer 2/3 (L2/L3) processing and switching of such multi-protocol, multi-bit- rate traffic, which can only rely on an excessive deployment of expensive electronic hardware, suffering from high power consumption, man- agement complexity, and large office real estate. The problem is further exacerbated by the fact that existing networks are multidomain, meaning that a large-scale network is partitioned into dif- ferent administrative domains deploying equip- ment from different vendors, whereas at their (multi-granular) interfaces, inter-operability is required to ensure effective end-to-end service delivery. This network paradigm cannot scale cost-effectively. To address this bottleneck, architectures with fewer layer transitions and reduced layer 2/3 processing leading to flatter, that is, less hierar- chical, network structures are sought. Flat net- works allow for enormous savings in operating expenditures (OPEX) and capital expenditures (CAPEX), and it is the only network architec- ture that can deliver long-term economically scalable growth. Along these lines, a two-tier core network architecture is proposed where the rigid distinctions between metro, outer core, and inner core become blurred significantly, alleviat- ing the processing burden, when traffic crosses core nodes or when it traverses different domains. Recently, a considerable number of new approaches for constructing a dynamic core net- work were proposed around various optical burst switching/optical packet switching (OBS/OPS) solutions. In the OBS/OPS schemes, the lack of effective optical buffering presents a hard dilem- ma between high loss (when adopting one-way reservation) and high delay (when using a cir- cuit-switch such as two-way reservation). Specifi- cally, OBS solutions with little or no buffering at the nodes have been extensively investigated, for example [1–3], and shown to suffer from heavy burst loss even at low utilization levels due to their ambitious on-the-fly act. The problems arise from conflicts due to the temporal proper- ties of data bursts entering the OBS nodes, which the method can alter only poorly due to lack of buffering. As a result, losses grow expo- nentially with each node the traffic must cross. In an alternative OPS-based solution described in [4], a metropolitan area network (MAN) consisted of a number of optical rings where traffic was stored in buffers located in the ABSTRACT The explosion of current demand has brought the contemporary multidomain core network paradigm to its limit. In the quest for new approaches that exploit recent developments in optical technology, a novel network architecture that obviates most of the expensive and loss- prone centralized all-optical switches is described in this work. It is based on clustered architecture for nodes in optical networks and features a rec- onciliation between dynamic resource allocation and guaranteed end-to-end network perfor- mance in a multidomain network. This article enhances the distributed, collision-free slot aggregation inside domains of clustered core nodes with dynamic switching of slots/frames between the domains. Thus, it can support dynamic sub-wavelength allocations between network domains, using standard burst-switching techniques. This extends the high efficiency and multiplexing gain into the inter-domain network even under highly bursty traffic. It features both low-cost optical add/drop edge nodes exploiting WDM transmission and agile and modular cen- tralized electro-optical switches that are present- ed in conjunction with the overall network architecture. Its performance exhibits very low burst loss probability traded for a higher but tol- erable and bounded delay. MULTIDOMAIN OPTICAL NETWORKS: ISSUES AND CHALLENGES Alexandros Stavdas, Theofanis G. Orphanoudakis, Helen C. Leligou, Konstantinos Kanonakis, Chris Matrakidis, Andreas Drakos, and John D. Angelopoulos, University of Peloponnese Andrew Lord, British Telecommunications Dynamic CANON: A Scalable Multidomain Core Network