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spacer The research activities in optical networking at OpNeAR Lab are organized around the following main areas:

  • Fast Bandwidth Provisioning in Metro Networks
  • Constrained Routing in Multi-Hop Multi-Rate Optical Networks
  • Differentiated Reliability (DiR)
  • Restoration Techniques in IP over WDM Networks

    The innovative solutions proposed and theoretical results derived in the various projects are tested and verified using a testbed which allows to implement some key aspects of the optical network control layer.

    The main areas are briefly summarized next.

  • spacer Fast Bandwidth Provisioning in Metro Networks
    spacer As opposed to current SONET/SDH solutions that provide static tributary signals, modern access and metro networks require bandwidth-dynamic architectures that can efficiently handle the transmission of users' data bursts. In this project a novel bandwidth-efficient multi-wavelength ring architecture that allows user to set up and tear down optical circuits on-demand has been proposed, termed LightRing. A unique distributed multi-token reservation mechanism is used in the LightRing to set up optical circuits in a contention-free manner. i.e., once initiated, the reservation is always completed successfully. A tell-and-go approach is therefore possible whereby data transmission is initiated while the circuit is being set up.

    The LightRing's distributed control yields fast set-up times that are below the ring round trip propagation time, fair blocking probability and high bandwidth utilization even in presence of relatively short optical bursts of data.

    spacer Constrained Routing in Multi-Hop Multi-Rate Optical Networks
    spacer Propagation of the optical signal across fibers and optical nodes - the so called transparency of optical networks - provides the network designers with a number of alternative network architectures to choose from, e.g., wavelength routing, broadcast-and-select, and photonic slot routing networks. The common objective of these architectures is to eliminate, or significantly reduce, the relatively slow and cumbersome electronic processing of the transmitted signal at the intermediate nodes.

    Among these architectures, wavelength routing networks make use of Wavelength Division Multiplexing (WDM) to create multiple coarse-bandwidth channels (i.e., wavelengths) in the fiber. To efficiently exploit each wavelength bandwidth, traffic grooming is thus required. It is important to observe that with current technology traffic grooming is possible only using electronics. Three classes of traffic grooming solutions are briefly summarized.

    A. In conventional First Generation (FG) optical networks, i.e., SONET/SDH, traffic grooming is performed at each intermediate node, thus potentially achieving bandwidth-efficient solutions at the cost of a large number of Optical Terminals (OT).

    B. In Single-Hop (SH) optical networks, less OTs are used as they are required only at the end nodes of the optical circuit or lightpath. Once transmitted, the optical signal propagates along the lightpath without requiring O/E and E/O conversion, until it is received at the destination node. Grooming is limited among the tributary signals that share the same source-destination pair.

    C. A generalization of both FG and SH network architectures is the Multi-hop and Multi-rate (M&M) architecture in which the tributary signal is transmitted from source to destination through multiple lightpaths, or optical hops, and the transmission rate of each hop may differ from the others'. Multi-hop transmission yields reduced number of OTs (and associated electronics) when compared to FG architecture and reduced number of wavelengths when compared to SH architecture. In addition, the multi-rate feature provides the network designer with the flexibility to select the most cost effective OT on a per-lightpath basis, as opposed to single-rate solutions in which all lightpaths must be transmitted at the same rate. It has been shown that in WDM ring, the M&M architecture has the potential to yield significant cost reductions when compared to FG and SH rings.

    Such cost reductions may be however affected by transmission impairments induced by available fibers and optical components, that may significantly restrain the signal transparency and must be taken into account during the network design. Examples of such impairments are Group Velocity Dispersion (GVD), Self-Phase Modulation (SPM), and Polarization Mode Dispersion (PMD). In presence of the above undesirable effects the quality of the optical signal may degrade significantly and, practically speaking, the maximum span of a lightpath may be constrained. In other words, the transparency degree of the network may be limited if no countermeasures are taken to compensate for such signal degradation.

    The goal of this project is to assess the impact of a number of transmission impairments on the overall design and cost of FG, SH, and M&M networks.

    spacer Differentiated Reliability (DiR)

    Current optical networks (as any other type of network) typically offer two degrees of service reliability: full protection in presence of a single fault in the network, and no protection at all. This situation reflects the historical duality that has its roots in the once divided telephone and data environment. The circuit oriented service requires protection, i.e., provisioning of readily available spare resources to replace working resources in case of a fault. The datagram oriented service relies upon restoration, i.e., dynamic search for and reallocation of affected resources via actions as routing table updates.

    The current development trend, however, is gradually driving the design of networks towards a unified solution that will jointly support traditional voice and data services as well as a variety of novel multimedia applications. The growing importance of concepts, such Quality of Service (QoS) and Differentiated Services - that provide multiple levels of service performance in the same network - evidences this trend.

    Consistently with this pattern, the concept of Differentiated Reliability (DiR) was formally introduced by the OpNeAR research team and applied to provide multiple reliability degrees (or classes) in the same network layer using a common protection mechanism, e.g., path switching. According to the DiR concept, each connection in the layer under consideration is guaranteed a minimum reliability degree, or equivalently a maximum failure probability allowed for that connection. The reliability degree chosen for a given connection is thus determined by the application requirements, and not by the actual network topology, design constraints, robustness of the network components, and span of the connection. Efficient algorithms have been proposed to design the Wavelength Division Multiplexing (WDM) layer of ring networks and have illustrated the advantages of DiR.

    Restoration Techniques in IP over WDM Networks

    One of the expected benefits of Wavelength Division Multiplexing (WDM) is the possibility to provide an optical layer (OL) with built-in capabilities to survive to network component faults, e.g., fiber cut. Several schemes exist to design a reliable OL. These schemes are generally divided into protection and restoration techniques. Protection schemes reserve in advance a dedicated backup path and a wavelength that are readily available upon disruption of the working path. Restoration schemes, on the contrary, dynamically look for backup paths of spare wavelengths upon failure occurrence. While protection schemes have been extensively used in the telephone industry for many years due to their prompt reaction to faults, restoration schemes are preferred candidates for the OL as, in principle, they yield more efficient and flexible resource reservation. The latter schemes, however, typically require a relatively long time to be completed due to the heavy signaling that originates upon network failure. In high capacity WDM networks, the presence of many connections concurrently seeking restoration exacerbates the above problem as, in existing restoration schemes, coordination among several restoration attempts may further slow down the process completion.

    The research team of the OpNeAR Lab has recently proposed a fast and efficient path restoration scheme called Preplanned Weighted Restoration (PWR). In the PWR scheme distinct restoration paths are precomputed at the source node during the connection setup. Upon failure, one of the preplanned paths is randomly selected depending on specific weights calculated by the source node at the time of the fault occurrence. The proposed scheme requires limited signaling upon failure occurrence as coordination among the source nodes involved in the restoration process is not required. Therefore, the scheme is fast and scalable in terms of number of network nodes, link or fiber capacity, and number of connections. Yet, the PWR scheme may considerably decrease the blocking probability of the restoration attempts when compared to other schemes such as the alternate routing.

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    Open Networking Advanced Research Lab
    Dept. of Electrical Engineering
    The University of Texas at Dallas. USA.