Yizhi Xiong

A study of waveband switching with multilayer multigranular optical cross-connects

Waveband switching (WBS) has attracted attention from the optical networking industry for its practical importance in reducing port count, the associated control complexity, and cost of optical cross-connects (OXCs). However, WBS-related problems of theoretical interest have not been addressed thoroughly by the research community and many issues are still wide open. In particular, WBS is different from wavelength routing and, thus, techniques developed for wavelength-routed networks (including for example, those for traffic grooming) cannot be directly applied to effectively address WBS-related problems. In this paper, we first develop an integer linear programming (ILP) model, which for a given set of lightpath requests, determines the routes and assigns wavelengths to the lightpaths so as to minimize the number of ports needed. Since the optimal WBS problem of minimizing the port count in WBS networks contains an instance of routing and wavelength assignment (RWA), which is NP-complete, we adopt a powerful waveband assignment strategy and develop an efficient heuristic algorithm called balanced path routing with heavy-traffic first waveband assignment (BPHT). Both the ILP and the heuristic algorithm can handle the case with multiple fibers per link. We conduct a comprehensive evaluation of the benefits of WBS through detailed analysis and simulations. For small networks, our results indicate that the performance of the BPHT heuristic is close to that achievable by using the ILP model and, hence verifying its near-optimality. We show that for larger networks, BPHT can perform better than its variation called balanced traffic routing with maximum-hop first waveband assignment and much better than another heuristic based on optimal (but waveband oblivious) RWA that minimizes wavelength resources. We also show that WBS using BPHT is even more beneficial in multifiber networks than in single-fiber networks in terms of reducing the port count. Our analytical and simulation results provide valuable insights into the effect of wavelength band granularity, as well as the tradeoffs between the wavelength-hop and the port count required in WBS networks.

Novel algorithms for shared segment protection

The major challenges in designing survivable schemes are how to allocate a minimal amount of spare resources (e.g., bandwidth) using fast (e.g., polynomial-time) algorithms, and, in case a failure occurs, to be able to recover quickly from it. All existing approaches invariably make tradeoffs. We propose novel shared segment protection algorithms which make little or no compromise . We develop an elegant integer linear programming (ILP) model to determine an optimal set of segments to protect a given active path. Although the ILP approach is useful for a medium-size network, it is too time consuming for large networks. Accordingly, we also design a fast heuristic algorithm based on dynamic programming to obtain a near-optimal set of segments. Although the heuristic algorithm has a polynomial time complexity, it can achieve a bandwidth efficiency as high as some best-performing shared path protection schemes and, at the same time, much faster recovery than these shared path protection schemes. The proposed scheme is also applicable to a wide range of networking technologies, including Internet Protocol and wavelength-division multiplexing networks under the generalized multiprotocol label switched framework.

Achieving fast and bandwidth-efficient shared-path protection

Dynamic provisioning of restorable bandwidth guaranteed paths is a challenge in the design of broad-band transport networks, especially next-generation optical networks. A common approach is called (failure-independent) path protection, whereby for every mission-critical active path to be established, a link (or node) disjoint backup path (BP) is also established. To optimize network resource utilization, shared path protection should be adopted, which often allows a new BP to share the bandwidth allocated to some existing BPs. However, it usually leads the backup paths to use too many links, with zero cost in term of additional backup bandwidth, along its route. It will violate the restoration time guarantee. In this paper, we propose novel integer linear programming (ILP) formulations by introducing two parameters (/spl epsi/ and /spl mu/) in both the sharing with complete information (SCI) scheme and the distributed partial information management (DPIM) scheme. Our results show that the proposed ILP formulations can not only improve the network resource utilization effectively, but also keep the BPs as short as possible.

Novel models for efficient shared-path protection

Summary form only given. As widespread deployment of high-capacity dense wavelength-division multiplexing (DWDM) systems is envisioned, achieving efficient shared-path protection, which protects a bandwidth guaranteed connection from a single link (node) failure using a link (node) disjoint pair of active path (AP) and backup path (BP), becomes a key design consideration. We have introduced two parameters (/spl epsi/ and /spl mu/) into the objective function of integer linear programming (ILP) models for dynamic provisioning of restorable, bandwidth guaranteed connections using shared-path protection. By using /spl epsi/ (0 < /spl epsi/ < 1), we can obtain a lower total bandwidth (TBW) consumption than using /spl epsi/ = 1 as in the existing models. In addition, using 1 /spl ges/ /spl mu/ > 0 can avoid the pitfall of choosing very long backup paths that is common in existing models (where /spl mu/ = 0), thus reducing the BP length and consequently restoration time. One of the pleasantly surprising results is that, by combining an appropriate /spl epsi/ (e.g., 0.1) and /spl mu/ (e.g., 0.2), the proposed models can achieve a TBW consumption with shorter BP than the existing ones.

Waveband Switching: A New Frontier in Optical WDM Networks

The rapid advances in dense wavelength division multiplexing (DWDM) technology with hundreds of wavelengths per fiber and world-wide fiber deployment have brought about a tremendous increase in the size (i.e., the number of ports) of photonic cross-connects, as well as in the cost and difficulty associated with controlling such large cross-connects. Waveband switching (WBS) has attracted attention for its practical importance in reducing the port count, associated control complexity, and cost of photonic cross-connects. In this chapter, we show that WBS is different from traditional wavelength routing, and thus techniques developed for wavelength-routed networks (including, for example, those for traffic grooming) cannot be directly applied to effectively address WBS-related problems. We describe a Three-layer multi-granular optical cross-connect (MG-OXC) architecture for WBS. By using this MG-OXC in conjunction with intelligent WBS algorithms, we show that one can achieve considerable savings in the port count. We present various WBS schemes and lightpath grouping strategies. Finally we discuss issues related to waveband conversion and failure recovery in WBS networks.

Ultrafast Potential-Backup-Cost (PBC)-Based Shared Path Protection Schemes

This paper describes a novel ultrafast heuristic algorithm to address an NP-hard optimization problem. One of the significant and nonintuitive results is that the schemes based on the heuristic can achieve a better overall performance than their time-consuming integer-linear-programming-based counterparts. More specifically, the proposed schemes are useful in providing an online shared path protection by establishing survivable connections in high-speed networks. The advantage of our heuristic algorithm over the existing heuristic algorithms in finding a pair of link-disjoint paths, which are called active and backup paths, comes from the following salient feature. It uses a so-called potential-backup-cost (PBC) function when selecting an active path in the first phase to take into consideration the backup bandwidth needed by the corresponding backup path, which has yet to be chosen in the second phase. The PBC function is derived mathematically based on the statistical analysis of experimental data. While the use of PBC only requires partial aggregate information on how the existing connections are established in a network, it can also be applied even more effectively when complete information is available.