Xiaojun Cao

A study of the routing and spectrum allocation in spectrum-sliced Elastic Optical Path networks

In OFDM-based optical networks, multiple subcarriers can be allocated to accommodate various size of traffic demands. By using the multi-carrier modulation technique, subcarriers for the same node-pair can be overlapping in the spectrum domain. Compared to the traditional wavelength routed networks (WRNs), the OFDM-based Spectrum-sliced Elastic Optical Path (SLICE) network has higher spectrum efficiency due to its finer granularity and frequency-resource saving. In this work, for the first time, we comprehensively study the routing and spectrum allocation (RSA) problem in the SLICE network. After proving the NP-hardness of the static RSA problem, we formulate the RSA problem using the Integer Linear Programming (ILP) formulations to optimally minimize the maximum number of sub-carriers required on any fiber of a SLICE network. We then analyze the lower/upper bounds for the sub-carrier number in a network with general or specific topology. We also propose two efficient algorithms, namely, balanced load spectrum allocation (BLSA) algorithm and shortest path with maximum spectrum reuse (SPSR) algorithm to minimize the required sub-carrier number in a SLICE network. The results show that the proposed algorithms can match the analysis and approximate the optimal solutions using the ILP model.

Assembling TCP/IP packets in optical burst switched networks

Optical burst switching (OBS) is a promising paradigm for the next-generation Internet infrastructure. We study the performance of TCP traffic in OBS networks and in particular, the effect of assembly algorithms on TCP traffic. We describe three assembly algorithms in this paper and compare them using the same TCP traffic input. The results show that the performance of the proposed adaptive-assembly-period (AAP) algorithm is better than that of the min-burstlength-max-assembly-period (MBMAP) algorithm and the fixed-assembly-period (FAP) algorithm in terms of goodput and data loss rate. The results also indicate that burst assembly mechanisms affect the behavior of TCP in that the assembled TCP traffic becomes smoother in the short term, and more suitable for transmission in optical networks.

Traffic statistics and performance evaluation in optical burst switched networks

Optical burst switching (OBS) is a promising switching technology to exploit the potential benefits of optical communication and, at the same time, support statistical multiplexing of data traffic at a fine granularity. To quantify its benefits, the paper describes several typical burst assembly algorithms and studies their impact on the assembled burst traffic characteristics as well as the performance of TCP traffic. Also described is a proactive burst scheduling algorithm, called burst overlap reduction algorithm (BORA), which schedules locally assembled bursts in such a way as to reduce burst contention at downstream nodes in OBS networks. Furthermore, to provide analytical insights into performance evaluation of OBS networks, a burst loss model at an OBS node and its extension to different reservation protocols are presented.

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.

Waveband switching in optical networks

The rapid advances in dense wavelength-division multiplexing technology with hundreds of wavelengths per fiber and worldwide fiber deployment have brought about a tremendous increase in the size (i.e., 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. 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 two multigranular OXC architectures for WBS. By using the multilayer MG-OXC in conjunction with intelligent WBS algorithms for both static and dynamic traffic, we show that one can achieve considerable savings in the port count. We also present various WBS schemes and lightpath grouping strategies, and discuss issues related to waveband conversion and failure recovery in WBS networks.

Framework for waveband switching in multigranular optical networks: part I-multigranular cross­ connect architectures (Invited)

Feature Issue on Waveband Switching, Routing, and GroomingOptical networks using wavelength-division multiplexing (WDM) are the foremost solution to the ever-increasing traffic in the Internet backbone. Rapid advances in WDM technology will enable each fiber to carry hundreds or even a thousand wavelengths (using dense-WDM, or DWDM, and ultra-DWDM) of traffic. This, coupled with worldwide fiber deployment, will bring about a tremendous increase in the size of the optical cross-connects, i.e., the number of ports of the wavelength switching elements. Waveband switching (WBS), wherein wavelengths are grouped into bands and switched as a single entity, can reduce the cost and control complexity of switching nodes by minimizing the port count. This paper presents a detailed study on recent advances and open research issues in WBS networks. In this study, we investigate in detail the architecture for various WBS cross-connects and compare them in terms of the number of ports and complexity and also in terms of how flexible they are in adjusting to dynamic traffic. We outline various techniques for grouping wavelengths into bands for the purpose of WBS and show how traditional wavelength routing is different from waveband routing and why techniques developed for wavelength-routed networks (WRNs) cannot be simply applied to WBS networks. We also outline how traffic grooming of subwavelength traffic can be done in WBS networks. In part II of this study [Cao , submitted to J. Opt. Netw.], we study the effect of wavelength conversion on the performance of WBS networks with reconfigurable MG-OXCs. We present an algorithm for waveband grouping in wavelength-convertible networks and evaluate its performance. We also investigate issues related to survivability in WBS networks and show how waveband and wavelength conversion can be used to recover from failures in WBS networks.

Detection of Faults and Attacks Including False Data Injection Attack in Smart Grid Using Kalman Filter

By exploiting the communication infrastructure among the sensors, actuators, and control systems, attackers may compromise the security of smart-grid systems, with techniques such as denial-of-service (DoS) attack, random attack, and data-injection attack. In this paper, we present a mathematical model of the system to study these pitfalls and propose a robust security framework for the smart grid. Our framework adopts the Kalman filter to estimate the variables of a wide range of state processes in the model. The estimates from the Kalman filter and the system readings are then fed into the χ2-detector or the proposed Euclidean detector. The χ2-detector is a proven effective exploratory method used with the Kalman filter for the measurement of the relationship between dependent variables and a series of predictor variables. The χ2-detector can detect system faults/attacks, such as DoS attack, short-term, and long-term random attacks. However, the studies show that the χ2-detector is unable to detect the statistically derived false data-injection attack. To overcome this limitation, we prove that the Euclidean detector can effectively detect such a sophisticated injection attack.

Optimized scheduling for data aggregation in wireless sensor networks

The most important issue in wireless sensor networks is energy consumption. To this end, many networking schemes attempt to minimize the amount of data transmitted by using data aggregation. This trades off data freshness for a savings in energy, because reports from sensor nodes that arrive at an aggregating node may have to be held there for some period of time before being reported so that additional reports may reach the aggregator from slower nodes. We propose to use an intelligent timer and some high-level knowledge of the network to implement an efficient aggregation timing control protocol. Our protocol aims to dynamically change the data aggregation period according to the aggregation quality. A request from the data sink will include the maximum latency for a certain number of reports. If this number of reports can be returned in less time than the maximum, then the maximum time will not be reached. If, however, the specified number of responses can be returned in anywhere between zero-time (instantaneous) and the maximum latency, the timing scheme will find the minimum aggregation period that satisfies the sinks request.

Ubiquitous WSN for Healthcare: Recent Advances and Future Prospects

Wireless sensor networks (WSNs) have witnessed rapid advancement in medical applications from real-time telemonitoring and computer-assisted rehabilitation to emergency response systems. In this paper, we present the state-of-the-art research from the ubiquity perspective, and discuss the insights as well as vision of future directions in WSN-based healthcare systems. First, we propose a novel tiered architecture that can be generally applied to WSN-based healthcare systems. Then, we analyze the IEEE 802 series standards in the access layer on their capabilities in setting up WSNs for healthcare. We also explore some of the up-to-date work in the application layer, mostly on the smartphone platforms. Furthermore, in order to develop and integrate effective ubiquitous sensing for healthcare (USH), we highlight four important design goals (i.e., proactiveness, transparency, awareness, and trustworthiness) that should be taken into account in future systems.

Towards elastic and fine-granular bandwidth allocation in spectrum-sliced optical networks

To overcome the inefficiency of the rigid frequency allocation in traditional wavelength division multiplexing (WDM) networks, the idea of slicing the optical spectrum for elastic and flexible bandwidth allocation has attracted significant interest recently. The resulting network, namely, the spectrum-sliced elastic optical path (SLICE) network, can facilitate both the super-wavelength and sub-wavelength traffic accommodation by allocating an appropriate number of sub-carriers. Compared to traditional wavelength routed WDM networks (WRNs), SLICE networks have the advantages of higher spectrum efficiency (through the elimination of spectrum gaps or guard-bands when possible) and better signal quality (by overcoming various impairments), thanks to the orthogonal frequency division multiplexing technology. To accommodate traffic demands in SLICE networks, the process of routing and spectrum allocation (RSA) has to be employed, which is different from and more challenging than the traditional routing and wavelength assignment problem in WRNs. In this work, we comprehensively study the RSA problem assuming the presence of known static or off-line traffic. We formally define the static RSA problem and show the NP-hardness of the optimal RSA problem. Integer linear programing models are then formulated to achieve different optimization goals in SLICE networks. We further analyze the lower/upper bound of the spectrum resources (i.e., sub-carriers) in SLICE networks with uniform traffic demands. To efficiently resolve the RSA problem in a large-scale network, we also propose two efficient algorithms, namely, the shortest path with maximum spectrum reuse algorithm, and the balanced load spectrum allocation algorithm, to minimize the required number of sub-carriers in a SLICE network. Our results show that the proposed algorithms can match the analysis and approximate the optimal solutions from the integer linear programing model.