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As we move from traditional power grids to smart grids, new threats arise. We focus on cybersecurity in power grids, highlighting the threats to and vulnerabilities of those cyberphysical systems and presenting future solutions. [ABSTRACT FROM AUTHOR]

We study carrier assignment in a single-cell multi-user OFDM multi-carrier system so as to satisfy user rate requirements with minimal resources. Different users experience different quality in different carriers due to frequency selectivity of users' propagation channels and due to non-co-located user receivers that perceive different interference from neighboring cells across carriers. We study a static instance of the problem, specified by user carrier qualities and rate requirements. Adaptive modulation at the transmitter differentiates carriers for each user. In good quality carriers, the user satisfies per-frame rate requirements with few slots (or equivalently it satisfies its per-slot rate requirements with small occupied time slot portion). We study integral and fractional assignment, where a user is assigned to only one or several carriers. Fractional assignment is formulated as a linear programming problem. For integral assignment, we introduce two classes of iterative heuristics that use carrier reassignment to users and user substitution in carriers respectively and may be viewed as resulting from corresponding optimal fractional assignment algorithms. We use Lagrangian relaxation to obtain performance bounds and show that the two classes of heuristics arise from two relaxations. Our approach identifies efficient feasible solutions and is amenable to distributed implementation. Index Terms--Wireless OFDM systems, multi-carrier systems, carrier allocation, optimization.

Space division multiple access (SDMA) with an antenna array at the transmitter is a promising means for increasing system capacity and supporting rate-demanding services. However, the presence of an antenna array at the physical layer raises significant issues at higher layers. In this paper, we attempt to capture the impact of SDMA on access layer channel allocation, reflected on channel reuse. This impact obtains different twists in TDMA, CDMA and OFDMA due to the different nature of co-channel and cross-channel interference and the different interaction of user spatial channel characteristics with system channels, namely time slots, codes and subcarriers. We consider these access schemes in a generalized unified framework and propose heuristic algorithms for channel allocation, downlink beamforming and transmit power control so as to increase total provisioned system rate and provide QoS to users in the form of minimum rate guarantees. We study the class of greedy algorithms that rely on criteria such as induced or received interference and signal-to-interference ratio (SIR), and a class of SIR balancing algorithms. Results show superior performance for SIR balancing resource allocation and expose the performance benefits of cross-layer design. Index Terms--Beamforming, SDMA, power control, cross-layer design, resource allocation, T/C/OFDMA.

A spatially distributed set of sources is creating data that must be delivered to a spatially distributed set of sinks. A network of wireless nodes is responsible for sensing the data at the sources, transporting them over a wireless channel, and delivering them to the sinks. The problem is to find the optimal placement of nodes, so that a minimum number of them is needed. The critical assumption is made that the network is massively dense, i.e., there are so many sources, sinks, and wireless nodes, that it does not make sense to discuss in terms of microscopic parameters, such as their individual placements, but rather in terms of macroscopic parameters, such as their spatial densities. Assuming a particular interference-limited, capacity-achieving physical layer, and specifying that nodes only need to transport the data (and not to sense them at the sources, or deliver them at the sinks once their location is reached), the optimal node placement induces a traffic flow that is identical to the electrostatic field created if the sources and sinks are replaced by a corresponding distribution of positive and negative charges. Assuming a general model for the physical layer, and specifying that nodes must not only transport the data, but also sense them at the sources and deliver them at the sinks, the optimal placement of nodes is given by a scalar nonlinear partial differential equation found by calculus of variations techniques. The proposed formulation and derived equations can help in the design of large wireless sensor networks that are deployed in the most efficient manner, not only avoiding the formation of bottlenecks, but also striking the optimal balance between reducing congestion and having the data packets follow short routes. Index Terms--Capacity, electrostatics, node placement, physical layer, sensor networks, wireless ad hoc networks.

A single commodity network that models the information flow in an arbitrary topology sensor field that collects and forwards information to a backbone through certain designated gateway nodes is considered. Resilient operation in overload stress situations caused by unpredictable traffic or topology variations is investigated. That amounts to studying the network in instability mode, where the traffic load distribution is outside the throughput region. A fluid model is adopted where superflows model traffic forwarding and backlog formations at the network level. Quantitative performance metrics of the overload including throughput, lexicographic minimization, most balanced allocation, and amount of lost traffic due to buffer overflow are considered to capture the information loss process due to overflow in the network. Optimal superflows with respect to these metrics are characterized and a distributed asynchronous algorithm that computes such superflows is given. The characterization of the optimal superflow amounts to obtaining a structural decomposition of the network in a sequence of disjoint subregions with decreasing overload such that traffic flows only from regions of higher overload to regions of lower overload. The optimal superflow represents the smoothest trajectory to overflow, followed by the network in case of instability. Index Terms--Distributed algorithms, lexicographic optimization, lost traffic minimization, most balanced allocation, network control, overload response, sensor networks, throughput maximization.

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.comnet.2005.03.001 Byline: Ozgur Ercetin (a), Leandros Tassiulas (b) Keywords: Network pricing; Market-based resource allocation; Game theoretical modeling of communication networks; Internet service classes under competition Abstract: In order to reduce the user latency, the web sites disseminate some of their information to surrogate caches located closer to the users. We discuss a simple differentiated service type architecture for content delivery networks. A pricing scheme is next proposed to complement this architecture and provide fair service to the subscribed publishers. Then, we show that the suggested pricing scheme is also the optimal (revenue maximizing) pricing scheme for a monopolistic surrogate. We investigate the performance of the pricing scheme in a duopoly and show that under certain conditions the competition results in peering of the surrogates. Finally, we suggest methods to determine the optimal number of service classes offered to the users. Author Affiliation: (a) Faculty of Engineering and Natural Sciences, Sabanci University, Istanbul 34956, Turkey (b) Department of Computer Engineering and Telecommunications, University of Thessaly, 38221 Volos, Greece Article History: Received 7 June 2004; Revised 24 January 2005; Accepted 2 March 2005 Article Note: (footnote) [star] This work was in part done, when the authors were with the Department of Electrical and Computer Engineering, University of Maryland College Park.

We study the fair allocation of bandwidth in multicast networks with multirate capabilities. In multirate transmission, each source encodes its signal in layers. The lowest layer contains the most important information and all receivers of a session should receive it. If a receiver's data path has additional bandwidth, it receives higher layers which leads to a better quality of reception. The bandwidth allocation objective is to distribute the layers fairly. We present a computationally simple, decentralized scheduling policy that attains the maxmin fair rates without using any knowledge of traffic statistics and layer bandwidths. This policy learns the congestion level from the queue lengths at the nodes, and adapts the packet transmissions accordingly. When the network is congested, packets are dropped from the higher layers; therefore, the more important lower layers suffer negligible packet loss. We present analytical and simulation results that guarantee the maxmin fairness of the resulting rate allocation, and upper bound the packet loss rates for different layers. Index Terms--Back pressure, maxmin fairness, multicast, scheduling.

Sharing the common spectrum among the links in a vicinity is a fundamental problem in wireless ad-hoc networks. Lately, some scheduling approaches have been proposed that guarantee fair share of bandwidth among the links. The quality of service perceived by the applications however depends on the end-to-end bandwidth allocated to the multihop sessions. We propose an algorithm that provides provably maxmin fair end-to-end bandwidth to sessions. The algorithm combines a link scheduling that avoids collisions, a fair session service discipline per link, and a hop-by-hop window flow control. All the stages of the algorithm are implementable based on local information, except the link scheduling part that needs some network-wide coordination.

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.comnet.2004.07.007 Byline: Thanasis Korakis, Leandros Tassiulas Keywords: 802.11 Wireless LAN; Scheduling; QoS Abstract: Quality of Service (QoS) support is a key attribute of a broadband communication system and is missing in 802.11-based wireless LANs. Recognizing the need, Task Group e (TGe) is working towards 802.11e, an evolution of the standard that provides the means for defining desirable QoS requirements for the traffic streams as well as a framework for scheduling schemes to achieve them. In this paper we propose a scheduling scheme for QoS guarantee provisioning that exploits and is fully compliant with the specifications of 802.11e. The scheduler operates at the access point and relies on the measurement and control parameters available in the fields of the 802.11e header to determine access during the evolution of PCF called HCF. Our scheme takes channel conditions under consideration in making these decisions, since information about the channel state can be conveyed now with the recent modifications adopted by TGh. Evaluation of our algorithm through simulation study shows significant performance improvement compared to earlier schemes that could not take channel conditions into consideration in making scheduling decisions. Author Affiliation: Computer Engineering and Telecommunications Department, University of Thessaly, 38221 Volos, Greece

We focus on the modeling and evaluation of the bandwidth requirements of the next generation of satellite communication technologies, which support future aeronautical applications. For modeling purposes, we chose the real-time delivery of high resolution weather maps to the cockpit as a particularly demanding future application. Focusing on this application, we investigate the use of low and geosynchronous Earth orbit (LEO and GEO) satellite networks for efficient delivery of the data. We propose a joint uni-cast and broadcast communication technique that offers bandwidth reduction. We quantify our results through simulation and compare the techniques with other methods.

We consider the problem of scheduling packets over channels with time-varying quality. This problem has received a lot of attention lately in the context of devising methods for providing quality of service in wireless communications. Earlier work dealing with this problem considered two cases. One case is that the arrival rate vector is in the throughput region and then policies that stabilize the system are pursued. The other case is that all packet queues are saturated and then policies that optimize an objective function of the channel throughputs are investigated. In this paper, we address the case where no assumption on the arrival rates is made. We obtain a scheduling policy that maximizes the weighted sum of channel throughputs. Under the optimal policy, in the general case, the system may operate in a regime where some queues are stable, while the other become saturated. If stability for the whole system is at all possible, it is always achieved. The optimal policy is a combination of a criterion that gives priorities based on queue lengths and a strict priority rule. The scheduling mechanism switches between the two criteria based on thresholds on the queue lengths and is modulated by the availability of the channels. The analysis of the operation of the system involves the study of a vector process which in steady state has some of its components stable while others are unstable. We adopt a novel model for time-varying channel availability that dispenses with the statistical assumptions and makes a rigorous description of system dynamics possible. Index Terms--Burstiness-constrained models, channel-aware scheduling, packet arrival dynamics, stability, throughput maximization, varying channel conditions, wireless packet networks.

We address the problem of attaining proportionally fair rates using Aloha protocols at the medium access layer. We consider a wireless network where all nodes need not be in transmission ranges of each other. We show how the attempt probabilities in Aloha protocols should be set so that the achieved rates are globally proportionally fair. For both slotted and unslotted Aloha, we argue that each node can compute its optimal attempt probability just by knowing some minimal information about the network topology in its two-hop radius. Index Terms--Aloha networks, fairness, local information.

An analysis of a class of collision-resolution protocols with retransmission diversity is presented.

We investigate the problem of replication in large wireless networks that employ caching in the case of a single file whose popularity varies with time. As opposed to the case of static popularity, in this case for the network resources to be efficiently allocated the replication should vary with time. In this study, we first outline the low-level operations of wireless networks with caching, which involve decisions of combinatorial complexity, such as about the contents of all network caches. To overcome this complexity, we approximate the network optimization with a formulation based on the frequency of file replication across the network--a high-level perspective, amenable to mathematical analysis. We present a solution that is based on looking ahead into the future and has a simple graphical representation. [ABSTRACT FROM AUTHOR]