David Starobinski

Rateless Deluge: Over-the-Air Programming of Wireless Sensor Networks Using Random Linear Codes

Over-the-air programming (OAP) is a fundamental service in sensor networks that relies upon reliable broadcast for efficient dissemination. As such, existing OAP protocols become decidedly inefficient (with respect to energy, communication or delay) in unreliable broadcast environments, such as those with relatively high node density or noise. In this paper, we consider OAP approaches based on rateless codes, which significantly improve OAP in such environments by drastically reducing the need for packet rebroadcasting. We thus design and implement two rateless OAP protocols, rateless Deluge and ACKless Deluge, both of which replace the data transfer mechanism of the established OAP Deluge protocol with rateless analogs. Experiments with Tmote Sky motes on single-hop networks with packet loss rates of 7% show these protocols to save significantly in communication over regular Deluge (roughly 15-30% savings in the data plane, and 50-80% in the control plane), and multi-hop experiments reveal similar trends. Simulations further shows that our new protocols scale better than standard Deluge (in terms of communication and energy) to high network density. TinyOS code for our implementation can be found at http://nislab.bu.edu.

Robust location detection in emergency sensor networks

We propose a new framework for providing robust location detection in emergency response systems, based on the theory of identifying codes. The key idea of this approach is to allow sensor coverage areas to overlap in such a way that each resolvable position is covered by a unique set of sensors. In this setting, determining a sensor-placement with a minimum number of sensors is equivalent to constructing an optimal identifying code, an NP-complete problem in general. We thus propose and analyze a new polynomial-time algorithm for generating irreducible codes for arbitrary topologies. We also generalize the concept of identifying codes to incorporate robustness properties that are critically needed in emergency networks and provide a polynomial-time algorithm to compute irreducible robust identifying codes. Through analysis and simulation, we show that our approach typically requires significantly fewer sensors than existing proximity-based schemes. Alternatively, for a fixed number of sensors, our scheme can provide robustness in the face of sensor failures or physical damage to the system.

RTS/CTS-induced congestion in ad hoc wireless LANs

The RTS/CTS mechanism is widely used in wireless networks in order to avoid packet collisions and, thus, achieve high throughput. In ad hoc networks, however the current implementation of the RTS/CTS mechanism may lead to interdependencies so that nodes become unable to transmit any packets during long periods of time. This effect manifests itself in the form of congestion where, after a certain point, the network throughput decreases with increasing load instead of maintaining its peak value. In this paper, we describe and analyze this problem in detail and provide a backward-compatible solution, called RTS validation. Our simulations show that this solution leads to a 60% gain in the peak throughput in addition to stabilizing the throughput at high load.

On the scalability of data synchronization protocols for PDAs and mobile devices

Personal digital assistants and other mobile computing devices rely on synchronization protocols in order to maintain data consistency. These protocols operate in environments where network resources such as bandwidth, memory and processing power are limited. We examine a number of popular and representative synchronization protocols, such as Palms HotSync, Pumatechs Intellisync and the industry-wide SyncML initiative. We investigate the scalability performance of these protocols as a function of data and network sizes and compare them to a novel synchronization approach, CPISync, which addresses some of their scalability concerns. The conclusions of this survey are intended to provide guidance for handling scalability issues in synchronizing data on large, heterogeneous, tetherless networks.

New call blocking versus handoff blocking in cellular networks

In cellular networks, blocking occurs when a base station has no free channel to allocate to a mobile user. One distinguishes between two kinds of blocking, the first is called new call blocking and refers to blocking of new calls, the second is called handoff blocking and refers to blocking of ongoing calls due to the mobility of the users. In this paper, we first provide explicit analytic expressions for the two kinds of blocking probabilities in two asymptotic regimes, i.e., for very slow mobile users and for very fast mobile users, and show the fundamental differences between these blocking probabilities. Next, an approximation is introduced in order to capture the system behavior for moderate mobility. The approximation is based on the idea of isolating a set of cells and having a simplifying assumption regarding the handoff traffic into this set of cells, while keeping the exact behavior of the traffic between cells in the set. It is shown that a group of 3 cells is enough to capture the difference between the blocking probabilities of handoff call attempts and new call attempts.

Robust location detection with sensor networks

We propose a novel framework for location detection with sensor networks, based on the theory of identifying codes. The key idea of this approach is to allow sensor coverage areas to overlap so that each resolvable position is covered by a unique set of sensors. In this setting, determining a sensor-placement with a minimum number of sensors is equivalent to constructing an optimal identifying code, an NP-complete problem in general. We, thus, propose and analyze new polynomial-time algorithms for generating irreducible (but not necessarily optimal) codes for arbitrary topologies. Our algorithms incorporate robustness properties that are critically needed in harsh environments. We further introduce distributed versions of these algorithms, allowing sensors to self-organize and determine a (robust) identifying code without any central coordination. Through analysis and simulation, we show that our algorithms produce nearly optimal solutions for a wide range of parameters. In addition, we demonstrate a tradeoff between system robustness and the number of active sensors (which is related to the expected lifetime of the system). Finally, we present experimental results, obtained on a small testbed, that demonstrate the feasibility of our approach.

Application of network calculus to general topologies using turn-prohibition

Network calculus is known to apply in general only to feedforward routing networks, i.e., networks where routes do not create cycles of interdependent packet flows. In this paper, we address the problem of using network calculus in networks of arbitrary topology. For this purpose, we introduce a novel graph-theoretic algorithm, called turn-prohibition (TP), that breaks all the cycles in a network and, thus, prevents any interdependence between flows. We prove that the TP-algorithm prohibits the use of at most 1/3 of the total number turns in a network, for any network topology. Using analysis and simulation, we show that the TP-algorithm significantly outperforms other approaches for breaking cycles, such as the spanning tree and up/down routing algorithms, in terms of network utilization and delay bounds. Our simulation results also show that the network utilization achieved with the TP-algorithm is within a factor of two of the maximum theoretical network utilization, for networks of up to 50 nodes of degree four. Thus, in many practical cases, the restriction of network calculus to feedforward routing networks may not represent a too significant limitation.

Spot pricing of secondary spectrum access in wireless cellular networks

Recent deregulation initiatives enable cellular providers to sell excess spectrum for secondary usage. In this paper, we investigate the problem of optimal spot pricing of spectrum by a provider in the presence of both nonelastic primary users, with long-term commitments, and opportunistic, elastic secondary users. We first show that optimal pricing can be formulated as an infinite horizon average reward problem and solved using stochastic dynamic programming. Next, we investigate the design of efficient single pricing policies. We provide numerical and analytical evidences that static pricing policies do not perform well in such settings (in sharp contrast to settings where all the users are elastic). On the other hand, we prove that deterministic threshold pricing achieves optimal profit amongst all single-price policies and performs close to global optimal pricing. We characterize the profit regions of different pricing policies, as a function of the arrival rate of primary users. Under certain reasonable assumptions on the demand function, we prove that the profit region of threshold pricing is optimal and independent of the specific form of the demand function, and that it includes the profit region of static pricing. In addition, we show that the profit function of threshold pricing is unimodal in price. We determine a restricted interval in which the optimal threshold lies. These properties enable very efficient computation of the optimal threshold policy, which is far faster than that of the global optimal policy.

Performance of wireless networks with hidden nodes: a queuing-theoretic analysis

Hidden nodes are a fundamental problem that can potentially affect any wireless network where nodes cannot hear each other. Although the hidden node problem is well known, so far only few papers have quantified its effects in a comprehensive manner. This paper represents a first step towards getting a quantitative insight into the impact of hidden nodes on the performance of wireless networks. We first carry out an exact queuing-theoretic analysis for a 4-node segment and derive analytical expressions for the probability of packet collision, the mean packet delay, and the maximum throughput, based on a model that closely follows the IEEE 802.11 standard. We then extend the analysis and provide an approximation for a general linear topology that is asymptotically exact at low load. Finally, we perform detailed simulations to validate our analytical results and show their applicability to predict the performance of IEEE 802.11 networks with hidden nodes. The simulation and analysis closely match. Moreover, they reveal that the impact of hidden nodes propagates through the network causing some nodes to saturate at load as low as 15% of the capacity.

Evaluation of the masked node problem in ad hoc wireless LANs

IEEE 802.11 wireless networks employ the so-called RTS/CTS mechanism in order to avoid DATA packet collisions. The main design assumption is that all the nodes in the vicinity of a sender and a rec...