objective is to extract useful global information by collecting individual sensor readings. Conventionally, this is done using in-network aggregation on a spanning tree from sensors to data sink. However, the spanning tree structure is not robust against communication errors; when a packet is lost, so is a complete subtree of values. Multipath routing can mask some of these errors, but on the other hand, may aggregate individual sensor values multiple times. This may produce erroneous results when dealing with duplicate-sensitive aggregates, such as SUM, COUNT, and AVERAGE. In this paper, we present and analyze two new fault tolerant schemes for duplicate-sensitive aggregation in WSNs: (1) Cascaded RideSharing and (2) Diffused RideSharing. These schemes use the available path redundancy in the WSN to deliver a correct aggregate result to the data sink. Compared to state-of-the-art, our schemes deliver results with lower root mean square (RMS) error and consume much less energy and bandwidth. RideSharing can consume as much as 50 % less resources than hash-based schemes, such as SKETCHES and Synopsis Diffusion, while achieving lower RMS for reasonable link error rates. I.

Time division multiple access (TDMA) based medium access control (MAC) protocols can provide QoS with guaranteed access to the wireless channel. However, in multihop wireless networks, these protocols may introduce scheduling delay if, on the same path, an outbound link on a router is scheduled to transmit before an inbound link on that router. The total scheduling delay can be quite large since it accumulates at every hop on a path. This paper presents a method that finds conflict-free TDMA schedules with minimum scheduling delay. We show that the scheduling delay can be interpreted as a cost, in terms of transmission order of the links, collected over a cycle in the conflict graph. We use this observation to formulate an optimization, which finds a transmission order with the minmax delay across a set of multiple paths. The min-max delay optimization is NP-complete since the transmission order of links is a vector of binary integer variables. We devise an algorithm that finds the transmission order with the minimum delay on overlay tree topologies and use it with a modified Bellman-Ford algorithm, to find minimum delay schedules in polynomial time. The simulation results in 802.16 mesh networks confirm that the proposed algorithm can find effective min-max delay schedules.

We present the discrete beeping communication model, which assumes nodes have minimal knowledge about their environment and severely limited communication capabilities. Specifically, nodes have no information regarding the local or global structure of the network, do not have access to synchronized clocks and are woken up by an adversary. Moreover, instead on communicating through messages they rely solely on carrier sensing to exchange information. This model is interesting from a practical point of view, because it is possible to implement it (or emulate it) even in extremely restricted radio network environments. From a theory point of view, it shows that complex problems (such as vertex coloring) can be solved efficiently even without strong assumptions on properties of the communication model. We study the problem of interval coloring, a variant of vertex coloring specially suited for the studied beeping model. Given a set of resources, the goal of interval coloring is to assign every node a large contiguous fraction of the resources, such that neighboring nodes have disjoint resources. A k-interval coloring is one where every node gets at least a 1/k fraction of the resources. To highlight the importance of the discreteness of the model, we contrast it against a continuous variant described in [17]. We present an O(1) time algorithm that with probability 1 produces a O(∆)-interval coloring. This improves an O(log n) time algorithm with the same guarantees presented in [17], and accentuates the unrealistic assumptions of the continuous model. Under the more realistic discrete model, we present a Las Vegas algorithm that solves O(∆)-interval coloring in O(log n) time with high probability and describe how to adapt the algorithm for dynamic networks where nodes may join or leave. For constant degree graphs we prove a lower bound of Ω(log n) on the time required to solve interval coloring for this model against randomized algorithms. This lower bound implies that our algorithm is asymptotically optimal for constant degree graphs.

Abstract—Sleep scheduling is a widely used mechanism in wireless sensor networks (WSNs) to reduce the energy consumption since it can save the energy wastage caused by the idle listening state. In a traditional sleep scheduling, however, sensors have to start up numerous times in a period, and thus consume extra energy due to the state transitions. The objective of this paper is to design an energy efficient sleep scheduling for low data-rate WSNs, where sensors not only consume different amounts of energy in different states (transmit, receive, idle and sleep), but also consume energy for state transitions. We use TDMA as the MAC layer protocol, because it has the advantages of avoiding collisions, idle listening and overhearing. We first propose a novel interference-free TDMA sleep scheduling problem called contiguous link scheduling, which assigns sensors with consecutive time slots to reduce the frequency of state transitions. To tackle this problem, we then present efficient centralized and distributed algorithms that use time slots at most a constant factor of the optimum. The simulation studies corroborate the theoretical results, and show the efficiency of our proposed algorithms.

This paper presents an integrated routing and MAC scheduling algorithm (IRMA) for improving system performance in multihop wireless mesh networks. The IRMA approach is motivated by the fact that conventional contention-based MAC protocols such as 802.11 do not perform well in combination with independent ad hoc routing protocols such as DSR, DSDV or AODV due to interactions between neighboring nodes in the network. In IRMA, a centralized algorithm is used to allocate resources to each flow based on traffic flow specifications and the network compatibility graph based on a generalized n-hop interference model. Joint routing and MAC eliminates contention between radio nodes and assigns traffic flows to alternate paths based on actual traffic demand, thereby providing significant increases in network capacity. Two alternative algorithms are described and evaluated using ns-2 simulations: 1) Link Scheduling with Min Hop Routing (IRMA-MH) which uses real-time flow information to select paths and to set up complete endto -end TDMA schedules; 2) Link Scheduling with BandwidthAware Routing (IRMA-BR) which uses local information about available MAC bandwidth to route around congested areas. Simulation results for both schemes are presented, showing up to 300% improvement in network throughput when compared with baseline 802.11-based multihop networks with independent routing.

Current and future wireless standards use TDMA to provide guaranteed Quality-of-Service (QoS) in the network. While these standards specify how transmissions should occur, they do not discuss scheduling algorithms to find when transmissions should oc-cur (transmission schedules). Despite the technological advances, the question of finding transmission schedules has existed for the past twenty years without a satisfactory an-swer. This thesis presents a new class of scheduling algorithms for Time Division Mul-tiple Access (TDMA) wireless multihop networks. These algorithms have three major advantages. First, they take into account overhead and delay. With reduced overhead, transmission schedules have much higher throughput than what is possible with previous approaches. The algorithms can also be customized to produce schedules with specific delay properties. Scheduling to achieve a specific delay opens up a new dimension in wireless scheduling that was previously not possible. Second, the algorithms provide a simple and computationally efficient way to specify exact constraints on end-to-end flows in the network. These constraints provide us with a way to solve two important cross-layer design problems in TDMA wireless multihop networks. Third, the algorithms

journal homepage: www.elsevier.com/locate/adhoc Locally scheduled packet bursting for data collection

This dissertation studies real-time application support in wireless ad-hoc and sensor networks. Real-time applications are performance critical applications that require bounded service latency. In multi-hop wireless ad-hoc and sensor networks, communication delays are dominant over processing delays. Therefore, to enable real-time applications in such networks, the communication latency must be bounded. The shared nature of the communication medium makes the delay characteristics of the medium access control (MAC) protocol in use very important. Furthermore, it is desirable that the MAC protocols for such networks be distributed and be able to spatially reuse the communication channel for scalability and efficiency. In this dissertation, we derive expressions of real-time capacity that characterize the ability of a network to deliver data on time as well as develop network proto-cols that achieve this capacity. We introduce a hexagonal network topology based architecture for wireless ad-hoc and sensor networks for real-time applications. We present addressing and constant time routing protocols for the hexagonal network. We develop

apport de recherche

Abstract—For energy conservation, a wireless sensor network is usually designed to work in a low-duty-cycle mode, in which a sensor node keeps active for a small percentage of time during its working period. In applications where there are multiple data delivery tasks with high data rates and time constraints, low-duty-cycle working mode may cause severe transmission congestion and data loss. In order to alleviate congestion and reduce data loss, the tasks need to be carefully scheduled to balance the workloads among the sensor nodes in both spatial and temporal dimensions. This paper studies the load balancing problem, and proves it is NP-Complete in general network graphs. Two efficient scheduling algorithms to achieve load balance are proposed and analyzed. Furthermore, a task scheduling protocol is designed relying on the proposed algorithms. To the best of our knowledge, this paper is the first one to tackle multiple task scheduling for low-duty-cycled sensor networks. The simulation results show that the proposed algorithms greatly improve the network performance in most scenarios. I.