In this study we investigate the interaction between TCP and MAC layer in a wireless multi-hop network. Using simulation, we provide new insight into two critical problems of TCP over wireless multi-hop. The first is the conflict between TCP data packets and TCP ACKs, which causes performance to degrade for window sizes greater than 1 packet. The second is the interaction between TCP and MAC layer backoff timers which causes severe unfairness and capture conditions. We show that these effects are particularly pronounced in two families of MAC protocols that have been extensively used in ad-hoc network simulation and implementations, namely CSMA and FAMA (a descendent of MACA). We then demonstrate that these problems are in part overcome by using MACAW, a MAC layer protocol which extends MACA by adding link level ACKs and a less aggressive backoff policy. We argue that link level protection, backoff policy and selective queue scheduling are critical elements for efficient and fair opera...

Significant TCP unfairness in ad hoc wireless networks has been reported during the past several years. This unfairness results from the nature of the shared wireless medium and location dependency. If we view a node and its interfering nodes to form a “neighborhood”, the aggregate of local queues at these nodes represents the distributed queue for this neighborhood. However, this queue is not a FIFO queue. Flows sharing the queue have different, dynamically changing priorities determined by the topology and traffic patterns. Thus, they get different feedback in terms of packet loss rate and packet delay when congestion occurs. In wired networks, the Randomly Early Detection (RED) scheme was found to improve TCP fairness. In this paper, we show that the RED scheme does not work when running on individual queues in wireless nodes. We then propose a Neighborhood RED (NRED) scheme, which extends the RED concept to the distributed neighborhood queue. Simulation studies confirm that the NRED scheme can improve TCP unfairness substantially in ad hoc networks. Moreover, the NRED scheme acts at the network level, without MAC protocol modifications. This considerably simplifies its deployment.

Abstract. This paper presents the Real-Time Event Detection Services middleware which is a component of the Data Service Middleware (DSWare). DSWare is a data-centric and group-based service for sensor networks. The real-time event service includes unreliability of individual sensor reports, correlation among different sensor observations, and inherent real-time characteristics of events. The event service supports confidence functions which are designed based on data semantics, including relative importance of sub-events and historical patterns. When the failure rate is high, the event service enables partial detection of critical events to be reported in a timely manner. It can also be applied to differentiate between the occurrences of events and false alarms. 1 Introduction Sensor networks are large-scale wireless networks that consist of numerous sen-sor and actuator nodes used to monitor and interact with physical environments [11][14]. From one perspective sensor networks are similar to distributeddatabase systems. They store environmental data on distributed nodes and respond to aperiodic and long-lived periodic queries [7][15][20]. Data interest canbe pre-registered to the sensor network so that the corresponding data is collected and transmitted only when needed. These specified interests are similarto views in traditional databases because they filter the data according to the application's data semantics and shield the overwhelming volume of raw datafrom applications [8][26]. Sensor networks also have inherent real-time properties. The environmentthat sensor networks interact with is usually dynamic and volatile. The sensor data usually has an absolute validity interval of time after which the data valuesmay not be consistent with the real environment. Transmitting and processing "stale " data wastes communication resources and can result in wrong decisionsbased on the reported out-of-date data. Besides data freshness, often the data must also be sent to the destination by a deadline. To date, not much researchhas been performed on real-time data services in sensor networks.

Preventing denial-of-service attacks in wireless sensor networks is difficult primarily because of the limited resources available to network nodes and the ease with which attacks are perpetrated. Rather than jeopardize design requirements which call for simple, inexpensive, mass-producible devices, we propose a coping strategy that detects and maps jammed regions. We describe a mapping protocol for nodes that surround a jammer which allows network applications to reason about the region as an entity, rather than as a collection of broken links and congested nodes. This solution is enabled by a set of design principles: loose group semantics, eager eavesdropping, supremacy of local information, robustness to packet loss and failure, and early use of results. Performance results show that regions can be mapped in 1 – 5 seconds, fast enough for real-time response. With a moderately connected network, the protocol is robust to failure rates as high as 25 percent. 1.

A central challenge in ad hoc networks is the design of routing protocols that can adapt their behavior to frequent and rapid changes in the network. The performance of proactive and reactive routing protocols varies with network characteristics, and one protocol may outperform the other in different network conditions. The optimal routing strategy depends on the underlying network topology, rate of change, and traffic pattern, and varies dynamically. This paper introduces the Sharp Hybrid Adaptive Routing Protocol (SHARP), which automatically finds the balance point between proactive and reactive routing by adjusting the degree to which route information is propagated proactively versus the degree to which it needs to be discovered reactively. SHARP enables each node to use a different application-specific performance metric to control the adaptation of the routing layer. This paper describes application-specific protocols built on top of SHARP for minimizing packet overhead, bounding loss rate, and controlling jitter. Simulation studies show that the resulting protocols outperform the purely proactive and purely reactive protocols across a wide range of network characteristics. 1

Ad hoc networks rely on the cooperation of the nodes participating in the network to forward packets for each other. A node may decide not to cooperate to save its resources while still using the network to relay its tra#c. If too many nodes exhibit this behavior, network performance degrades and cooperating nodes may find themselves unfairly loaded. Most previous e#orts to counter this behavior ([4],[5],[6],[21]) have relied on further cooperation between nodes to exchange reputation information about other nodes. If a node observes another node not participating correctly, it reports this observation to other nodes who then take action to avoid being a#ected and potentially punish the bad node by refusing to forward its tra#c. Unfortunately, such second-hand reputation information is subject to false accusations and requires maintaining trust relationships with other nodes. The objective of OCEAN is to avoid this trust-management machinery and see how far we can get simply by using direct first-hand observations of other nodes' behavior. We find that, in many scenarios, OCEAN can do as well as, or even better than, schemes requiring second-hand reputation exchanges. This encouraging result could possibly help obviate solutions requiring trust-management for some contexts.

In this paper, we investigate the impact of radio irregularity on wireless sensor networks. Radio irregularity is a common phenomenon which arises from multiple factors, such as variance in RF sending power and different path losses depending on the direction of propagation. From our experiments, we discover that the variance in received signal strength is largely random; however, it exhibits a continuous change with incremental changes in direction. With empirical data obtained from the MICA2 and MICAZ platforms, we establish a radio model for simulation, called the Radio Irregularity Model (RIM). This model is the first to bridge the discrepancy between spherical radio models used by simulators and the physical reality of radio signals. With this model, we investigate the impact of radio irregularity on several upper layer protocols, including MAC, routing, localization and topology control. Our results show that radio irregularity has a relatively larger impact on the routing layer than the MAC layer. It also shows that radio irregularity leads to larger localization errors and makes it harder to maintain communication connectivity in topology control. To deal with these issues, we present eight solutions to deal with radio irregularity. We evaluate three of them in detail. The results obtained from both the simulations and a running testbed demonstrate that our solutions greatly improve system performance in the presence of radio irregularity.

Geographic routing protocols allow stateless routing in mobile ad hoc networks (MANETs) by taking advantage of the location information of mobile nodes and thus are highly scalable. A central challenge in geographic routing protocols is the design of scalable distributed location services that track mobile node locations. A number of location services have been proposed, but little is known about the relative performance of these location services. In this paper, we perform a detailed performance comparison of three rendezvous-based location services that cover a range of design choices: a quorum-based protocol (XYLS) which disseminates each node's location to O( # N) nodes, a hierarchical protocol (GLS) which disseminates each node's location to O(log N) nodes, and a geographic hashing based protocol (GHLS) which disseminates each node's location to O(1) nodes.

In this study we investigate the interaction between TCP and MAC layer in a wireless multi-hop network. This type of network has traditionally found applications in the military (automated battlefield), law enforcement (search and rescue) and disaster recovery (flood, earthquake), where there is no fixed wired infrastructure. More recently, wireless "ad-hoc" multi-hop networks have been proposed for nomadic computing applications. Key requirements in all the above applications are reliable data transfer and congestion control, features that are generally supported by TCP. Unfortunately, TCP performs on wireless in a much less predictable way than on wired protocols. Using simulation, we provide new insight into two critical problems of TCP over wireless multi-hop. The first is the conflict between data packets and ACKs, which causes TCP performance to degrade for window sizes greater than 1 packet. The second is the interaction between MAC and TCP layer backoff timers which causes severe unfairness and capture conditions. In the paper, we identify these problems in several representative simulation runs on various topologies and traffic patterns and indicate possible remedies to improve TCP efficiency over a wireless multi-hop network. 1.

This paper presents a framework to address new data management challenges introduced by data-intensive, perva-sive computing environments. These challenges include a spatio-temporal variation of data and data source availability, lack of a global catalog and schema, and no guarantee of reconnection among peers due to the serendipitous nature of the environment. An important aspect of our solution is to treat devices as semi-autonomous peers guided in their interactions by profiles and context. The profiles are grounded in a semantically rich language and represent informa-tion about users, devices and data described in terms of “beliefs”, “desires”, and “intentions”. We present a prototype implementation of this framework over combined Bluetooth and Ad-Hoc 802.11 networks, and present experimental and simulation results that validate our approach and measure system performance.