Many safety applications rely on multi-hop broadcasting to disseminate safety messages. In most existing multi-hop broadcasting protocols, one next forwarder is selected through contention among forwarder candidates based on their different waiting times. In this paper, we first analyze the latency and collision of the existing protocols, and point out two problems: 1) unnecessary delay occurs in the contention process due to the lack of considering the distribution of vehicles and 2) the short difference between waiting times of forwarder candidates may allow redundant broadcasts to collide with each other. Secondly, we propose a new multi-hop broadcast protocol called RObust and Fast Forwarding (ROFF) to mitigate both problems. ROFF solves the first problem of unnecessary delay by allowing a forwarder candidate to use the waiting time which is inversely proportional to its forwarding priority. A forwarder candidate acquires its forwarding priority using the novel concept of ESD bitmap, which describes the distribution of empty spaces between vehicles. In addition, ROFF prevents the waiting time difference from being shorter than the predefined lower bound in order to avoid collisions, thus solving the second problem. Our extensive simulations reveal that ROFF achieves faster and more reliable broadcasting as compared to the other protocols.

Recently, multi-sink wireless sensor networks (WSNs) are envisioned to solve the hot spot problem caused by traditional single-sink WSNs. Routing protocols should be able to distribute network traffic evenly to multiple sinks to prolong network lifetime and they should be scalable. Gradient-based routing protocols are known to be suitable for the networks, where each node maintains its gradient representing the direction toward a neighbor node to reach a sink. In particular, existing protocols allow a sensor node to construct its gradient using the cumulative traffic load of a path for load-balancing. However, they have a critical drawback that a sensor node cannot efficiently avoid using the path with the most overloaded node. Hence, this paper introduces a new gradient-based routing protocol for load-balancing (GLOBAL) with a new gradient model to maximize network lifetime. In GLOBAL, the least-loaded path which also avoids the most overloaded sensor node is selected for forwarding. Through ns-2 simulation, we verify that GLOBAL achieves better performance than the shortest path routing protocol and load-aware gradient-based routing one.

Abstract In ad hoc networks, the spatial reuse property limits the number of packets which can be spatially transmitted over a path. In standard Transmission Control Protocol (TCP), however, a TCP sender keeps transmitting packets without taking into account this property. This causes heavy contention for the wireless channel, resulting in the performance degradation of TCP flows. Hence, two techniques have been proposed independently in order to reduce the contention. First, a TCP sender utilizes a congestion window limit (CWL), by considering the spatial reuse property. This prevents the TCP sender from transmitting more than CWL number of packets at one time. Second, a delayed ack (DA) strategy is exploited in order to mitigate the contention between the TCP ACK and DATA packets. Recently, although TCP‐DAA (Dynamic Adaptive Acknowledgment) attempts to utilize a CWL‐based DA strategy, TCP‐DAA overlooks a dynamic correlation between these two techniques. This paper, therefore, reveals the dynamic correlation and also proposes a protocol which not only reduces the frequency of the TCP ACK transmissions but also determines a CWL value dynamically, according to network conditions. Simulation studies show that our protocol performs the best in various scenarios, as compared to TCP‐DAA and standard TCP (such as TCP‐NewReno). Copyright © 2009 John Wiley & Sons, Ltd.

In MANETs, ACK loss can affect TCP communication performance, since end-to-end reliable delivery of data packets is guaranteed by using ACK packets sent from a TCP receiver. This ACK loss may force a TCP sender to produce unnecessary retransmissions (called spurious retransmissions). These spurious retransmissions can also degrade the performance of TCP flows in the network because they will create more channel contention and interferences. These issues have been overlooked and hence require attention. We, therefore, propose new solutions to reduce such spurious retransmissions. Our scheme allows a TCP sender to be correctly informed of the sequence number that the TCP receiver expects to receive (i.e., ACK sequence). This ACK sequence is piggybacked onto routing control packets during a new route discovery process invoked by the TCP receiver. Using the ACK sequence tossed by the routing layer, the TCP sender can reduce spurious retransmissions. We evaluated and compared our scheme with TCP-Reno via simulation on ns-2. Simulation results showed that our proposed solutions outperformed general TCP (for both static and dynamic networks cases under multiple cross TCP traffic) in terms of aggregate throughput and number of spurious retransmissions.

A MANET (mobile ad hoc network) requires an efficient network-wide broadcasting service in route discovery as well as in many applications. Recently, many broadcasting protocols have been proposed to avoid the well-known broadcast-storm problem, or to provide reliability over wireless broadcast media. However, since it is assumed that all nodes in the network have the same radio ranges (i.e. homogeneous radio ranges), the protocols do not work in a realistic environment where all nodes have different fixed radio ranges or they use dynamic radio ranges intentionally to save their energy in the network (i.e. heterogeneous radio ranges). We therefore propose an efficient stateless broadcasting technique to support the heterogeneous radio ranges. An additional area covered through a node's rebroadcasting is calculated by considering different radio ranges of nodes. Then, nodes with larger coverage area will rebroadcast earlier than other nodes with smaller area. When determining rebroadcasting schedule, the DFD (dynamic forwarding delay) concept introduced in DDB (dynamic delayed broadcasting) is exploited in our protocol. Extensive simulation using ns-2 proves that our scheme is more suitable for a realistic environment.