Hongseok Yoo

Dynamic Duty-Cycle Scheduling Schemes for Energy-Harvesting Wireless Sensor Networks

In this letter, we propose two novel dynamic duty-cycle scheduling schemes (called DSR and DSP) in order to reduce sleep latency, while achieving balanced energy consumption among sensor nodes in wireless sensor networks (WSNs) with energy harvesting capability. In DSR, each sensor node is allowed to adjust its duty-cycle according to the current amount of residual energy only. Since the residual energy of nodes in energy-harvesting WSNs can increase over time due to their harvesting opportunity, the estimation of prospective increase in their residual energy is useful to achieve our goal. Hence, DSP allows each of sensor nodes to reduce its duty-cycle more aggressively in proportion to such an increase. Through NS-2 simulations, we verified that our proposed schemes outperform the duty-cycle scheduling scheme used in a representative existing MAC protocol such as RI-MAC.

GLOBAL: A Gradient-based routing protocol for load-balancing in large-scale wireless sensor networks with multiple sinks

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.

Repetition-based cooperative broadcasting for vehicular ad-hoc networks

In vehicular ad hoc networks, most of critical applications involved with safety rely on reliable broadcast communications with low latency. Recently, repetition-based protocols have been proposed to meet the requirements of timeliness and reliability for broadcasting. In these protocols, a sender repeatedly retransmits the broadcast message during the lifetime of the message. However, existing protocols face serious problems such as deterioration of the signal quality caused by wireless fading. In particular, since excessive repetitions might cause network congestion and waste channel resources, reliability of broadcasting should be achieved with as small a number of repetitions as possible. In this paper, we therefore propose a novel repetition-based broadcast protocol which exploits a cooperative diversity technique (called RB-CD) making a small number of repetitions robust for wireless fading. To support this cooperative diversity, neighboring nodes transmit the same message almost simultaneously (that is, using the same repetition pattern for each other) in order to form a virtual antenna array. The virtual antenna array achieves a diversity gain at the receivers. In the RB-CD protocol, the virtual antenna array consists of the source and some of its neighbors (called relays) which participate in repeating the transmission of a broadcast message. In addition, a new distributed relay selection algorithm is introduced in the RB-CD protocol. From the ns-2 simulation results, we verified that RB-CD provides a more reliable broadcasting service due to its capability of exploiting cooperative diversity.

ROFF: RObust and Fast Forwarding in Vehicular Ad-Hoc Networks

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.

Unicast geographic routing protocols for inter-vehicle communications: a survey

Vehicular ad hoc network (VANET) is a new type of network that has promised countless applications including safety, comfort, infotainment, etc. The high mobility of vehicles in VANET makes routing protocol design a challenging task. Geographic routing is considered a suitable approach for VANET. The proposed geographic routing protocols for VANET, primarily, differ in the forwarding strategy. In this article we classify state-of-the-art unicast geographic routing protocols on the basis of forwarding strategies. We describe each protocol in terms of some qualitative metric. Finally the selected unicast geographic routing protocols are analyzed through a comparative discussion.

A scalable multi-sink gradient-based routing protocol for traffic load balancing

Wireless sensor networks have been assumed to consist of a single sink and multiple sensor nodes which do not have mobility. In these networks, sensor nodes near the sink dissipate their energy so fast due to their many-to-one traffic pattern, and finally they die early. This uneven energy depletion phenomenon known as the hot spot problem becomes more serious as the number of sensor nodes (i.e., their scale) increases. Recently, multi-sink wireless sensor networks have been envisioned to solve the hot spot problem. Gradient routing protocols are known to be appropriate for the networks in that network traffic is evenly distributed to multiple sinks to prolong network lifetime and they are scalable. Each node maintains its gradient representing the direction toward a neighbor node to reach one of the sinks. 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 routing protocol for LOad-BALancing (GLOBAL) with a new gradient model to maximize network lifetime.The proposed gradient model considers both of the cumulative path load and the traffic load of the most overloaded node over the path in calculating each nodes gradient value. Therefore, packets are forwarded over the least-loaded path, which avoids the most overloaded node. In addition, it is known that assigning a unique address to each sensor node causes much communication overhead. Since the overhead increases as the network scales, routing protocols using an address to indicate the receiver in forwarding a packet are not scalable. Thus, GLOBAL also includes an addressing-free data forwarding strategy. Through ns-2 simulation, we verify that GLOBAL achieves better performance than the shortest path routing and load-aware gradient routing ones.

Dynamic Channel Coordination Schemes for IEEE 802.11p/1609 Vehicular Networks: A Survey

IEEE 802.11p/1609-based vehicular networks utilize a multichannel architecture to support vehicle-to-vehicle and vehicle-to-infrastructure communications. In the multi-channel architecture, the available channels in the 5 GHz spectrum are divided into one control channel (CCH) and multiple service channels (SCHs). Multiple SCHs are defined for nonsafety data transfer, while the CCH is used to broadcast safety messages called beacons and control messages (i.e., service advertisement messages). According to the channel coordination scheme, a radio interface alternately switches between the CCH and a specific SCH. The intervals during which a radio interface stays tuned to the CCH and SCH are called CCH and SCH intervals, respectively. Both intervals are set to a fixed value (i.e., 50 ms) in the standard. However, since the fixed-length intervals cannot be effective for dynamically changing traffic load, some dynamic interval division protocols have been recently proposed to support the dynamic adjustment of the CCH/SCH intervals for improving channel utilization. In this paper, we therefore provide a survey of dynamic interval division protocols for VANETs, discuss the advantages and disadvantages of them, and define some open issues and possible directions of future research.

A dynamic safety interval protocol for VANETs

In IEEE 802.11p/1609-based vehicular networks, the channel access time of a synchronization interval is divided into two fixed-length intervals, i.e. control channel (CCH) and service channel (SCH) intervals. Since the fixed-length intervals cannot be effective for dynamically changing traffic load, some protocols have been recently proposed to support variable-length CCH intervals in order to improve channel utilization. In existing protocols, the CCH interval is subdivided into safety and non-safety intervals, and each interval is dynamically determined based on traffic load. However, they do not consider the presence of hidden nodes. Hence, each interval has no additional room for transmissions from hidden nodes. Consequently, messages transmitted in each interval are likely to overlap with simultaneous transmissions (i.e. interference) from hidden nodes. Particularly, life-critical safety messages can be unreliably delivered due to such interference, which deteriorates QoS of safety applications such as cooperative collision warning. In this paper, we therefore propose a new dynamic safety interval (DSI) protocol. DSI calculates the number of vehicles sharing the channel with the consideration of hidden nodes. The safety interval is derived based on the measured number of vehicles. From simulation study using the ns-2, we verified that DSI achieves better delivery ratio of periodic messages such as beacon.

EMDOR: Emergency message dissemination with ACK-overhearing based retransmission

In vehicular ad hoc networks (VANETs), providing drivers with safety service such as the reliable delivery of an emergency message is a challenge. To propagate the emergency message quickly to all vehicles, the broadcasting technique can be utilized, however, the broadcasting is vulnerable to unreliability. In this paper, we therefore propose a scheme to improve the broadcast reliability by allowing nodes to overhear an ACK message. A selected node transmits an ACK message on behalf of other receiving nodes. If a node overhears an ACK message, but has not received the corresponding message, the node considers that the emergency message is lost and requests the selected node to rebroadcast the emergency message. Using ns-2 simulator, we verify that our proposed scheme provides more reliable service than other existing solutions.

Agriculture Sensor-Cloud Infrastructure and Routing Protocol in the Physical Sensor Network Layer

Nowadays, wireless sensor networks (WSNs) are used in a variety of areas. However, it is difficult to efficiently manage a large number of sensor nodes and their sensing data owing to the limitations of WSNs. Particularly, in agricultural applications, a WSN installed in a specific region is used for multiple services (i.e., greenhouse environment monitoring and control). However, the network resources (i.e., channel, battery, etc.) are currently being utilized for redundant operations requested from multiple service users owing to the lack of an efficient system for managing large WSN data. In this paper, we propose an agriculture sensor-cloud infrastructure (ASCI) to effectively provide various agricultural services using WSNs. In addition, we propose hierarchical source routing (HSR), aggregation gradient routing (AGR), and a priority-based data transmission technique in order to allow packets to be delivered to the destination fast and reliably in large-scale WSNs.