Woosik Lee

Experimental link channel characteristics in wireless body sensor systems

In the resource-constrained environment such as wireless body sensor networks (WBSNs), all sensor nodes operating with limited battery need the very-low-power wireless technologies. The representative approach to reduce energy consumption in WBSNs is transmission power control (TPC). The TPC technique is used to maintain acceptable packet delivery performance with the low power consumption. So, it has been widely researched as the important subject in WSNs. However, the TPC algorithms which have been researched in WSNs are not directly applicable in WBSNs which deploy sensor nodes in, on, or around a human body. This is because instantaneous channel condition is frequently changed due to sensor mobility resulting from human movements in WBSNs. Therefore, we need a sophisticated TPC technology to achieve reliable communication in WBSNs. To do this, in this paper, we investigate link layer channel characteristics in WBSNs. For understanding link characteristics in terms of power and placement, we conducted diverse experiments with real sensor devices. Through the analysis of experimental results, we knew that RSSI values are deeply depended on the transmission power and sensor placement in WBSNs.

Hybrid Transmission Power Control for Wireless Body Sensor Systems

In wireless body sensor network systems (WB-SNSs), the sensor nodes have very limited battery power because they are tiny, lightweight, and wearable or implantable. As a result, WB-SNSs require a very efficient transmission power control (TPC) algorithm for effectively reducing energy consumption and extending the lifetime of sensor nodes. To achieve this goal, we propose a novel TPC algorithm referred to as hybrid TPC. The hybrid TPC algorithm adaptively selects a conservative or an aggressive control mechanism depending on current channel conditions. The conservative control mechanism, which slowly changes transmission power level (TPL), is suitable in a dynamic environment. On the other hand, the aggressive control mechanism, which rapidly changes TPL, is ideal in a static environment. In order to evaluate the effectiveness of the hybrid TPC algorithm, we implemented various TPC algorithms and compared their performances against the hybrid TPC algorithm in different channel environments. The experimental results showed that the hybrid TPC algorithm outperformed other TPC algorithms in all channel environments.

Different Characteristics of Radio Modules in Wireless Body Sensor Network Systems

Wireless body sensor network systems (WB-SNSs) can use diverse radio modules. However, previous studies did not consider different characteristics of radio modules, which deeply impact the performance of WB-SNSs. In this paper, we analyze the performance of WB-SNSs using the representative radio modules CC2420 and CC1000. In this environment, we collected log data from real sensor devices deployed on the human body. After log data collection, we first show that CC2420 and CC1000 have different radio characteristics from diverse views, such as the received signal strength indication (RSSI) average and deviation, transmission power levels, and body movement. Through the analysis, we also find that an efficient transmission power control (TPC) algorithm should consider these diverse factors due to different radio modules.

Prime Block Design for Asynchronous Wake-Up Schedules in Wireless Sensor Networks

Due do the various requirements of sensor applications, it is desired to design a neighbor discovery protocol that supports both the symmetric and asymmetric duty cycles. This letter proposes a new block-based neighbor discovery protocol for asymmetric sensor networks by adapting the theory of balanced incomplete block designs and the Chinese remainder theorem. Through the simulation study, it is demonstrated that the proposed block-based neighbor discovery protocol outperforms other neighbor discovery methods, such as Disco, U-Connect, SearchLight, Hedis, and Todis.

Block cycle length-based asymmetric rendezvous protocol for IoT applications: poster abstract

In the Internet of Things (IoT) environment, a sensor network protocol must satisfy various application-specific requirements such as synchronous, asynchronous, symmetric, and asymmetric operations. Most block design-based neighbor discovery protocols can not support asymmetric operations. In this paper, we propose a new block cycle length-based asymmetric rendezvous protocol for neighbor discovery. The proposed protocol has better energy efficiency and shorter latency than existing asynchronous neighbor discovery protocols since it is based on optimized block designs.

Block combination–based asynchronous wake-up schedule in wireless sensor networks

In wireless sensor networks, when sensor nodes are operated with different ratios of active slots, this is called asymmetric duty cycles. Furthermore, cycles with the same ratio of active slots per cycle for all nodes are called symmetric duty cycles. In wireless sensor networks, most applications require both symmetric and asymmetric duty cycles. The balanced incomplete block design–based wake-up schedule is known to be the optimal solution for symmetric duty cycles. However, because this schedule cannot support asymmetric duty cycles, the balanced incomplete block design–based wake-up schedule is not suitable for wireless sensor networks. Herein, we propose a new scheme called the block combination–based asynchronous wake-up schedule to resolve this issue for asymmetric duty cycles. Block combination–based asynchronous wake-up schedule combines different blocks using a block combination operation. The combined schedule guarantees common active slots between sensor nodes in asymmetric duty cycles. To demonstrate the superior performance of block combination–based asynchronous wake-up schedule, we implement a TOSSIM-based simulation and compare the experimental results with recent neighbor discovery protocols such as balanced incomplete block design, prime-based block design, Disco, U-Connect, SearchLight, Hedis, and Todis. We then prove that block combination–based asynchronous wake-up schedule outperforms the others.

Security Policy Scheme for an Efficient Security Architecture in Software-Defined Networking

In order to build an efficient security architecture, previous studies have attempted to understand complex system architectures and message flows to detect various attack packets. However, the existing hardware-based single security architecture cannot efficiently handle a complex system structure. To solve this problem, we propose a software-defined networking (SDN) policy-based scheme for an efficient security architecture. The proposed scheme considers four policy functions: separating, chaining, merging, and reordering. If SDN network functions virtualization (NFV) system managers use these policy functions to deploy a security architecture, they only submit some of the requirement documents to the SDN policy-based architecture. After that, the entire security network can be easily built. This paper presents information about the design of a new policy functions model, and it discusses the performance of this model using theoretical analysis.

Block Combination Selection Scheme for Neighbor Discovery Protocol

A neighbor discovery protocol (NDP) is one of the critical research subjects in wireless sensor networks (WSNs) for efficient energy management of sensor nodes. A block design concept can be applied to find a neighbor discovery schedule that guarantees at least one common active slot between any pair of sensor nodes. However, the block design -- based solutions in the literature are not flexible enough because, due to the lack of a general block design scheme, only a limited set of block designs is available for some duty cycles. In this paper, a new approach for the block construction that can easily generate a set of neighbor discovery schedules with a diverse set of duty cycles is introduced. Moreover, we propose a block combination selection scheme (BCS) to choose a near-optimal block combination among the set of candidate schedules. The BCS algorithm first generates set of the candidate block combinations whose duty cycle is similar to the desired duty cycle. Then, from the set of candidates, BCS picks a block combination which has the minimum duty cycle and latency product. In this simulation study, we evaluate the performance of the BCS algorithm and compare it to the performance of other NDPs, such as Random, U-Connect, Disco, and Quorum with a target duty cycle. According to our simulation results, the BCS algorithm always picks the best block combination from the set. If the desired duty cycle is 1%, the block combination selected by BCS performs up to 56.29% better than other NDPs in terms of worst-case latency. In terms of energy consumption, the BCS algorithm performs is 58.8% better than other NDPs.

The Characteristic Analysis of the Body Conditions for Efficient Transmission Power Control Algorithms in Wearable Healthcare Systems

In wearable healthcare systems (WHCSs), sensor devices, which can be deployed around parts of the human body and have limited lifetimes, control its transmission power level (TPL) based on transmission power control (TPC) algorithms for saving the energy consumption. A transmission and a reception part of WHCSs bring more energy consumption than other modules such as a sensing and a logging. Therefore, if sensor devices can reduce a number of transmission and reception packets, they can increase their lifetimes as much as reduced packets. Because many times sensed data packets are negligible, in this case sensor devices cannot send data packets. In this paper, we first analyze the characteristics of the human body conditions using the real-collected body condition data set to show the existence of the negligible period during communication times. Then, we implement the TPC algorithm to compare the performance of with or without the consideration of the human body conditions.

OR-based block combination for asynchronous asymmetric neighbor discovery protocol

The neighbor discovery protocol (NDP) is one of the critical research subjects in wireless sensor networks (WSNs) for efficient energy management of sensor nodes. A block design concept can be applied to find a neighbor discovery schedule that guarantees at least one common active slot between any pair of sensor nodes. However, the block design-based solutions in literature are not flexible enough because block design cannot support asymmetric asynchronous operations. In this paper, we introduce a new approach for asymmetric asynchronous neighbor discovery protocol that combines two optimal block designs using the OR operation. OR-based block combination solves the problem of original block design, which cannot support asymmetric scenarios. We evaluate the performance of OR-based block combinations using a simulation study. According to our simulation results, OR-based block combination performs up to 71% better than other asymmetric algorithms, such as U-Connect and Disco with 10% and 1% duty cycles.