Sangsoon Lim

A Fire Detection and Rescue Support Framework with Wireless Sensor Networks

Recently, wireless sensor network (WSN) technology has been widely used in various fields such as public safety applications since it provides a wide range of surveillance and monitoring applications with an inexpensive price. In this paper, we investigate the use of wireless sensor network technology for fire detection and rescue system. We propose a framework, which can detect fires promptly and support rescue activities, using wireless sensor devices. Our framework consists of fire detection sensor network, information gathering layer, middleware, and escape support system. We have implemented a test-bed and evaluated the performance of fire detection via experiments.

Dual wake-up low power listening for duty cycled wireless sensor networks

Energy management is an interesting research area for wireless sensor networks. Relevant dutycycling (or sleep scheduling) algorithm has been actively studied at MAC, routing, and application levels. Low power listening (LPL) MAC is one of effective dutycycling techniques. This paper proposes a novel approach called dual wake-up LPL (DW-LPL). Existing LPL scheme uses a preamble detection method for both broadcast and unicast, thus suffers from severe overhearing problem at unicast transmission. DW-LPL uses a different wake-up method for unicast while using LPL-like method for broadcast; DW-LPL introduces a receiver-initiated method in which a sender waits a signal from receiver to start unicast transmission, which incurs some signaling overhead but supports flexible adaptive listening as well as overhearing removal effect. Through analysis and Mote (Telosb) experiment, we show that DW-LPL provides more energy saving than LPL and our adaptive listening scheme is effective for energy conservation in practical network topologies and traffic patterns.

StreetSense: Effect of Bus Wi-Fi APs on Pedestrian Smartphone

Recently, we have received a growing number of reports that complain about poor and unstable internet connections at bus stops in metro Seoul. Careful analyses led us to conclude that Wi-Fi APs equipped on buses instigate the trouble. According to the ambitious free Wi-Fi expansion plan by the city of Seoul, public buses started to equip Wi-Fi APs. As buses with APs stop and go, they actualize intermittent connection opportunities to riders waiting at the bus stops. However, the connection durations are too short such that bus APs are a nuisance rather than a convenience. We collected the basic statistics such as AP inter-arrival and sojourn times and measured link level performance metrics. We observed the effect of frequent frame losses on the TCP congestion control and eventually on the TCP throughput. We also measured the performance of applications such as PLT (Page Load Time). The measurement results showed that passing APs are useful only for some applications in very limited situations while they are virtually useless and just irritations in many cases. We also discovered that poor Wi-Fi connections pervert MPTCP; MPTCP performs worse than the generic single path TCP over the LTE network. We expect that our results will be used as the reference data in redesigning Wi-Fi offloading mechanisms as well as in planning and deploying urban Wi-Fi networks.

Design, Analysis and Implementation of Energy-efficient Broadcast MAC Protocols for Wireless Sensor Networks

In wireless sensor networks (WSNs), most energy saving asynchronous MAC protocols are custom tailored for unicast communications only. However, broadcast protocols are very commonly used in WSNs for a variety of functionalities, such as gathering network topology information, event monitoring and query processing. In this paper, we propose a novel low-power asynchronous broadcast MAC protocol called Alarm Broadcast (A-CAST). A-CAST employs the strobe preamble that specifies the residual waiting time for the following data transmission. Each receiver goes back to sleep upon hearing the strobe preamble for the residual time duration, to conserve energy and to wake up just before data transmission starts. We compute the energy consumption of A-CAST via rigorous mathematical analysis. The analytic results show that A-CAST outperforms B-CAST, a simple broadcast extension of the well-known B-MAC. We also implement A-CAST on sensor motes and evaluated its performance through real experiments. Our experimental results show that A-CAST reduces the energy consumption by up to 222% compared to the previously proposed protocols.

CoSense: Interference resilient ZigBee detection in heterogeneous wireless networks

The concurrent deployment of heterogeneous wireless networks such as Wi-Fi, Bluetooth, and ZigBee has led to the severe interference problems in the 2.4GHz ISM band. In particular, ZigBee networks are susceptible to the interferences from other wireless technologies; For example, strong Wi-Fi signals trigger false alarms to ZigBee device that is performing low power idle listening and cause appreciable energy waste. In this paper, we propose a novel ZigBee signal detection scheme, called CoSense that accurately identifies ZigBee signals in the presence of the cross-technology interferences. CoSense, which is a highly reliable signal correlation technique, enjoys the following three advantages: First, CoSense reduces false wake-ups, which typically consume energy unnecessarily. Secondly, CoSense is robust against heterogeneous interference scenarios because its signal correlation feature has been shown to work well in bad channel conditions. Third, CoSense is backward-compatible and does not require to change the traditional ZigBee networks. We have implemented CoSense on the USRP/GNURadio platform in order to prove its feasibility. The results show that, under typical setting, CoSense indeed reduces the false alarm rate and its overhead is tolerable. We can conclude that CoSense saves energy by up to 63% in heterogeneous network environments where weak ZigBee signals are overwhelmed by strong signals such as Wi-Fi.

NBP: light-weight Narrow Band Protection for ZigBee and Wi-Fi coexistence

The recent development of various wireless technologies in the 2.4GHz ISM band has led to the co-channel coexistence of heterogeneous wireless devices, such as Wi-Fi, Bluetooth, and ZigBee. This sharing of the common channel results in the challenging problem of cross-technology interference, since the wireless devices generally use diverse PHY/MAC specifications. In particular, the less capable ZigBee device may often experience unpredictably low throughput due to the interference from the powerful Wi-Fi. The ZigBee protector is an attractive solution, since it can reserve the channel on behalf of the weak ZigBee devices. The protector method, however, has a few limitations; (i) it may cause significant overhead to both ZigBee and Wi-Fi, and (ii) the ZigBee control packets are still vulnerable to the Wi-Fi interference. In this paper, we propose a novel time reservation scheme called Narrow Band Protection (NBP), that uses a protector to guard the ongoing ZigBee transmission. The key contributions are threefold: First, NBP autonomously detects any ongoing ZigBee transmissions by cross-correlating the ZigBee’s packets with the pre-defined Pseudo-random Noise (PN) sequences. By using this cross-correlation, it significantly reduces the control overhead. Second, due to the reliable cross-correlation, NBP is robust from the control packet collisions, which typically wastes channel time for both ZigBee and Wi-Fi. Third, NBP protects the burst of ZigBee packets by estimating the size of the burst, in turn, giving a semantic to the PN codebook. This is important because ZigBee is typically battery-powered and thus the long burst is advantageous for the low duty cycle operations. We first show the feasibility of NBP by implementing it on the real USRP/GNURadio platform. Then, we evaluate the performance of NBP through mathematical analysis and NS-2 simulations. The results show that NBP enhances the ZigBee throughput by up to 1.77x compared to the existing scheme.

The synergic enhancement of coexistence performance in wireless mobile combo-chips

This paper deals with the problem of severe wireless performance degradation when multiple wireless technologies are concurrently utilized in a same user device. This type of usage is already frequent in most smartphones and laptops, such as streaming Bluetooth audio while using a Wi-Fi download, and is more intensifying with IoT device deployment which triggers the coexistence of heterogeneous wireless technologies. To lower the form factor and the cost, chip vendors package multiple wireless interfaces into a single combo-chip where a common antenna is shared by multiple network technologies in a time division multiplexing manner. We issue that the careless operations of combo-chip design incur indeed performance degradation for in-device wireless coexistence and show the experimental results via TCP performance measurements in several smartphones and laptops. Our analysis reveals that the behavior negatively affects not only on the transmit power management of wireless access point, but also on the congestion control of TCP sender. We propose a cooperative switching scheme which incorporates TCP control behaviors for better coexistence and implement it on Android and Linux devices. Under the simultaneous use of in-device network interfaces, our approach led a WLAN throughput increment up to eight times without the mentioned issues. Further, this does not require any modification of TCP sender and wireless access point. Thus, the approach is directly applicable to existing mobile devices and also easily extendable to the combination of other in-device wireless technologies.

Design, Analysis and Evaluation of A New Energy Conserving MAC Protocol for Wireless Sensor Networks

Low power listening (LPL) MAC protocols based on duty-cycling mechanism have been studied extensively to achieve ultra low energy consumption in wireless sensor networks (WSNs). Especially, recent ACK-based LPL schemes such as X-MAC employ strobe preambles and an early ACK, and show fair performances in communications and energy efficiencies. However, the state-of-the-art ACK-based LPL scheme still suffers from collision problems due to the protocol incompleteness. These collision effects are not trivial and make WSNs unstable, aggravate energy consumptions. In this paper, we propose two novel schemes; (i) ?-duration CCA to mitigate the collision problem in ACK-based LPL MAC protocols. (ii) Short Preamble Counter (SPC) to conserve more energy by reducing unnecessary overhearing. We demonstrate the performance improvement of our scheme via a mathematical analysis and real-time experiments. Both analysis and experimental results confirm that our proposed scheme saves energy by up to 36% compared to the naive ACK-based LPL MAC protocol thanks to ?-duration CCA and SPC.

BlueCoDE: Bluetooth coordination in dense environment for better coexistence

Dense Wi-Fi and Bluetooth (BT) environments become increasingly common so that the coexistence issue between Wi-Fi and BT is imperative to solve. In this paper, we propose BlueCoDE, a coordination scheme for multiple neighboring BT piconets, to make them collision-free and less harmful to Wi-Fi. BlueCoDE reuses BTs existing PHY and MAC design, thus making it practically feasible. We implement a prototype of BlueCoDE on Ubertooth One platform and corroborate the performance gain via analysis, NS-3 simulations, and prototype-based experiments. Our experimental results show that with merely 10 legacy BT piconets, neighboring Wi-Fi network becomes useless achieving under 1 Mb/s throughput, while BlueCoDE enables the Wi-Fi throughput always remain above 12 Mb/s. We expect BlueCoDE to be a breakthrough solution for coexistence in dense Wi-Fi and BT environments.

GreenAir: Harmonic CTI relaxation under massive heterogeneous wireless devices

Recently, the simultaneous usage of various wireless technologies, such as Wi-Fi, Bluetooth, ZigBee and so on, becomes common and the propensity tends to be more complicated with the prevalence of smart and IoT devices. Such technology increments, however, intensify challenging Cross-Technology Interference (CTI) issues in a shared wireless band. To circumvent the hurdle, we propose a way which minimizes the medium occupation of wireless devices, called GreenAir. In the range of preserving network performances like connectivity and delay, GreenAir supports to generate the lesser communication links among homogeneous devices by utilizing a duty-cycle control. We experiment whether such approach is feasible to reduce CTI under the mixture of heterogeneous devices. Through retransmission and throughput measures in the combined use of Wi-Fi, ZigBee and Microwave oven (aggressive offender in ISM band), we validate that there exist large margins to reduce CTI via OPNET modeler. For the better applicability in IoT surroundings where the addition/removal of devices is arbitrary and frequent, we further tailor the operation of GreenAir to be distributed and autonomous. As our approach only requires local communications among homogeneous devices, the usability of GreenAir is device-agnostic.