Taekyoung Kwon

An experimental study on the capture effect in 802.11a networks

In wireless networks, a frame collision does not necessarily result in all the simultaneously transmitted frames being lost. Depending on the relative signal power and the arrival timing of the involved frames, one frame can survive the collision and be successfully received by the receiver. Using our IEEE 802.11a wireless network testbed, we carry out a measurement study that shows the terms and conditions (timing, power difference, bit rate) under which this capture effect takes place. Recent measurement work on the capture effect in 802.11 networks [10] argues that the stronger frame can be successfully decoded only in two cases: (1) The stronger frame arrives earlier than the weaker frame, or (2) the stronger frame arrives later than the weaker frame but within the preamble time of the weaker frame. However, our measurement shows that the stronger frame can be decoded correctly regardless of the timing relation with the weaker frame. In addition, when the stronger frame arrives later than the weaker frames arrival, the physical layer capture exhibits two very distinct patterns based on whether the receiver has been successfully synchronized to the previous weak frame or not. In explaining the distinct cases we observe that the successful capture of a frame involved in a collision is determined through two stages: preamble detection and the frame body FCS check.

Mobile Agent Based Wireless Sensor Networks

Recently, mobile agents have been proposed for efficient data dissemination in sensor networks. In the traditional client/server-based computing architecture, data at multiple sources are transferred to a destination; whereas in the mobile-agent based computing paradigm, a task specific executable code traverses the relevant sources to gather data. Mobile agents can be used to greatly reduce the communication cost, especially over low bandwidth links, by moving the processing function to the data rather than bringing the data to a central processor. This paper proposes to use the mobile agent paradigm for reducing and aggregating data in a planar sensor network architecture. The proposed architecture is called mobile agent based wireless sensor network (MAWSN). Extensive simulation shows that MAWSN exhibits better performance than client/server communications in terms of energy consumption and the packet delivery ratio. However, MAWSN has a longer end-to-end latency than client/server communications in certain conditions.

Fast-handoff support in IEEE 802.11 wireless networks

With the advance of wireless local area network (WLAN) technology, handoff support has become one of the most important issues in IEEE 802.11 WLANs. However, the current IEEE 802.11 specification does not provide the fast handoff required for real-time multimedia applications. To support fast handoff in IEEE 802.11 networks, a number of fast-handoff schemes have been proposed in the literature. In this article we review these fast-handoff schemes and analyze their advantages and disadvantages qualitatively. After that, important design considerations for mobility support in future IEEE 802.11 networks are suggested. Also, we introduce a fast-handoff framework which adaptively meets different application requirements via a cross-layer approach.

Optimal multi-sink positioning and energy-efficient routing in wireless sensor networks

In wireless sensor networks, the sensors collect data and deliver it to a sink node. Most of the existing proposals deal with the traffic flow problem to deliver data to the sink node in an energy-efficient manner. In this paper, we extend this problem into a multi-sink case. To maximize network lifetime and to ensure fairness, we propose (i) how to position multiple sink nodes in a sensor network and (ii) how to route traffic flow from all of the sensors to these multiple sink nodes. Both of the problems are formulated by the linear programming model to find optimal locations of the multiple sink nodes and the optimal traffic flow rate of routing paths in wireless sensor networks. The improved lifetime and fairness of our scheme are compared with those of the multi-sink aware minimum depth tree scheme.

Mobile agent-based directed diffusion in wireless sensor networks

In the environments where the source nodes are close to one another and generate a lot of sensory data traffic with redundancy, transmitting all sensory data by individual nodes not only wastes the scarce wireless bandwidth, but also consumes a lot of battery energy. Instead of each source node sending sensory data to its sink for aggregation (the so-called client/server computing), Qi et al. in 2003 proposed a mobile agent (MA)-based distributed sensor network (MADSN) for collaborative signal and information processing, which considerably reduces the sensory data traffic and query latency as well. However, MADSN is based on the assumption that the operation of mobile agent is only carried out within one hop in a clustering-based architecture. This paper considers MA in multihop environments and adopts directed diffusion (DD) to dispatch MA. The gradient in DD gives a hint to efficiently forward the MA among target sensors. The mobile agent paradigm in combination with the DD framework is dubbed mobile agent-based directed diffusion (MADD). With appropriate parameters set, extensive simulation shows that MADD exhibits better performance than original DD (in the client/server paradigm) in terms of packet delivery ratio, energy consumption, and end-to-end delivery latency.

A novel cluster-based cooperative MIMO scheme for multi-hop wireless sensor networks

A cluster-based cooperative multiple-input-multiple-output (MIMO) scheme is proposed to reduce the adverse impacts caused by radio irregularity and fading in multi-hop wireless sensor networks. This scheme extends the LEACH protocol to enable the multi-hop transmissions among clusters by incorporating a cooperative MIMO scheme into hop-by-hop transmissions. Through the adaptive selection of cooperative nodes and the coordination between multi-hop routing and cooperative MIMO transmissions, the scheme can gain effective performance improvement in terms of energy efficiency and reliability. Based on the energy consumption model developed in this paper, the optimal parameters to minimize the overall energy consumption are found, such as the number of clusters and the number of cooperative nodes. Simulation results exhibit that the proposed scheme can effectively save energy and prolong the network lifetime.

Energy-efficient differentiated directed diffusion (EDDD) in wireless sensor networks

A number of routing protocols [1] have been proposed for wireless sensor networks in recent years. Considering energy-efficiency as the primary objective, most of routing protocols focus on reducing the number of packet transmissions by clustering, leveraging geographical information, and so on. These routing protocols in sensor networks have the limitation of lacking application contexts for filtering or aggregation. To remedy this, Directed Diffusion (DD) [2], which utilizes application contexts in data dissemination, is proposed. However, DD cannot support time-sensitive traffic nor perform energy-balancing to increase network lifetime. To bridge this gap, this paper extends DD as follows: (1) real-time (RT) filters to provide better end-to-end (ETE) delay performance for real-time traffic, (2) best-effort (BE) filters to achieve global energy balance and to prolong network lifetime, (3) RT-repairs to fast recover node/link failure for RT traffic. The extended DD is dubbed energy-efficient differentiated directed diffusion (EDDD). Comprehensive simulation experiments show that EDDD has the following advantages: (1) differentiates dissemination service for RT and BE traffic, (2) achieves lower delay for RT traffic than DD, (3) exhibits substantially longer network lifetime than DD. dubbed energy-efficient differentiated directed diffusion (EDDD). Comprehensive simulation experiments show that EDDD has the following advantages: (1) differentiates dissemination service for RT and BE traffic, (2) achieves lower delay for RT traffic than DD, (3) exhibits substantially longer network lifetime than DD.

A Hybrid Query Tree Protocol for Tag Collision Arbitration in RFID systems

In this paper, we propose a hybrid query tree protocol that combines a tree based query protocol with a slotted backoff mechanism. The proposed protocol decreases the average identification delay by reducing collisions and idle time. To reduce collisions, we use a 4-ary query tree instead of a binary query tree. To reduce idle time, we introduce a slotted backoff mechanism to reduce the number of unnecessary query commands. For static scenarios of tags, we extended the proposed protocol by adopting two phases. First, in leaf query phase for existing tags, the interrogator queries leaf-nodes directly to reuse query strings in the previous session. Second, in root query phase for new arriving tags, the interrogator starts the query process from the roof-node. Simulation reveals that the proposed protocol achieves lower identification delay than existing tag collision arbitration protocols regardless of whether tags are mobile or not.

SNC: a selective neighbor caching scheme for fast handoff in IEEE 802.11 wireless networks

Mobility support is one of the most challenging issues in IEEE 802.11 networks. In the proactive neighbor caching (PNC) scheme, when a mobile host is connected to an access point (AP), its context (e.g. security association or QoS information) is propagated in advance to all of the APs neighbors to reduce handoff processing time. In this paper, we propose a selective neighbor caching (SNC) scheme, which propagates a mobile hosts context only to the selected neighbor APs considering handoff patterns. Therefore, the SNC scheme can reduce the message overhead on the links among APs. We evaluate the performance of the SNC and PNC schemes in terms of the cache hit probability and the signaling cost. Especially, we investigate the effect of mobility and cache size through extensive simulations. The results reveal that the SNC scheme provides a comparable cache hit probability while significantly reducing the signaling overhead in IEEE 802.11 networks. Moreover, although the SNC propagates relatively a small number of contexts to neighbor APs, the SNC scheme outperforms the PNC scheme when the cache size is small and the mobility is low.

Route Optimization Using Tree Information Option for Nested Mobile Networks

Mobile IP is the basic solution to provide host mobility, whereas network mobility refers to the concept of collective mobility of a set of nodes. In the simplest scenario, a mobile network moves as a single unit with one mobile router (MR) that connects it to the global Internet. Also, multiple mobile networks can be nested in a hierarchical form, e.g., a wireless personal area network (PAN) in a vehicular network. In a nested mobile network, multiple MRs form a tree hierarchy in which the root MR is called the top-level mobile router (TLMR). Nested mobile networks exhibit the pinball routing problem, which becomes worse in proportion to the number of nested levels in the hierarchy. To solve this problem, we propose a routing optimization scheme using a tree information option (ROTIO) that extends the NEMO basic support protocol. In the ROTIO scheme, each MR in the nested mobile network sends two binding updates (BUs): one to its home agent and the other to the TLMR. The former BU contains the TLMRs home address, while the latter contains routing information between the issuing MR and the TLMR. This alleviates the pinball routing problem significantly. Now, a packet from a correspondent node only needs to visit two transit nodes (the home agents of the MR and the TLMR), regardless of the degree of nesting. Moreover, the ROTIO scheme provides location privacy and mobility transparency. We also extend ROTIO to perform routing between two mobile network nodes inside the same nested mobile network more efficiently and to substantially reduce the disruption when a mobile network hands off