Yumi Takaki

Distributed association control considering user utility and user guidance in IEEE802.11 networks

Public WLANs based on IEEE802.11 standard are now widely deployed. In an environment offering several APs (Access Points), a UTE (User Terminal Equipment) needs to choose which AP to associate with. Conventionally, the UTE associates with the AP with the strongest RSSI (Received Signal Strength Indicator). This simple approach, however, can degrade the efficiency of network utilization since the UTE is likely choose a heavily loaded AP, even though choosing a more lightly loaded AP would be more effective. So far, some AP association schemes have been proposed. To the best of our knowledge, however, these existing schemes do not take into account of individual difference among UTEs in terms of user utility, physical data rate, traffic characteristics such as traffic demand and frame length. In this paper, we propose a distributed user association control scheme that does considering such UTE characteristics. The proposed scheme needs to estimate uplink and downlink throughput, so that we introduce a throughput estimation method that considers UTE diversities. Through numerical simulations, we confirm that our distributed control scheme can improve user utility and fairness.

Development of 27MHz/40MHz bands maritime wireless ad-hoc networks

The maritime MANET (Mobile Ad-hoc Network) system using 27MHz/40MHz bands was developed under the technical committee of the Japanese Ministry of Internal Affairs and Communications for “development of technical examination services concerning frequency crowding.” The maritime MANET is based on AODV (Ad-hoc On-demand Distance Vector) with a transmission power control. After three years study including simulations and demonstrative experiments at sea off Nishinomiya and Kushimoto, the committee has reported the technical standards for the maritime ad-hoc networks using 27MHz/40MHz bands. In this paper, we describe a part of the activities and achievements of the working group constituted under the committee. In the maritime experiments at sea off Kushimoto, five-hop communication was achieved and the effectiveness of the transmission power control was also verified.

Side trip multi-path routing protocol for spatial node failure tolerance in data-gathering WSNs.

In severe disaster environments, wireless sensor networks (WSNs) need to be tolerant to spatial node failure due to external factors such as bomb attack and river overflow. The aim of this paper is to realize data-gathering WSNs that keep the data collection ratio as high as possible while keeping system lifetime as long as possible. This paper proposes not only a multipath data gathering protocol called “Side Trip (ST)” scheme to enhance spatial node-failure tolerance, but also an improved version of ST scheme called “Side Trip with Network Coding (STNC)” scheme which utilizes inter-flow XOR network coding to reduce redundant packets without affecting the system lifetime significantly. In data-gathering WSNs, nodes further from a sink node tend to relay fewer packets. In the ST scheme, a node establishes another secondary path which, for some distance, passes through nodes the same hop count away from the sink in order to spatially separate the path from the primary path. In the STNC scheme, redundant packets from different source nodes are network-coded into an encoded packet in order to reduce the number of redundant packets. We compare the ST scheme, STNC scheme, and two existing multipath protocols, i.e., H-SPREAD (hybrid-secure protocol for reliable data delivery) scheme and Sub-branch Multipath Routing Protocol (SMRP) scheme, by means of a self-developed simulator. The results show that STNC scheme and ST scheme offer enhanced tolerance to node-failure compared to H-SPREAD scheme and SMRP scheme with no significant impact on system lifetime.

Efficient urban broadcast protocol for V2V communications with Relay Control

In Vehicle-to-Vehicle (V2V) communications, broadcast is expected to disseminate information to other vehicles efficiently. However, due to shadowing of buildings, especially in urban areas, the V2V communications at an intersection is unreliable. Thus it is difficult to disseminate information to all directions at an intersection. Moreover, most of existing protocols which consider intersections in urban area need to use digital map, which is costly and hard to update frequently. Also, they depend on periodic beacons to judge whether or not nodes rebroadcast. They require a lot of control overheads, especially in urban areas, which prejudice efficiency and reliability of message delivery. In this paper, an efficient urban broadcast scheme is proposed for V2V communications in urban environment, which does not rely on the digital map and periodic beacon exchanges. Without digital map, we identify intersection by the difference of a theoretical RSSI (Received Signal Strength Indicator) value and a measured RSSI value. We decide RAD (Random Assessment Delay) by difference of RSSIs as well as a distance between a sender and a receiver to achieve low end-to-end delay. Also we use angle between vectors and vertical distance to get high reception ratio. Both of them are significant for safety critical services of ITS. Simulation results show that proposal scheme achieves higher reception ratio and shorter delivery time than existing broadcast protocols in different loads and vehicle densities.

A study on cache suppression control according to content popularity for in-network guidance

Content servers have become burdened by the transfer of large content sets such as high-definition video and music. To mitigate this problem, we propose a Mapping Server with Cache-location Resolution with Cache Suppression (MSCR/CS), which suppresses the unnecessary duplication of popular contents in network caches.Simulations show that MSCR/CS decreases content-server loads.

A Simple Approach on Mitigating the Gray Zone Effect to Improve Throughput in IEEE802.11 Based Ad-Hoc Network

In wireless ad-hoc networks, one of the reasons thatincrease the data packet loss rate is gray zone effect. Thegray zone effect can happen when broadcasting controlpackets with lower rate while transmitting the data packetswith such higher rate or the same PHY data rate. In thiscase, nodes are able to find valid routes but some datapackets may not reach the destination, which causes thehigh packet loss rate. Most research efforts so far haveattempted to modify the routing protocol to take the grayzone effect into consideration. This method, however, couldhave a detrimental performance owning to its high computationalcomplexity. In this paper, by adjusting the suitablebroadcast rate and data transmission rate, we can obtainhigher throughput than the original way. By simulations,we show that our simple approach can improve throughputsignificantly.

Efficient and reliable packet transfer protocol for wireless multihop bidirectional communications

In wireless multihop bidirectional communication environment, there is a possibility that packet collision and retransmission owing to the hidden node problem decrease efficiency of throughput. The aim of this article is to achieve efficient and reliable packet transmissions in such environments. To do so, we propose a packet transmission scheme named inter-flow network coding with passive acknowledgment. In inter-flow network coding with passive acknowledgment, it is necessary to optimize the encoding latency and to avoid passive acknowledgment packet collision, so we address these issues in this article. Finally, we also confirm that the inter-flow network coding with passive acknowledgment scheme is effective in terms of the collection ratio and delay through simulations.

A study on penetration strategy of on-board unit taking account of the type of vehicles in V2V communications

Recent years have seen a great deal of research into Vehicle-to-Vehicle (V2V) and Vehicle-to-Infrastructure (V2I) communications, which will contribute to advances in driving support systems. In contrast to V2I communications, V2V communications offer the advantage of not requiring infrastructure, but introduction of such systems is not expected to be effective unless there is sufficient penetration by On-Board Units (OBUs). Since this factor is, in itself, an obstacle to the penetration of V2V communications, it is essential to determine what extent of penetration should be encouraged in the initial stage in order to attain effectiveness and stimulate wider usage. Therefore, we focused on the fact that even though Large Vehicles (LVs) such as large commercial freight carriers are large and tall, and are obstacles blocking V2V communications from the viewpoint of Small Vehicles (SVs), antennas on LVs could also be used to relay communications, thereby facilitating the success of such systems. This paper presents an experiment simulating a traffic environment consisting of mixed LVs and SVs, and examined how information dissemination using V2V communication varied with changes of the percentage of vehicles equipped with OBUs. This simulation experiment revealed that prioritizing LVs for OBU installation is the effective means of increasing the packet reception ratio (PRR), and clarified some guidelines for policies that could encourage wider usage.

Control of transmission power and carrier sense threshold to enhance throughput and fairness for dense WLANs

Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) adopted in IEEE 802.11 WLAN suffers from serious problems of hidden terminal and exposed terminal. This will get worse if the environment becomes highly dense. The rise of channel interference among stations (STA) and the deterioration of the total network throughput as the result of these terminal problems need to be mitigated. A possible solution is to enable STAs to control transmission power and carrier sense threshold based on the information retrieved from the access point (AP). However, broadcasting message frames from AP to all STAs for identification purpose will cause much overhead. The proposed control scheme applies Bloom filter to distinguish associated STAs within the same basic service set (BSS) from the others while minimizing the overhead. The aim of this proposed scheme is to enhance the total throughput of the network and improve fairness of each STA in dense WLAN.

Side Trip Multipath Routing Protocol for Spatial Node Failure in Data-Gathering WSNs

In severe disaster environments, data-gathering wireless sensor networks possibly suffer from spatial node failure due to external factors such as land slide and river overflow. In order to enhance tolerance to spatial node failure, this paper proposes not only a multipath data gathering protocol called “Side Trip (ST)” scheme but also an improved version of ST scheme called “Side Trip with Network Coding (STNC)” scheme. In the ST scheme, a node establishes a secondary path which passes through nodes almost the same hop count away from the sink in order to spatially separate the path from the primary path and enhance spatial node-failure tolerance. In the STNC scheme, redundant packets from different source nodes are network-coded into an encoded packet in order to reduce the number of redundant packets and the impact on the system lifetime. We compare the ST scheme, STNC scheme, and two existing multipath protocols (i.e., H-SPREAD (Hybrid-Secure Protocol for REliable dAta Delivery) scheme and SMRP (Subbranch Multipath Routing Protocol) scheme) by means of a self-developed simulator. Simulation results show that STNC scheme and ST scheme enhance tolerance to node-failure compared to H-SPREAD scheme and SMRP scheme with no significant impact on system lifetime.