Mobile Access Points Research Articles

A Time-Varying Opportunistic Approach to Lifetime Maximization of Wireless Sensor Networks

In this paper, we examine the advantages of transmission scheduling by medium access control (MAC) protocols for energy-limited wireless sensor networks (WSN) as a means of maximizing network lifetime. We consider transmission scheduling for sensor networks with a mobile access point, where each sensor transmits its measurement directly to an access point through a fading channel. WSN lifetime maximization depends almost exclusively on the channel-state information (CSI) and the residual-energy information (REI) of each sensor in the network. We discuss distributed protocols which exploit local CSI and REI. We present a novel protocol for distributed transmission scheduling, dubbed the time-varying opportunistic protocol (TOP), for maximizing the network lifetime. TOP prioritizes sensors with better channels when the network is young, by exploiting local CSI to reduce transmission energy. However, TOP prefers sensors with higher residual energy when the network is old by exploiting local REI to reduce the wasted energy. We show that the relative performance loss of TOP compared to the optimal centralized protocol in terms of network lifetime decreases as the initial energy stored in the sensors increases. Furthermore, TOP significantly simplifies the implementation of carrier sensing compared to other distributed MAC protocols. We also explore the case of large-scale wireless sensor networks, where the activated sensors are picked randomly and modify the implementation of TOP for such networks. Simulation results show that TOP outperforms other distributed MAC protocols that have been proposed recently.

System-of-Systems Modeling and Simulation of a Ship Environment With Wireless and Intelligent Maintenance Technologies

Modeling and simulation environments are needed to support decision making in Navy Warfighters, which are emergent systems that pose a challenge to operations management. Ships consist of complex interconnected systems such as the infrastructure, crew, and workflow. A system-of-systems approach using agent-based modeling is applied here to develop workflow simulations involving a ship's crew conducting routine maintenance, watch duty, and reporting functions. Simple models are used to describe basic behavioral traits and intelligence in crew members; machinery including sensors for intelligent maintenance; equipment consuming power; mobile and stationary communication network access points; models for data transfer over the network; crew mobility models; power distribution and trimming models for the electrical system; and a fire model to simulate emergency scenarios. The simulation results demonstrate an increase in machine availability due to the implementation of intelligent maintenance systems. The effects of wireless-network usage on crew resource utilization and overall ship capability in normal operational scenarios are also demonstrated. A simple rescheduling algorithm is used to improve crew utilization and estimate manning requirements. The effects of emergency scenarios such as fires in different locations are also studied. Sensitivity analysis is presented to verify the developed model, and a note on validation is given.

シナリオベースドデザインを支援する Mobile AP II システム開発

シナリオベースドデザインを支援する Mobile AP II システム開発

Dynamic load balancing through association control of mobile users in WiFi networks

With the development of IEEE 802.11 MAC (medium access control) protocols, efficient utilization of wireless local area networks (WLANs) has become a very important issue. In typical IEEE 802.11 networks, the association between mobile users (MUs) and access point (AP) is based on the signal strength information. As a result, it often results in the extremely unfair bandwidth allocation among MUs. In this paper, we propose a distributed association algorithm to achieve load balancing among the APs. The proposed algorithm gradually balances the AP loads for the available multiple bit rate choices in a distributed manner. We analyze the stability and overhead of the proposed algorithm, and show the improvement of the fairness via computer simulation. Additionally, we have implemented a prototype on a testbed to prove its feasibility.

Information Retrieval and Processing in Sensor Networks: Deterministic Scheduling Versus Random Access

We investigate the effect of medium-access control (MAC) used in information retrieval by a mobile access point (AP) on information processing in large-scale sensor network, where sensors are unreliable and subject to outage. We focus on a 1-D sensor network and assume that the location information is available locally at each sensor and unavailable to the AP. For a fixed collection interval, two types of MAC schemes are considered: the deterministic scheduling, which collects data from predetermined sensors locations, and random access, which collects data from random locations. We compare the signal estimation performance of the two MACs, using the expected maximum distortion as the performance measure. For large sensor networks with fixed density, we show that there is a critical threshold on the sensor outage probability P <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">out</sub> -For _P <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">out</sub> < e <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-lambda(1+o(1))</sup> , where lambda is the throughput of the random access protocol, the deterministic scheduling provides better reconstruction performance. However, for P <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">out</sub> > e <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-lambda(1+o(1))</sup> , the performance degradation from missing data samples due to sensor outage does not justify the effort of scheduling; simple random access outperforms the optimal scheduling.

On the Error Exponent and the Use of LDPC Codes for Cooperative Sensor Networks With Misinformed Nodes

The problem of retrieving information by a mobile access point from a sensor network where sensors cooperatively transmit messages using a common codebook is considered. It is assumed that there is a probability that a sensor is misinformed with a wrong message, which complicates the information retrieval process. The access point uses the capacity achieving stay- <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</i> scheduler that schedules a sensor to transmit for <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</i> consecutive code-letters before switching to a new sensor. The random coding exponent is derived as a function of <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</i> , and it is shown that there is an optimal <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</i> that gives the largest error exponent. The application of low-definition parity-check (LDPC) codes is considered next. It is shown in simulations that the optimal <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</i> of the stay- <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</i> scheduler for LDPC codes can be inferred from that for the random coding exponent.

Sensor Networks With Mobile Access: Energy and Capacity Considerations

Sensor network with mobile access (SENMA) is an architecture in which randomly deployed low-power sensors are orchestrated by a few powerful mobile access points. This paper considers SENMA from energy-efficiency and information-theoretic perspectives. By allowing sensors to propagate data directly to mobile access points over multiaccess channels and relieving sensors from energy-consuming network functions, SENMA has the potential of offering orders of magnitude of improvement in energy efficiency over the multihop ad hoc architecture, as demonstrated by our analysis on scalability. Optimization configurations of SENMA such as the altitude, the trajectory, and the coverage of access points are considered next, using the sum-rate as the performance metric. Optimal strategies for single and multiple access points are determined. For multiple access points, the possibility of and the gain due to cooperation (i.e., joint decoding of signals received at different access points) are investigated.

Periodic properties of user mobility and access-point popularity

Understanding user mobility and its effect on access points (APs) is important in designing location-aware systems and wireless networks. Although various studies of wireless networks have provided useful insights, it is hard to apply them to other situations. Here we present a general methodology for extracting mobility information from wireless network traces, and for classifying mobile users and APs. We used the Fourier transform to reveal important periods and chose the two strongest periods to serve as parameters to a classification system based on Bayes' theory. Analysis of 1-month traces shows that while a daily pattern is common among both users and APs, a weekly pattern is common only for APs. Analysis of 1-year traces revealed that both user mobility and AP popularity depend on the academic calendar. By plotting the classes of APs on our campus map, we discovered that their periodic behavior depends on their proximity to other APs.

A survey of current architectures for connecting wireless mobile ad hoc networks to the Internet

AbstractConnecting wired and wireless networks, and particularly mobile wireless ad hoc networks (MANETs) and the global Internet, is attractive in real‐world scenarios due to its usefulness and praticality. Because of the various architectural mismatches between the Internet and MANETs with regard to their communication topology, routing protocols, and operation, it is necessary to introduce a hybrid interface capable of connecting to the Internet using Mobile IP protocol and to MANETs owing to an ad hoc routing protocol. Specifically, the approaches available in the literature have introduced updated versions of Mobile IP agents or access points at the edge of the Internet to help MANET nodes get multi‐hop wireless Internet access. The main differences in the existing approaches concern the type of ad hoc routing protocol as well as the switching algorithm used by MANET nodes to change their current Mobile IP agents based on specific switching criteria. This paper surveys a variety of approaches to providing multi‐hop wireless Internet access to MANET nodes. Copyright © 2006 John Wiley &amp; Sons, Ltd.

A mutual authentication and privacy mechanism for WLAN security

AbstractIEEE 802.11 wireless local area networks (WLAN) has been increasingly deployed in various locations because of the convenience of wireless communication and decreasing costs of the underlying technology. However, the existing security mechanisms in wireless communication are vulnerable to be attacked and seriously threat the data authentication and confidentiality. In this paper, we mainly focus on two issues. First, the vulnerabilities of security protocols specified in IEEE 802.11 and 802.1X standards are analyzed in detail. Second, a new mutual authentication and privacy scheme for WLAN is proposed to address these security issues. The proposed scheme improves the security mechanisms of IEEE 802.11 and 802.1X by providing a mandatory mutual authentication mechanism between mobile station and access point (AP) based on public key infrastructure (PKI), offering data integrity check and improving data confidentiality with symmetric cipher block chain (CBC) encryption. In addition, this scheme also provides some other new security mechanisms, such as dynamic session key negotiation and multicast key notification. Hence, with these new security mechanisms, it should be much more secure than the original security scheme. Copyright © 2006 John Wiley &amp; Sons, Ltd.

Estimation of the number of operating sensors in large-scale sensor networks with mobile access

This paper investigates the estimation of the number of operating sensors in a sensor network in which the data collection is made by a mobile access point. In this paper, an estimator based on the Good-Turing estimator of the missing mass is proposed and it is generalized to other related problems such as the estimation of the distribution of energy available at sensors. The estimator is analyzed using the theory of large deviations. Closed-form bounds on the large deviation exponent are presented and confidence intervals for the estimator are characterized.

Context-aware handover using active network technology

Context-aware computing can play a major role to improve the services of mobile networking systems. In this paper, we focus on optimizing handover decisions in heterogeneous environments, where the user has a choice among different mobile networks and access points. In our approach, the decision is not only based on the signal quality, but also on the knowledge about the context of mobile devices and networks. Since context information and context processing evolves fast, we propose a flexible, integrated approach for context management, which can adapt in several ways. Our architecture encompasses programmable platforms and distributed context management components in network nodes and mobile devices, as well as a service deployment scheme for network services. This flexible architecture is able to actively deploy different handover services. It manages dynamic context information and allows mobile devices to be always connected to the most suitable access network. Our architecture is validated in a prototype implementation and performance results are discussed.

Cooperative Sensor Networks With Misinformed Nodes

The communication capacity of Cooperative SEnsor Networks with Mobile Access (C-SENMA) is considered when some sensors may be misinformed with erroneous messages. It is assumed that a global message is first distributed to all the nodes, each node receiving the message correctly with probability 1-/spl beta/. The nodes cooperate in delivering the global message to the mobile access point. Three system configurations are discussed based on whether a polling channel and/or an energy constraint are present. The first type is C-SENMA with Polling with No Energy constraints (PNE), where the mobile access point has the ability to poll individual sensors. Without energy constraints, each sensor can transmit for an unlimited number of times. The second type is C-SENMA with No Polling with No Energy constraints (NPNE), where adaptive polling is not allowed and sensors have to transmit according to a predetermined schedule. The third type is C-SENMA with No Polling with an Energy constraint (NPE), where each node has a limit on the number of transmissions. The capacities of the three system configurations are analyzed. It is shown that, the capacity for C-SENMA PNE is the same as that when there are no misinformed sensors. For C-SENMA NPNE, with the absence of the polling channel, there is a loss on the achievable rate, proportional to /spl beta/, the probability that a sensor is misinformed. Results are extended to multiple simultaneous transmissions with the presence of channel fading. The optimal number of simultaneous transmissions is investigated under three different fading situations.

Sensitivity and Coding of Opportunistic ALOHA in Sensor Networks with Mobile Access

We consider a distributed medium access protocol, Opportunistic ALOHA, for reachback in sensor networks with mobile access points (AP). We briefly discuss some properties of the protocol, like throughput and transmission control for an orthogonal CDMA physical layer. We then consider the incorporation of necessary side information like location into the transmission control and numerically demonstrate the loss in throughput in the absence of such information. Through simulations, we discuss the robustness and sensitivity of the protocol under various modeling errors and propose strategies to allow for errors in estimation of some parameters without reduction in the throughput. For networks, where the sensors are allowed to collaborate, we consider three coding schemes for reliable transmission: spreading code independent, spreading code dependent transmission and coding across sensors. These schemes are compared in terms of achievable rates and random coding error exponents. The coding across sensors scheme has comparable achievable rates to the spreading code dependent scheme, but requires the additional transmission of sensor ID. However, the scheme does not require the mobile AP to send data through the beacon unlike the other two schemes. The use of these coding schemes to overcome sensitivity is demonstrated through simulations.

Energy efficiency of large-scale wireless networks: proactive versus reactive networking

An analytical approach to the characterization of energy consumption of large-scale wireless networks is presented. The radio model includes energy consumption of nodes at various operating states. We analyze the total energy consumption of the proactive and the reactive networking strategies, taking into account transmitting, listening, and sleeping energy. Scaling laws with respect to the increase of node density and geographical size are derived. Energy efficiency and overhead at the physical and the network layers are evaluated against message duty cycle, channel fading rate, and node mobility. The crossover point in message duty cycle below which reactive network has assured advantages is obtained. The analysis is then applied to large-scale sensor networks for applications involving data-centric and location-centric queries. The ad hoc sensor network architecture is compared with sensor networks with mobile access points.

Estimating sensor population via probabilistic sequential polling

A probabilistic sequential polling protocol (PSPP) is presented for the estimation of the sensor population in a large-scale sensor network with a mobile access point. It is shown that PSPP requires O(log/sub 2/N) sensor transmissions and a total of O((log/sub 2/N)/sup 2/) polls to achieve an arbitrarily predetermined level of accuracy.

Opportunistic Carrier Sensing for Energy-Efficient Information Retrieval in Sensor Networks

We consider distributed information retrieval for sensor networks with cluster heads or mobile access points. The performance metric used in the design is energy efficiency defined as the ratio of the average number of bits reliably retrieved by the access point to the total amount of energy consumed. A distributed opportunistic transmission protocol is proposed using a combination of carrier sensing and backoff strategy that incorporates channel state information (CSI) of individual sensors. By selecting a set of sensors with the best channel states to transmit, the proposed protocol achieves the upper bound on energy efficiency when the signal propagation delay is negligible. For networks with substantial propagation delays, a backoff function optimized for energy efficiency is proposed. The design of this backoff function utilizes properties of extreme statistics and is shown to have mild performance loss in practical scenarios. We also demonstrate that opportunistic strategies that use CSI may not be optimal when channel acquisition at individual sensors consumes substantial energy. We show further that there is an optimal sensor density for which the opportunistic information retrieval is the most energy efficient. This observation leads to the design of the optimal sensor duty cycle.

Shepherd: A lightweight statistical authentication protocol for access control in wireless LANs

With the increasing performance and dropping price of wireless networking equipment, wireless networking has revolutionized the way people work and play. Wi-Fi hot spots popping up all over the country provide a convenient means of Internet connectivity. For the ISPs of hot spots, authentication and accounting have been recognized as two crucial concerns. For authentication, IETF PANA 1 1 PANA stands for Protocol for Carrying Authentication for Network Access. WG, is working on a transport protocol for authenticating IP hosts for network access. However, PANA does not provide access control and per-packet authentication, which are desirable in accounting and access control. Instead of using high-overhead crypto-based mechanisms, such as IPSec or 802.11i, we propose a lightweight statistical authentication protocol, namely Shepherd, through which the access point (AP) in a wireless network authenticates the mobile nodes as a Shepherd discriminates among alike sheep according to their characteristics. In Shepherd, the legitimacy of a mobile node is determined by continuously checking a series of random authentication bits, where each bit in this stream is piggybacked to a packet. Such an authentication bit stream is generated by both the mobile node and access point using the same random number generator under the same shared seed as a key. We analyze this protocol under three synchronization schemes. Our analytical results show that this protocol performs well in terms of computational and communication cost, synchronization efficiency, and protocol operation secrecy. We also show that this new protocol is practical for implementation in wireless LANs.

Low-power system-on-chip architecture for wireless LANs

The authors present the architecture of a low-power system-on-chip (SoC) that implements baseband processing as well as the medium access control and data link control functionalities of a 5 GHz wireless system. The design is based on the HIPERLAN/2 wireless LAN standard, but it also covers critical processing requirements of the IEEE 802.11a standard. The options, constraints and motivations for the taken design decisions are presented, and the followed design steps, starting from the system specifications up to the architecture definition and the system implementation, are explained. The system's functionality covers both mobile terminal and access point devices. A critical design task in such systems is the assignment of the target system's tasks on the different types of processing elements available. Processor cores, dedicated hardware as well as memory elements and advanced bus architectures are used in order to achieve the target implementation. The architecture is targeted for a low-power SoC platform, due to the fact that power consumption is a critical parameter in electronic portable system design where excess power dissipation can lead to expensive and less reliable systems. A system prototype has been developed on a FPGA-based platform (including microprocessor modules). This FPGA-based prototype is currently being migrated to a SoC, which requires that the treatment of important issues such as clock handling, synthesis, testability and debugging is addressed.

Wireless incentive engineering

The successful operation of emerging public wireless local-area networks requires flexible network mechanisms that can support differentiated or tiered services for a variety of applications such as bursty transactional Web applications, as well as reservation demanding voice and video applications. Congestion pricing has been proposed as one promising solution to traffic control because it accurately models the cost that each user's traffic imposes on network congestion points. The underlying assumption of congestion pricing is that users will respond to a monetary-based price signal to maximize their own utility. We observe, however, that monetary service charge is largely a business concern and not a traffic control concern because operators and users prefer simple, predictable, and stable service charges such as flat-rate or block-rate charging. Therefore, we argue, that traffic control techniques such as congestion pricing should be decoupled from monetary service charges. Under such a new regime, however, a user's response to a nonmonetary price signal would be quite different from utility maximization, and without the appropriate incentives for users to cooperate such a regime would lead to the "tragedy of the commons" phenomenon, undermining any future differentiated service offerings by wireless Internet service providers. To address this problem, we propose wireless incentive engineering mechanisms for mobile devices (MDs) and access points (APs) that provide incentives for mobile users to cooperatively use different classes of services without relying on monetary service charging. Wireless incentive engineering possesses a number of beneficial properties including minimizing the algorithmic and protocol overhead on MDs and APs, Nash bargaining fairness, and incentive compatibility for mobile users promoting the truthful selection of service class and bandwidth declaration. We use analysis, simulation, and experimental results from a wireless testbed to demonstrate the effectiveness of wireless incentive engineering.