Artificial neural networks (ANNs) constitute a class of flexible nonlinear models designed to mimic biological neural systems. In this entry, we introduce ANN using familiar econometric terminology and provide an overview of ANN modeling approach and its implementation methods. † Correspondence: Chung-Ming Kuan, Institute of Economics, Academia Sinica, 128 Academia Road, Sec. 2, Taipei 115, Taiwan; ckuan@econ.sinica.edu.tw. †† I would like to express my sincere gratitude to the editor, Professor Steven Durlauf, for his patience and constructive comments on early drafts of this entry. I also thank Shih-Hsun Hsu and Yu-Lieh Huang for very helpful suggestions. The remaining errors are all mine.

Artificial neural networks

Digital processing of speech signals

Wireless sensor network (WSN) has emerged as one of the most promising technologies for the future. This has been enabled by advances in technology and availability of small, inexpensive, and smart sensors resulting in cost effective and easily deployable WSNs. However, researchers must address a variety of challenges to facilitate the widespread deployment of WSN technology in real-world domains. In this survey, we give an overview of wireless sensor networks and their application domains including the challenges that should be addressed in order to push the technology further. Then we review the recent technologies and testbeds for WSNs. Finally, we identify several open research issues that need to be investigated in future.

Our survey is different from existing surveys in that we focus on recent developments in wireless sensor network technologies. We review the leading research projects, standards and technologies, and platforms. Moreover, we highlight a recent phenomenon in WSN research that is to explore synergy between sensor networks and other technologies and explain how this can help sensor networks achieve their full potential. This paper intends to help new researchers entering the domain of WSNs by providing a comprehensive survey on recent developments.

Wireless sensor networks: a survey on recent developments and potential synergies

Space-air-ground integrated network (SAGIN), as an integration of satellite systems, aerial networks, and terrestrial communications, has been becoming an emerging architecture and attracted intensive research interest during the past years. Besides bringing significant benefits for various practical services and applications, SAGIN is also facing many unprecedented challenges due to its specific characteristics, such as heterogeneity, self-organization, and time-variability. Compared to traditional ground or satellite networks, SAGIN is affected by the limited and unbalanced network resources in all three network segments, so that it is difficult to obtain the best performances for traffic delivery. Therefore, the system integration, protocol optimization, resource management, and allocation in SAGIN is of great significance. To the best of our knowledge, we are the first to present the state-of-the-art of the SAGIN since existing survey papers focused on either only one single network segment in space or air, or the integration of space-ground, neglecting the integration of all the three network segments. In light of this, we present in this paper a comprehensive review of recent research works concerning SAGIN from network design and resource allocation to performance analysis and optimization. After discussing several existing network architectures, we also point out some technology challenges and future directions.

Space-Air-Ground Integrated Network: A Survey

Consider a discrete source producing a sequence of message letters from a finite alphabet. A single-letter distortion measure is given by a non-negative matrix (d ij ). The entryd ij measures the ?cost? or ?distortion? if letteriis reproduced at the receiver as letterj. The average distortion of a communications system (source-coder-noisy channel-decoder) is taken to bed= ? i.j P ij d ij whereP ij is the probability ofibeing reproduced asj. It is shown that there is a functionR(d) that measures the ?equivalent rate? of the source for a given level of distortion. For coding purposes where a leveldof distortion can be tolerated, the source acts like one with information rateR(d). Methods are given for calculatingR(d), and various properties discussed. Finally, generalizations to ergodic sources, to continuous sources, and to distortion measures involving blocks of letters are developed. In this paper a study is made of the problem of coding a discrete source of information, given afidelity criterionor ameasure of the distortionof the final recovered message at the receiving point relative to the actual transmitted message. In a particular case there might be a certain tolerable level of distortion as determined by this measure. It is desired to so encode the information that the maximum possible signaling rate is obtained without exceeding the tolerable distortion level. This work is an expansion and detailed elaboration of ideas presented earlier [1], with particular reference to the discrete case. We shall show that for a wide class of distortion measures and discrete sources of information there exists a functionR(d) (depending on the particular distortion measure and source) which measures, in a sense, the equivalent rateRof the source (in bits per letter produced) whendis the allowed distortion level. Methods will be given for evaluatingR(d) explicitly in certain simple cases and for evaluatingR(d) by a limiting process in more complex cases. The basic results are roughly that it is impossible to signal at a rate faster thanC/R(d) (source letters per second) over a memoryless channel of capacityC(bits per second) with a distortion measure less than or equal tod. On the other hand, by sufficiently long block codes it is possible to approach as closely as desired the rateC/R(d) with distortion leveld. Finally, some particular examples, using error probability per letter of message and other simple distortion measures, are worked out in detail.

Coding Theorems for a Discrete Source With a Fidelity CriterionInstitute of Radio Engineers, International Convention Record, vol. 7, 1959.

The advances in satellite networking and material science enabled the deployment of low cost and low power small sized sensors that can transmit information through satellite channel to remotely located host. Earth stations represent sink nodes of the sensor network and often, for safety reasons, they are rebounded. In this environment, controlled information distribution methods may play a crucial role and represent a sensible field of investigation. In particular, distribution methods proposed in the paper are aimed at guaranteeing the lower energy consumption and the lower level of congestion in the network. The paper includes: the presentation of the application environment composed of sensor networks and sink Earth stations; the introduction of the sinks management functions and the description of the flooding-based techniques used to propagate information through the network; an introductive comparison of the performance in terms of energy consumption and total time spent in the sensor network

Comparative Performance Evaluation for Information Distribution Methods in Satellite-based Sensor Networks

Resource allocation schemes dedicated to satellite environment often considers Internet traffic as the superposition of traffic sources without distinguishing between TCP and UDP flows, even if TCP and UDP imply very different traffic characteristics. The basic idea of this work is that a resource allocation algorithm where the allocation function is conscious of the difference may result more efficient and fair. To reach the aim, the paper introduces: a system control architecture with three types of flows entering the network (constant bit rate (CBR), UDP and TCP) and a cost function including an analytical measure of the packet loss for TCP flows The work joins the novelties introduced with a bandwidth allocation control already in the literature (called CAP-ABASC) and derives a new allocation scheme called E-CAP-ABASC (extended-CAP-ABASC). Performance evaluation includes the comparison of the two allocation strategies mentioned above

Bandwidth Allocation in Geostationary Satellite Faded Channels for Internet Traffic

This paper focuses on countering data hiding in images. Specifically, we refer to the active warden problem, a process which aims at disrupting the covert communications that are possibly taking place within host media. Although this has been extensively studied in the literature, typical state of the art approaches treat the hidden message as a noise-like component, proposing de- noising techniques, lossy compression or noise addition as a countermeasure. In this paper we present novel approach called Lesser Components Distortion (LCD) that aims at disrupting the covert communication while minimizing distortion on the host medium. Although stemming from insights gained from notable works in the spread spectrum steganography field, we prove through extensive testing that it shows remarkable effectiveness against steganographic techniques in general, compared to common attacks.

A Novel Active Warden Technique for Image Steganography

: A telecommunication network is composed of a number of nodes. Each node can be either a terminal host or a web cache location. Objects are downloaded throughout the network among the nodes. An object can represent either a file or any other resource to be shared (e.g. machine time). For memory saving and for safety reasons, each object is composed of a number of portions and not all the portions are located within the same node, because no node should have the complete knowledge of each object. It means that a single node can have only a part of the file. It is strongly recommendable because if a single node should be either accessed without authorization, information retrieved is not sufficient to detect the overall content. Concerning networking viewpoint the following main issues will characterize the performance of object exchange: Position of the information, Strategy to reach the information, Algorithm to download information, Capacity Planning. The paper proposes a control architecture that consider the mentioned issues. It is composed of three layers: Local, Network and Planning Controller. The Local Controller acts locally to each node at object downloading time scale (seconds/minutes) and, after an "advanced flooding" signaling query to get informed about the object portions' position, decides from which node (or nodes) each portion needs to be downloaded; the Network Controller may change the distribution of the object portions among the nodes, acts with larger time scale (hours/day) and it is centralized; the Planning Controller may change the dimension of each single portion and increase/decrease the physical link and node capacities. The order of magnitude of its intervention is weeks/months. The problem is modeled through a mathematical formulation and a specific cost function, which takes into account all the necessary details is introduced for each controller as well as a minimization procedure.

Resource Allocation over a GRID Military Network

This paper considers video transmissions through Smartphones in scenarios of emergency networks where satellite links are employed. Unfortunately, satellite channels may be affected by attenuation and fading, causing significant and frequent variations of the link quality. For this reason, static compression, coding and resource allocation are not optimal solutions to guarantee a satisfactory level of Quality of Experience: we propose to apply two techniques: i) a dynamic transmission rate allocation able to assign resources to the links according to their conditions; ii) an application layer joint coder for video transmission capable of adaptively compressing and protecting the transmitted video frames so recovering losses. The performance investigation of the proposal has been carried out through emulation and the obtained results are satisfactory and open the doors to future development of the proposal.

Igor Bisio

Papers

Comparative Performance Evaluation for Information Distribution Methods in Satellite-based Sensor Networks

Bandwidth Allocation in Geostationary Satellite Faded Channels for Internet Traffic

A Novel Active Warden Technique for Image Steganography

Resource Allocation over a GRID Military Network

Cooperative Application Layer Joint Coding and rate allocation techniques for video transmissions over satellite channels through smartphones