13442653
Establishing virtual enterprises by means of mobile agents
Electronic commerce is expanding from the simple notion of electronic stores to the notion of virtual enterprises (VE), where existing enterprises dynamically form temporary alliances, joining their businesses in order to share their costs, skills and resources in supporting certain activities. Two fundamental problems in VE are: (1) how a VE is established, and (2) how information is shared among the VE participants in a controlled and efficient manner. Currently, existing enterprises are using workflows to automate their operations by integrating their information systems and human resources. Thus, in this paper, we view the establishment of a VE as a problem of dynamically expanding and integrating workflows in decentralized, autonomous and interacting workflow management systems. The focus is on the idea of mobile agents called "adlets" and their use in establishing VEs that involves advertising, negotiating and exchanging control information and data as well as its management.
30095382
Performance Analysis of Generalized Selective Packet Discarding Schemes
Selective packet discarding schemes have been mostly developed and analyzed in the context of ATM networks. However, similar schemes could be applied to non-ATM or more general networks. The analysis of selective packet discarding schemes in a general packet switching environment has been limited to the basic dropping schemes, i.e., Partial Packet Discard (PPD) and Early Packet Discard (EPD), and for only the single traffic source case. In this paper, we extend the mathematical analysis of PPD and EPD for multiple sources and include simulation results to verify the analytical results. Finally, to place the performance of the generalized EPD and PPD in the context of other packet dropping schemes, we also provide simulation results of the Drop from Front (DfF), and preemptive Partial Packet Discard (pPPD) schemes.
14976055
A new token passing distributed mutual exclusion algorithm
Eliminating interference between concurrently executing activities through mutual exclusion is one of the most fundamental problems in computer systems. The problem of mutual exclusion in a distributed system is especially interesting owing to the lack of global knowledge in the presence of variable communication delays. In this paper, a new token-based distributed mutual exclusion algorithm is proposed. The algorithm incurs approximately three messages at high loads, irrespective of the number of nodes N in the system. At low loads, it requires approximately N messages. The paper also addresses failure recovery issues, such as token loss.
25726525
A survey of TCP-friendly router-based AQM schemes
Although the majority of the traffic over the Internet is still TCP-based, applications such as voice over IP and video-conferencing are changing this very rapidly. While TCP-based applications react to network congestion, UDP-based streaming applications usually do not have any type of flow and congestion control mechanisms. UDP, the transport layer protocol used by audio and video streaming applications, does not react to network congestion thus stealing bandwidth from the responsive TCP-based connections. Several solutions have been suggested to combat this TCP-friendliness problem, mostly applied either in the end systems (end-to-end) at the transport layer of the protocol stack or inside the network (routers) at the network layer. This article surveys the state-of-the-art in router-based mechanisms to address the TCP-friendliness problem and presents a description of the most important algorithms, design issues, advantages and disadvantages, and a performance evaluation. The article also describes ways to estimate the number of active flows traversing a core router and points to further sources on this subject, which is widely used by many mechanisms, including several described in this survey.
40446138
A distributed hierarchical management framework for heterogeneous WANs
The scope of network management is expanding in multiple dimensions. Local area networks (LANs) have more nodes than ever before, enterprise networks span national boundaries, wide area networks (WANs) cover the globe, and administrators want to manage their LANs all the way down to a PC’s network interface and application software. In order to centrally manage these networks, the network manager faces the complexity of heterogeneous management tools and the difficulty of managing vast amount of data generated by network elements.
12875295
A comparison of multicast feedback control mechanisms
In reliable multicast applications, packet loss needs to be reported by having group members send feedback messages. This results in the well-known feedback implosion problem. The available feedback control mechanisms can be classified as timer-based, hierarchy-based and router-assisted, among which the timer-based approach is more preferable due to its simplicity and flexibility. This paper compares the performance of a set of multicast protocols that use either the traditional timer-based feedback control or a combination of the traditional timers and some representatives in the feedback control. We investigate how the use of representatives and deterministic timers improves feedback control performance.
10850100
Low-coordination topologies for redundancy in sensor networks
Tiny, low-cost sensor devices are expected to be failure-prone and hence in many realistic deployment scenarios for sensor networks these nodes are deployed in higher than necessary densities to meet operational goals. In this paper we address the question of how nodes should be managed in such dense sensor deployments so that the network topology formed by the active sensors is able to provide connected-coverage to the entire area of interest and at the same time increase the lifetime of the network. In particular, we propose and study distributed, low-coordination node wakeup schemes to efficiently construct multiple independent (node-disjoint) sensor network topologies to achieve good fault tolerance. We propose and evaluate different distributed, random and pattern-based wakeup policies for sensor nodes to construct connected-covered topologies. Through analysis and simulations we demonstrate that in dense sensor deployment scenarios, these policies can construct near-optimal topologies (within 2.7% of the optimal) with zero coordination between nodes, as long as location information is available at the individual sensor nodes.Based on these observations, we develop and evaluate a few simple distributed, wakeup based topology construction algorithms that can realize similar performance bounds in realistic sensor deployments, with varying node densities. These algorithms differ in terms of the required level of coordination and the use of sensor location information, and generate connected-covered topologies efficiently, with very low message-exchange overhead.
16760269
Implementing XML Schema inside a relational database
XML Schema has emerged as a promising data model that unites structured and unstructured content. The Oracle database has led the commercial database community in integrating support for XML Schema inside an enterprise data server. The foundation for this was laid with the absorption of the SQL:1999 'objectrelational' type system in the database, which provided the necessary hierarchical abstractions necessary for representing XML. We look at how XML Schemas have been implemented in Oracle XML DB, what optimizations are available to cover the diverse use-cases for schema-based XML storage and retrieval, and how this technology contributes to richer data management.
8561751
Partially overlapped channels not considered harmful
Many wireless channels in different technologies are known to have partial overlap. However, due to the interference effects among such partially overlapped channels, their simultaneous use has typically been avoided. In this paper, we present a first attempt to model partial overlap between channels in a systematic manner. Through the model, we illustrate that the use of partially overlapped channels is not always harmful. In fact, a careful use of some partially overlapped channels can often lead to significant improvements in spectrum utilization and application performance. We demonstrate this through analysis as well as through detailed application-level and MAC-level measurements. Additionally, we illustrate the benefits of our developed model by using it to directly enhance the performance of two previously proposed channel assignment algorithms --- one in the context of wireless LANs and the other in the context of multi-hop wireless mesh networks. Through detailed simulations, we show that use of partially overlapped channels in both these cases can improve end-to-end application throughput by factors between 1.6 and 2.7 in different scenarios, depending on wireless node density. We conclude by observing that the notion of partial overlap can be the right model of flexibility to design efficient channel access mechanisms in the emerging software radio platforms.
2626768
MAR: a commuter router infrastructure for the mobile Internet
We introduce MAR, a commuter mobile access router infrastructure that exploits wireless diversity (e.g. channel diversity, network diversity, and technology diversity) to provide improved data performance for wireless data users. Our system design stems from the observation that rather than choosing a single wireless service provider (e.g. Sprint, AT&T, BT, Vodafone), a single technology (e.g. GPRS, UMTS, CDMA, 802.11), or a single wireless channel, users can obtain significant benefits by using the multiplicity of choices available. MAR is a wireless multi-homed device that can be placed in moving vehicles (e.g. car, bus, train) to enable high-speed data access. MAR dynamically instantiates new channels based on traffic demand, aggregates the bandwidth and dynamically shifts load from poor quality to better quality channels. MAR, thus, provides a faster, more stable, and reliable communication channel to mobile users.We have implemented and tested the MAR system in our testbed which spans the networks of three different cellular providers. Through our experiments we have performed a detailed evaluation to quantify the benefits of MAR for different protocols and applications. For example, even in highly mobile environments, MAR, on average, improves the end-user experience of web-browsing and streaming applications by a factor of 2.8 and 4.4 respectively. Our results show that significant benefits can be obtained by exploiting the diversity in coverage offered by many cellular operators, different technology networks (e.g. GPRS, CDMA), and diverse wireless channels.
2759775
Distributed channel management in uncoordinated wireless environments
Wireless 802.11 hotspots have grown in an uncoordinated fashion with highly variable deployment densities. Such uncoordinated deployments, coupled with the difficulty of implementing coordination protocols, has often led to conflicting configurations (e.g., in choice of transmission power and channel of operation) among the corresponding Access Points (APs). Overall, such conflicts cause both unpredictable network performance and unfairness among clients of neighboring hotspots. In this paper, we focus on the fairness problem for uncoordinated deployments. We study this problem from the channel assignment perspective. Our solution is based on the notion of channel-hopping, and meets all the important design considerations for control methods in uncoordinated deployments - distributed in nature, minimal to zero coordination among APs belonging to different hotspots, simple to implement, and interoperable with existing standards. In particular, we propose a specific algorithm called MAXchop, which works efficiently when using only non-overlapping wireless channels, but is particularly effective in exploiting partially-overlapped channels that have been proposed in recent literature. We also evaluate how our channel assignment approach complements previously proposed carrier sensing techniques in providing further performance improvements. Through extensive simulations on real hotspot topologies and evaluation of a full implementation of this technique, we demonstrate the efficacy of these techniques for not only fairness, but also the aggregate throughput, metrics.We believe that this is the first work that brings into focus the fairness properties of channel hopping techniques and we hope that the insights from this research will be applied to other domains where a fair division of a system's resources is an important consideration.