scispace - formally typeset
Search or ask a question

Showing papers on "Network topology published in 2001"


Proceedings ArticleDOI
22 Apr 2001
TL;DR: The per-session throughput for applications with loose delay constraints, such that the topology changes over the time-scale of packet delivery, can be increased dramatically when the nodes are mobile rather than fixed, by exploiting node mobility as a type of multiuser diversity.
Abstract: The capacity of ad-hoc wireless networks is constrained by the mutual interference of concurrent transmissions between nodes. We study a model of an ad-hoc network where n nodes communicate in random source-destination pairs. These nodes are assumed to be mobile. We examine the per-session throughput for applications with loose delay constraints, such that the topology changes over the time-scale of packet delivery. Under this assumption, the per-user throughput can increase dramatically when the nodes are mobile rather than fixed. This improvement can be achieved by exploiting node mobility as a type of multiuser diversity.

1,376 citations


Proceedings ArticleDOI
15 Aug 2001
TL;DR: The goal is to produce a topology generation framework which improves the state of the art and is based on the design principles of representativeness, inclusiveness, and interoperability.
Abstract: Effective engineering of the Internet is predicated upon a detailed understanding of issues such as the large-scale structure of its underlying physical topology, the manner in which it evolves over time, and the way in which its constituent components contribute to its overall function. Unfortunately, developing a deep understanding of these issues has proven to be a challenging task, since it in turn involves solving difficult problems such as mapping the actual topology, characterizing it, and developing models that capture its emergent behavior. Consequently, even though there are a number of topology models, it is an open question as to how representative the generated topologies they generate are of the actual Internet. Our goal is to produce a topology generation framework which improves the state of the art and is based on the design principles of representativeness, inclusiveness, and interoperability. Representativeness leads to synthetic topologies that accurately reflect many aspects of the actual Internet topology (e.g. hierarchical structure, node degree distribution, etc.). Inclusiveness combines the strengths of as many generation models as possible in a single generation tool. Interoperability provides interfaces to widely-used simulation applications such as ns and SSF and visualization tools like otter. We call such a tool a universal topology generator.

1,296 citations


Proceedings ArticleDOI
22 Apr 2001
TL;DR: This work proposes a simple distributed algorithm where each node makes local decisions about its transmission power and these local decisions collectively guarantee global connectivity and gives an approximation scheme in which the power consumption of each route can be made arbitrarily close to the optimal by carefully choosing the parameters.
Abstract: The topology of wireless multihop ad hoc networks can be controlled by varying the transmission power of each node. We propose a simple distributed algorithm where each node makes local decisions about its transmission power and these local decisions collectively guarantee global connectivity. Specifically, based on the directional information, a node grows it transmission power until it finds a neighbor node in every direction. The resulting network topology increases the network lifetime by reducing the transmission power and reduces traffic interference by having low node degrees. Moreover, we show that the routes in the multihop network are efficient in power consumption. We give an approximation scheme in which the power consumption of each route can be made arbitrarily close to the optimal by carefully choosing the parameters. Simulation results demonstrate significant performance improvements.

955 citations


Proceedings ArticleDOI
22 Apr 2001
TL;DR: This work develops several placement algorithms that use workload information, such as client latency and request rates, to make informed placement decisions, and evaluates the placement algorithms using both synthetic and real network topologies, as well as Web server traces.
Abstract: There has been an increasing deployment of content distribution networks (CDNs) that offer hosting services to Web content providers. CDNs deploy a set of servers distributed throughout the Internet and replicate provider content across these servers for better performance and availability than centralized provider servers. Existing work on CDNs has primarily focused on techniques for efficiently redirecting user requests to appropriate CDN servers to reduce request latency and balance load. However, little attention has been given to the development of placement strategies for Web server replicas to further improve CDN performance. We explore the problem of Web server replica placement in detail. We develop several placement algorithms that use workload information, such as client latency and request rates, to make informed placement decisions. We then evaluate the placement algorithms using both synthetic and real network topologies, as well as Web server traces, and show that the placement of Web replicas is crucial to CDN performance. We also address a number of practical issues when using these algorithms, such as their sensitivity to imperfect knowledge about client workload and network topology, the stability of the input data, and methods for obtaining the input.

895 citations


Proceedings ArticleDOI
27 Aug 2001
TL;DR: A 'crawler' is built to extract the topology of Gnutella's application level network, a topology graph is analyzed and the current configuration has the benefits and drawbacks of a power-law structure.
Abstract: Despite recent excitement generated by the P2P paradigm and despite surprisingly fast deployment of some P2P applications, there are few quantitative evaluations of P2P system behavior. Due to its open architecture and achieved scale, Gnutella is an interesting P2P architecture case study. Gnutella, like most other P2P applications, builds at the application level a virtual network with its own routing mechanisms. The topology of this overlay network and the routing mechanisms used have a significant influence on application properties such as performance, reliability, and scalability. We built a 'crawler' to extract the topology of Gnutella's application level network, we analyze the topology graph and evaluate generated network traffic. We find that although Gnutella is not a pure power-law network, its current configuration has the benefits and drawbacks of a power-law structure. These findings lead us to propose changes to the Gnutella protocol and implementations that bring significant performance and scalability improvements.

824 citations


Journal ArticleDOI
TL;DR: This paper presents a two-year study of Internet routing convergence through the experimental instrumentation of key portions of the Internet infrastructure, including both passive data collection and fault-injection machines at Internet exchange points, and describes several unexpected properties of convergence.
Abstract: This paper examines the latency in Internet path failure, failover, and repair due to the convergence properties of interdomain routing. Unlike circuit-switched paths which exhibit failover on the order of milliseconds, our experimental measurements show that interdomain routers in the packet-switched Internet may take tens of minutes to reach a consistent view of the network topology after a fault. These delays stem from temporary routing table fluctuations formed during the operation of the border gateway protocol (BGP) path selection process on the Internet backbone routers. During these periods of delayed convergence, we show that end-to-end Internet paths will experience intermittent loss of connectivity, as well as increased packet loss and latency. We present a two-year study of Internet routing convergence through the experimental instrumentation of key portions of the Internet infrastructure, including both passive data collection and fault-injection machines at major Internet exchange points. Based on data from the injection and measurement of several hundred thousand interdomain routing faults, we describe several unexpected properties of convergence and show that the measured upper bound on Internet interdomain routing convergence delay is an order of magnitude slower than previously thought. Our analysis also shows that the upper theoretic computational bound on the number of router states and control messages exchanged during the process of BGP convergence is factorial with respect to the number of autonomous systems in the Internet. Finally, we demonstrate that much of the observed convergence delay stems from specific router vendor implementation decisions and ambiguity in the BGP specification.

703 citations


Proceedings ArticleDOI
21 Oct 2001
TL;DR: This paper is the first description of the software architecture that supports named data and in- network processing in an operational, multi-application sensor-network and shows that approaches such as in-network aggregation and nested queries can significantly affect network traffic.
Abstract: In most distributed systems, naming of nodes for low-level communication leverages topological location (such as node addresses) and is independent of any application. In this paper, we investigate an emerging class of distributed systems where low-level communication does not rely on network topological location. Rather, low-level communication is based on attributes that are external to the network topology and relevant to the application. When combined with dense deployment of nodes, this kind of named data enables in-network processing for data aggregation, collaborative signal processing, and similar problems. These approaches are essential for emerging applications such as sensor networks where resources such as bandwidth and energy are limited. This paper is the first description of the software architecture that supports named data and in-network processing in an operational, multi-application sensor-network. We show that approaches such as in-network aggregation and nested queries can significantly affect network traffic. In one experiment aggregation reduces traffic by up to 42% and nested queries reduce loss rates by 30%. Although aggregation has been previously studied in simulation, this paper demonstrates nested queries as another form of in-network processing, and it presents the first evaluation of these approaches over an operational testbed.

677 citations


Proceedings ArticleDOI
01 Jan 2001
TL;DR: This paper presents a clustering scheme to create a hierarchical control structure for multi-hop wireless networks and presents an efficient distributed implementation of the clustering algorithm for a set of wireless nodes to create the set of desired clusters.
Abstract: In this paper we present a clustering scheme to create a hierarchical control structure for multi-hop wireless networks. A cluster is defined as a subset of vertices, whose induced graph is connected. In addition, a cluster is required to obey certain constraints that are useful for management and scalability of the hierarchy. All these constraints cannot be met simultaneously for general graphs, but we show how such a clustering can be obtained for wireless network topologies. Finally, we present an efficient distributed implementation of our clustering algorithm for a set of wireless nodes to create the set of desired clusters.

616 citations


Patent
01 May 2001
TL;DR: In this article, a new generation Fast Circuit Switch (packet/circuit) communication processors and platform which enables a new Internet Exchange Networking Processor Architecture at the edge and core of every communication system.
Abstract: A system and method for delivering increases speed, security, and intelligence to wireline and wireless systems. The present invention includes a new generation Fast Circuit Switch (packet/circuit) Communication processors and platform which enables a new Internet Exchange Networking Processor Architecture at the edge and core of every communication system, for next generation Web Operating System or Environment (WOE) to operate on with emphasis of a non-local processor or networking processor with remote web computing capabilities. A Unified Network Communication & Processor System or UniNet is a New generation network architecture of packet/circuit communication processors or Internet networking processor, that increases speeds over any communication channels and topologies, synchronizing, enabling, improving, controlling and securing all of the data transmission of web applications over existing wireline and wireless infrastructure while providing seamless integration to the legacy telecom & data corn backbone. The present invention is capable of operating on any topology with distributed intelligence and data switching/routing, which is located at the edge. This method not only alleviates the ever increasing data processing bottleneck which is currently done by the data communication and telecom switch and routers, but it also enables new and next generation Internet Processor architecture. The UniNet is also a flexible solution for the novel concept that the capability of a network interface should depend on the level of service assigned to a service access point, not the capacity of the total network, such as transaction services with a short burst of messages with short access delay. The present invention increases channel capacity by using a parallel or multi-channel structure in such wireless and wireline at the edge or the core of. This new architecture of the present invention uses parallel bitstreams in a flexible way and distributed switching/routing technique, is not only to avoid the potential bottlenet of centralized switches, but also to increase speed with intelligence that is seamlessly integrating into the Fiber Optic Backbone such as WDM and SONET of the MAN/WAN network with a Real-time guarantees, different types of traffic (such as Stringent synchronous, isochronous, and asynchronous data messages) with different demands, and privacy & security of multi access and integrated services environment.

567 citations


Journal ArticleDOI
24 Jun 2001
TL;DR: An information-theoretic constructive scheme for obtaining an achievable rate region in communication networks of arbitrary size and topology and communicating over a general vector discrete memoryless channel (DMC) is proposed.
Abstract: We study communication networks of arbitrary size and topology and communicating over a general vector discrete memoryless channel (DMC). We propose an information-theoretic constructive scheme for obtaining an achievable rate region in such networks. Many well-known capacity-defining achievable rate regions can be derived as special cases of the proposed scheme. A few such examples are the physically degraded and reversely degraded relay channels, the Gaussian multiple-access channel, and the Gaussian broadcast channel. The proposed scheme also leads to inner bounds for the multicast and allcast capacities. Applying the proposed scheme to a specific wireless network of n nodes located in a region of unit area, we show that a transport capacity of /spl Theta/(n) bit-meters per second (bit-meters/s) is feasible in a certain family of networks, as compared to the best possible transport capacity of /spl Theta/(/spl radic/n) bit-meters/s in Gupta et al. (2000), where the receiver capabilities were limited. Even though the improvement is shown for a specific class of networks, a clear implication is that designing and employing more sophisticated multiuser coding schemes can provide sizable gains in at least some large wireless networks.

512 citations


Journal ArticleDOI
TL;DR: This article addresses some of the quality of service issues for ad hoc networks which have started to receive increasing attention in the literature and focuses on QoS routing.
Abstract: Ad hoc wireless networks consist of mobile nodes interconnected by multihop communication paths. Unlike conventional wireless networks, ad hoc networks have no fixed network infrastructure or administrative support. The topology of the network changes dynamically as mobile nodes join or depart the network or radio links between nodes become unusable. This article addresses some of the quality of service issues for ad hoc networks which have started to receive increasing attention in the literature. The focus is on QoS routing. This is a complex and difficult issue because of the dynamic nature of the network topology and generally imprecise network state information. We present the basic concepts and discuss some of the results. The article concludes with some observations on the open areas for further investigation.

Proceedings ArticleDOI
Takahiro Hara1
22 Apr 2001
TL;DR: This work proposes three replica allocation methods to improve data accessibility by replicating data items on mobile hosts by taking into account the access frequency from mobile hosts to each data item and the status of the network connection.
Abstract: The advances in computer and wireless communication technologies have led to an increasing interest in ad hoc networks which are temporarily constructed by only mobile hosts. In ad hoc networks, since mobile hosts move freely, disconnections occur frequently, and this causes frequent network division. Consequently, data accessibility in ad hoc networks is lower than that in the conventional fixed networks. We propose three replica allocation methods to improve data accessibility by replicating data items on mobile hosts. In these three methods, we take into account the access frequency from mobile hosts to each data item and the status of the network connection. We also show the results of simulation experiments regarding the performance evaluation of our proposed methods.

Proceedings ArticleDOI
22 Apr 2001
TL;DR: This paper introduces the Bluetooth topology construction protocol (BTCP), an asynchronous distributed protocol for constructing scatternets which starts with nodes that have no knowledge of their surroundings and terminates with the formation of a connected network satisfying all connectivity constraints posed by the Bluetooth technology.
Abstract: Wireless ad hoc networks have been a growing area of research. While there has been considerable research on the topic of routing in such networks, the topic of topology creation has not received due attention. This is because almost all ad hoc networks to date have been built on top of a single channel, broadcast based wireless media, such as 802.11 or IR LANs. For such networks the distance relationship between the nodes implicitly (and uniquely) determines the topology of the ad hoc network. Bluetooth is a promising new wireless technology, which enables portable devices to form short-range wireless ad hoc networks and is based on a frequency hopping physical layer. This fact implies that hosts are not able to communicate unless they have previously discovered each other by synchronizing their frequency hopping patterns. Thus, even if all nodes are within direct communication range of each other, only those nodes which are synchronized with the transmitter can hear the transmission. To support any-to-any communication, nodes must be synchronized so that the pairs of nodes (which can communicate with each other) together form a connected graph. Using Bluetooth as an example, this paper first provides deeper insights into the issue to link establishment in frequency hopping wireless systems. It then introduces the Bluetooth topology construction protocol (BTCP), an asynchronous distributed protocol for constructing scatternets which starts with nodes that have no knowledge of their surroundings and terminates with the formation of a connected network satisfying all connectivity constraints posed by the Bluetooth technology. To the best of our knowledge, the work presented in this paper is the first attempt at building Bluetooth scatternets using distributed logic and is quite "practical" in the sense that it can be implemented using the communication primitives offered by the Bluetooth 1.0 specifications.

Journal ArticleDOI
TL;DR: An overview of the traffic grooming problem is given and some representative work in this area is surveyed, showing that grooming traffic in general mesh networks is an important emerging problem.
Abstract: The emergence of wavelength-division multiplexing technology has led to a tremendous increase In the available transmission capacity in wide area networks. Consequently, these networks may no longer be limited by the transmission bandwidth, but rather by the processing capability of electronic switches, routers, and multiplexers in the network. This realization has led to a new wave of research aimed at overcoming the electronic bottleneck by providing optical bypass at the WDM layer. Traffic grooming can be used as a bypass mechanism by which low-rate circuits are assigned to wavelengths in order to minimize the amount of electronic multiplexing equipment. This topic has received a significant amount of attention in both the research and commercial arenas. We give an overview of the traffic grooming problem and survey some representative work in this area. While most work has focused on grooming in SONET rings, grooming traffic in general mesh networks is an important emerging problem.

Journal ArticleDOI
TL;DR: Several aspects in the design of shallow water acoustic networks that maximize throughput and reliability while minimizing power consumption are considered.
Abstract: Underwater acoustic networks are generally formed by acoustically connected ocean bottom sensor nodes, autonomous underwater vehicles (AUVs), and surface stations that serve as gateways and provide radio communication links to on-shore stations. The quality of service of such networks is limited by the low bandwidth of acoustic transmission channels, high latency resulting from the slow propagation of sound, and elevated noise levels in some environments. The long-term goal in the design of underwater acoustic networks is to provide for a self-configuring network of distributed nodes with network links that automatically adapt to the environment through selection of the optimum system parameters. This article considers several aspects in the design of shallow water acoustic networks that maximize throughput and reliability while minimizing power consumption.

01 Apr 2001
TL;DR: The design, implementation and usage of the BRITE universal topology generation tool, a "universal topology generator", are discussed and the BRite Analysis Engine, BRIANA, is described, which is an independent piece of software designed and built upon BRITE design goals of flexibility and extensibility.
Abstract: Effective engineering of the Internet is predicated upon a detailed understanding of issues such as the large-scale structure of its underlying physical topology, the manner in which it evolves over time, and the way in which its constituent components contribute to its overall function. Unfortunately, developing a deep understanding of these issues has proven to be a challenging task, since it in turn involves solving difficult problems such as mapping the actual topology, characterizing it, and developing models that capture its emergent behavior. Consequently, even though there are a number of topology models, it is an open question as to how representative the topologies they generate are of the actual Internet. Our goal is to produce a topology generation framework which improves the state of the art and is based on design principles which include representativeness, inclusiveness, and interoperability. Representativeness leads to synthetic topologies that accurately reflect many aspects of the actual Internet topology (e.g. hierarchical structure, degree distribution, etc.). Inclusiveness combines the strengths of as many generation models as possible in a single generation tool. Interoperability provides interfaces to widely-used simulation applications such as ns and SSF as well as visualization applications. We call such a tool a "universal topology generator". In this paper we discuss the design, implementation and usage of the BRITE universal topology generation tool that we have built. We also describe the BRITE Analysis Engine, BRIANA, which is an independent piece of software designed and built upon BRITE design goals of flexibility and extensibility. The purpose of BRIANA is to act as a repository of analysis routines along with a user--friendly interface that allows its use on different topology formats.

Proceedings ArticleDOI
11 Jun 2001
TL;DR: In this article, the authors propose two protocols for forming connected scatternets, in which the number of roles each node can assume are limited to two or three (depending on the protocol), thus imposing low slave management overhead.
Abstract: Bluetooth is an open specification for short-range wireless communication and networking, mainly intended to be a cable replacement between portable and/or fixed electronic devices. The specification also defines techniques for interconnecting large number of nodes in scatternets, thus enabling the establishment of a mobile ad hoc network (MANET). While several solutions and commercial products have been introduced for one-hop Bluetooth communication, the problem of scatternet formation has not yet been dealt with. This problem concerns the assignment of the roles of master and slave to each node so that the resulting MANET is connected. We introduce two novel protocols for forming connected scatternets. In both cases, the resulting topology is termed a bluetree. In our bluetrees the number of roles each node can assume are limited to two or three (depending on the protocol), thus imposing low slave management overhead. The effectiveness of both protocols in forming MANETs is demonstrated through extensive simulations.

Journal ArticleDOI
TL;DR: Simulation experiments show that in the event of route failures, as the route reestablishment time increases, the use of feedback provides significant improvements in performance.
Abstract: Ad hoc networks are completely wireless networks of mobile hosts, in which the topology rapidly changes due to the movement of mobile hosts. This frequent topology change may lead to sudden packet losses and delays. Transport protocols like TCP, which have been designed for reliable fixed networks, misinterpret this packet loss as congestion and invoke congestion control, leading to unnecessary retransmissions and loss of throughput. To overcome this problem, a feedback scheme is proposed so that the source can distinguish between a route failure and network congestion. When a route is disrupted, the source is sent a route failure notification packet, allowing it to invalidate its timers and stop sending packets. When the route is reestablished, the source is informed through a route reestablishment notification packet, upon which it resumes packet transmissions. Simulation experiments show that in the event of route failures, as the route reestablishment time increases, the use of feedback provides significant improvements in performance.

Proceedings ArticleDOI
04 Dec 2001
TL;DR: A congestion control system which is arbitrarily scalable, in the sense that its stability is maintained for arbitrary network topologies and arbitrary amounts of delay is developed.
Abstract: Discusses flow control in networks, in which sources control their rates based on feedback signals received from the network links, a feature present in current TCP protocols. We develop a congestion control system which is arbitrarily scalable, in the sense that its stability is maintained for arbitrary network topologies and arbitrary amounts of delay. Such a system can be implemented in a decentralized way with information currently available in networks plus a small amount of additional signaling.

Proceedings ArticleDOI
22 Apr 2001
TL;DR: The time for end-to-end Internet convergence depends on the length of the longest possible backup autonomous system path between a source and destination node, and significant variation in the convergence behavior of Internet service providers is demonstrated, with the larger providers exhibiting the fastest convergence latencies.
Abstract: This paper examines the role inter-domain topology and routing policy play in the process of delayed Internet routing convergence. In previous work, we showed that the Internet lacks effective inter-domain path fail-over. Unlike circuit-switched networks which exhibit fail-over on the order of milliseconds, we found Internet backbone routers may take tens of minutes to reach a consistent view of the network topology after a fault. In this paper, we expand an our earlier work by exploring the impact of specific Internet provider policies and topologies on the speed of routing convergence. Based on data from the experimental injection and measurement of several hundred thousand inter-domain routing faults, we show that the time for end-to-end Internet convergence depends on the length of the longest possible backup autonomous system path between a source and destination node. We also demonstrate significant variation in the convergence behavior of Internet service providers, with the larger providers exhibiting the fastest convergence latencies. Finally, we discuss possible modifications to BGP and provider routing policies which if deployed, would improve inter-domain routing convergence.

Journal ArticleDOI
TL;DR: The goal is, given the failure probabilities of the paths, to find the optimal way to fragment and then distribute the blocks to the paths so that the probability of reconstructing the original information at the destination is maximized.
Abstract: In this article we propose a framework for multipath routing in mobile ad hoc networks and provide its analytical evaluation. The instability of the topology (e.g., failure of links) in these types of networks, due to nodal mobility and changes in wireless propagation conditions, makes transmission of time-sensitive information a challenging problem. To combat this inherent unreliability of these networks, we propose a routing scheme that uses multiple paths simultaneously by splitting the information among the multitude of paths, to increase the probability that the essential portion of the information is received at the destination without incurring excessive delay. Our scheme works by adding some overhead to each packet, which is calculated as a linear function of the original packet bits. The resulting packet (information and overhead) is fragmented into smaller blocks and distributed over the available paths. Our goal is, given the failure probabilities of the paths, to find the optimal way to fragment and then distribute the blocks to the paths so that the probability of reconstructing the original information at the destination is maximized. Our algorithm has low time complexity, which is crucial since the path failure characteristics vary with time and the optimal block distribution has to be recalculated in real time.

Proceedings ArticleDOI
29 Nov 2001
TL;DR: Fundamental details of network-based control and recent network- based control techniques for handling the network delays are presented, based on various concepts such as state augmentation, queuing and probability theory, nonlinear control and perturbation theory, and scheduling.
Abstract: There are two general structures to design a control system through a network. The first structure is to have several subsystems, in which each of the subsystem contains a set of sensors, a set of actuators, and a controller by itself. These system components are attached to the same control plant. In this case, a subsystem controller receives a set point from the central controller. Another structure is to connect a set of sensors and a set of actuators to a network directly. Sensors and actuators in this case are attached to a plant, while a controller is separated from the plant via a network connection to perform a closed-loop control over the network. A challenging problem in control of networked-based system is network delay effects. The time to read a sensor measurement and to send a control signal to an actuator through the network depends on network characteristics such as their topologies, routing schemes, etc. Therefore, the overall performance of a network-based control system can be significantly affected by network delays. The severity of the delay problem is aggravated when data loss occurs during a transmission. Moreover, the delays do not only degrade the performance of a network-based control system, but also can destabilize the system. This tutorial presents fundamental details of network-based control and recent network-based control techniques for handling the network delays. The techniques are based on various concepts such as state augmentation, queuing and probability theory, nonlinear control and perturbation theory, and scheduling. A general structure of a network-based control system, delay types, and delay behaviors are also described in this tutorial. In addition, advantages and disadvantages of these techniques are discussed.

Patent
17 Sep 2001
TL;DR: In this article, the authors present a method and an apparatus for analyzing a network configuration against a corporate network policy and determining violation(s) against the corporate network policies and adding an entry to its final report each time that it detects a violation against the network policy in the network configuration model.
Abstract: A method and an apparatus for analyzing a network configuration against a corporate network policy and determining violation(s) against the corporate network policy. A report indicating the violation(s) can be generated indicating instances of the violation(s). An analysis platform reads in a network policy. The analysis platform collects configuration files from the relevant network devices in the network and builds up an internal instance of a network configuration model based on the configuration files and the network topology. The analysis platform analyzes this network configuration model according to the network policy and adds an entry to its final report each time that it detects a violation against the network policy in the network configuration model. The data in the entries pinpoints the cause of the deviation(s) from the network policy.

Journal ArticleDOI
Kai Nagel1, M. Rickert
01 Nov 2001
TL;DR: This paper describes the parallel implementation of the TRansportation ANalysis and SIMulation System (TRANSIMS) traffic micro-simulation, and describes how information between domains is exchanged, and how the transportation network graph is partitioned.
Abstract: This paper describes the parallel implementation of the TRansportation ANalysis and SIMulation System (TRANSIMS) traffic micro-simulation. The parallelization method is domain decomposition, which means that each CPU of the parallel computer is responsible for a different geographical area of the simulated region. We describe how information between domains is exchanged, and how the transportation network graph is partitioned. An adaptive scheme is used to optimize load balancing. We then demonstrate how computing speeds of our parallel micro-simulations can be systematically predicted once the scenario and the computer architecture are known. This makes it possible, e.g., to decide if a certain study is feasible with a certain computing budget, and how to invest that budget. The main ingredients of the prediction are knowledge about the parallel implementation of the micro-simulation, knowledge about the characteristics of the partitioning of the transportation network graph, and knowledge about the interaction of these quantities with the computer system. In particular, we investigate the differences between switched and non-switched topologies, and the effects of 10 Mbit, 100 Mbit, and Gbit Ethernet. As an example, we show that with a common technology – 100 Mbit switched Ethernet – one can run the 20 000-link EMME/2-network for Portland (Oregon) more than 20 times faster than real time on 16 coupled Pentium CPUs.

Journal ArticleDOI
TL;DR: A multiple access scheme for the broadcast of control messages is designed, and a new access-based clustering protocol (ABCP) whose cluster formation is heavily influenced by the outcome of the multiple access is proposed, providing a generic, flexible, rapidly deployed and stable cluster architecture for the upper layer protocols.
Abstract: The ad hoc network is a temporary wireless system without a fixed (wired or wireless) infrastructure. Many clustering algorithms have been proposed to partition mobile users into clusters to support routing and network management. Most previous studies, however, focus on algorithm design, lacking an overall evaluation of clustering overheads. We design a multiple access scheme for the broadcast of control messages, and propose a new access-based clustering protocol (ABCP) whose cluster formation is heavily influenced by the outcome of the multiple access. By taking into account many delicate aspects in the clustering process, the ABCP provides a generic, flexible, rapidly deployed and stable cluster architecture for the upper layer protocols. Simulation is used to compare ABCP with the other clustering strategy in terms of cluster stability and overheads. Since ABCP makes clustering decision directly based on the result of channel access, it requires fewer control overheads and has shorter convergence time than the other clustering criteria. We also demonstrate that the resulting cluster structure by ABCP behaves stable in face of topology changes.

Proceedings ArticleDOI
22 Apr 2001
TL;DR: A formal analysis of the integrated approach and a method for deriving the appropriate link weights show that for any given set of optimal routes of the overlay approach with respect to a set of traffic demands, the Integrated approach can achieve exactly the same result by reproducing them as shortest paths.
Abstract: The overlay approach has been widely used by many service providers for traffic engineering in large Internet backbones. In the overlay approach, logical connections are set up between edge nodes to form a full mesh virtual network on top of the physical topology. IP routing is then run over the virtual network. Traffic engineering objectives are achieved through carefully routing logical connections over the physical links. Although the overlay approach has been implemented in many operational networks, it has a number of well-known scaling issues. This paper proposes a new approach, which we call the integrated approach, to achieve traffic engineering without full-mesh overlaying. In the integrated approach, IP routing runs natively over the physical topology rather than over the virtual network. Traffic engineering objectives are realized by setting appropriate link metrics in IP routing protocols. We first illustrate our approach with a simple network, then present a formal analysis of the integrated approach and a method for deriving the appropriate link weights. Our analysis shows that for any given set of optimal routes of the overlay approach with respect to a set of traffic demands, the integrated approach can achieve exactly the same result by reproducing them as shortest paths. We further extend the result to a more generic one: for any arbitrary set of routes, as long as they are not loopy, they can be converted to shortest-paths with respect to some set of positive link weights. A theoretical insight of our result is that the optimal routing (with respect to any objective function) is always shortest path routing with respect to some appropriate positive link weights.

Journal ArticleDOI
TL;DR: In this paper, the authors investigate a model that simultaneously optimizes facility locations and the design of the underlying transportation network, motivated by the simple observation that changing the network topology is often more cost-effective than adding facilities to improve service levels.
Abstract: Network location models have been used extensively for siting public and private facilities In this paper, we investigate a model that simultaneously optimizes facility locations and the design of the underlying transportation network Motivated by the simple observation that changing the network topology is often more cost-effective than adding facilities to improve service levels, the model has a number of applications in regional planning, distribution, energy management, and other areas The model generalizes the classical simple plant location problem We show how the model can be solved effectively We then use the model to analyze two potential transportation planning scenarios The fundamental question of resource allocation between facilities and links is investigated, and a detailed sensitivity analysis provides insight into the model's usefulness for aiding budgeting and planning decisions We conclude by identifying promising research directions

Journal ArticleDOI
TL;DR: It is proved that, under certain conditions, having equality degree constraints with multiple edges allowed in the design of logical topologies does not affect congestion and helps in reducing the dimensionality of the search space and hence speeds up the search for an optimal solution of the linear formulation.
Abstract: We consider the problem of constructing logical topologies over a wavelength-routed optical network with no wavelength changers. We present a general linear formulation which considers routing traffic demands, and routing and assigning wavelengths to lightpaths, as a combined optimization problem. The formulation also takes into account the maximum number of hops a lightpath is permitted to take, multiple logical links in the logical topology, multiple physical links in the physical topology, and symmetry/asymmetry restrictions in designing logical topologies. The objective is to minimize congestion. We show by examples how equality and inequality logical degree constraints have a bearing on congestion. We prove that, under certain conditions, having equality degree constraints with multiple edges allowed in the design of logical topologies does not affect congestion. This helps in reducing the dimensionality of the search space and hence speeds up the search for an optimal solution of the linear formulation. We solve the linear formulation for small examples and show the tradeoff between congestion, number of wavelengths available and the maximum number of hops a lightpath is allowed to take. For large networks, we solve the linear formulation by relaxing the integer constraints. We develop topology design algorithms for large networks based on rounding the solutions obtained by solving the relaxed problem. Since the whole problem is linearizable, the solution obtained by relaxation of the integer constraints yields a lower bound on congestion. This is useful in comparing the efficiency of our heuristic algorithms. Following Bienstock and Gunluk (1995), we introduce a cutting plane which helps in obtaining better lower bounds on congestion and also enables us to reduce the previously obtained upper bounds on congestion.

Proceedings ArticleDOI
22 Apr 2001
TL;DR: This work proposes and studies an approach based on overlaying a virtual infrastructure adaptation of the core, proposed by Sivakumar, Sinha and Bharghavan, on an ad hoc network and operating routing protocols over the infrastructure, which enables routing protocols to use only a subset of nodes in the network for route management and avoid the use of broadcast relays.
Abstract: Several routing algorithms for mobile ad hoc networks (MANETs) have been proposed previously. With the exception of a few, these protocols (i) involve all nodes in the route management process, (ii) rely on the use of broadcast relays for route computation, and (iii) are primarily reactive in nature. Related work has shown that the capacity utilization in ad hoc networks decreases significantly when broadcast relays or "broadcast storms" are performed frequently. This effect is compounded when all nodes in the network take part in the route computation. We propose and study an approach based on overlaying a virtual infrastructure adaptation of the core, proposed by Sivakumar, Sinha and Bharghavan (see IEEE Journal on Selected Areas in Communications, vol.17, no.8, p.1454-65, 1999) on an ad hoc network and operating routing protocols over the infrastructure. The core enables routing protocols to use only a subset of nodes in the network for route management and avoid the use of broadcast relays. Using the ns-2 simulator we evaluate the performance of two ad hoc routing protocols, dynamic source routing (DSR) and ad hoc on demand distance vector (AODV), when they are operated over the core and compare their performance against those of their basic versions.

Proceedings ArticleDOI
03 Sep 2001
TL;DR: This paper proposes a power-aware routing protocol and simulates the performance, showing that the routing protocol adapts routes to the available power and leads to a reduction in the total power used as well as more even power usage across nodes.
Abstract: Wireless sensor networks have become possible because of the on-going improvements in sensor technology and VLSI. One issue in smart sensor networks is achieving efficient operation because of the limited available power. For important classes of sensor networks, such as biomedical sensors, the locations of the sensing nodes are fixed and the placement can be pre-determined. In this paper, we consider the topology that best supports communication among these sensor nodes. We propose a power-aware routing protocol and simulate the performance, showing that our routing protocol adapts routes to the available power. This leads to a reduction in the total power used as well as more even power usage across nodes. We consider different routes and topologies, demonstrating the difference in performance and explaining the underlying causes.