Showing papers on "Network topology published in 1998"

Synchronous network for digital media streams

Abstract: A network adapter for a synchronous logical ring network operates on existing physical twisted-pair telephone topologies. Information propagates around the logical ring, reaching every device on each revolution around the network. Network devices are full-duplex, transmitting and receiving information on every clock cycle. Network devices arbitrate to be elected the network clock device. By synchronizing all network devices to a single reference clock, and providing fixed frames of information propagating around the network at consistent time intervals, the logical ring network ensures that information propagates from one device to another at consistent time intervals. The fixed-length frames are divided into two independent streams: a data stream for the distribution of real-time continuous digital media streams; and a system command stream for the distribution of system commands.

Cycle-oriented distributed preconfiguration: ring-like speed with mesh-like capacity for self-planning network restoration

Abstract: Cycle-oriented preconfiguration of spare capacity is a new idea for the design and operation of mesh-restorable networks. It offers a sought-after goal: to retain the capacity-efficiency of a mesh-restorable network, while approaching the speed of line-switched self-healing rings. We show that through a strategy of pre-failure cross-connection between the spare links of a mesh network, it is possible to achieve 100% restoration with little, if any, additional spare capacity than in a mesh network. In addition, we find that this strategy requires the operation of only two cross-connections per restoration path. Although spares are connected into cycles, the method is different than self-healing rings because each preconfigured cycle contributes to the restoration of more failure scenarios than can a ring. Additionally, two restoration paths may be obtained from each pre-formed cycle, whereas a ring only yields one restoration path for each failure it addresses. We give an optimal design formulation and results for preconfiguration of spare capacity and describe a distributed self-organizing protocol through which a network can continually approximate the optimal preconfiguration state.

Global state routing: a new routing scheme for ad-hoc wireless networks

Abstract: In an ad-hoc environment with no wired communication infrastructure, it is necessary that mobile hosts operate as routers in order to maintain the information about connectivity. However with the presence of high mobility and low signal/interference ratio (SIR), traditional routing schemes for wired networks are not appropriate, as they either lack the ability to quickly reflect the changing topology, or may cause excessive overhead, which degrades network performance. Considering these restrictions, we propose a new scheme especially designed for routing in an ad-hoc wireless environments. We call this scheme "global state routing" (GSR), where nodes exchange vectors of link states among their neighbors during routing information exchange. Based on the link state vectors, nodes maintain a global knowledge of the network topology and optimize their routing decisions locally. The performance of the algorithm, studied in this paper through a series of simulations, reveals that this scheme provides a better solution than existing approaches in a truly mobile, ad-hoc environment.

A hybrid multilevel inverter topology for drive applications

Abstract: The use of multilevel inverters has become popular in recent years for high power applications. Various topologies and modulation strategies have been reported for utility and drive applications in the recent literature. This paper is devoted to the investigation of a 500 hp induction motor drive based on a seven-level 4.5 kV hybrid inverter. The topological structure and operating principles of the proposed approach are presented. Various design criteria, spectral structures and other practical issues such as capacitor voltage balancing are discussed. The feasibility of the proposed approach is verified by computer simulations.

Strong approximations for Markovian service networks

Abstract: Inspired by service systems such as telephone call centers, we develop limit theorems for a large class of stochastic service network models. They are a special family of nonstationary Markov processes where parameters like arrival and service rates, routing topologies for the network, and the number of servers at a given node are all functions of time as well as the current state of the system. Included in our modeling framework are networks of M_t/M_t/n_t queues with abandonment and retrials. The asymptotic limiting regime that we explore for these networks has a natural interpretation of scaling up the number of servers in response to a similar scaling up of the arrival rate for the customers. The individual service rates, however, are not scaled. We employ the theory of strong approximations to obtain functional strong laws of large numbers and functional central limit theorems for these networks. This gives us a tractable set of network fluid and diffusion approximations. A common theme for service network models with features like many servers, priorities, or abandonment is “non-smooth” state dependence that has not been covered systematically by previous work. We prove our central limit theorems in the presence of this non-smoothness by using a new notion of derivative.

An optimal topology-transparent scheduling method in multihop packet radio networks

Abstract: Many transmission scheduling algorithms have been proposed to maximize the spatial reuse and minimize the time-division multiple-access (TDMA) frame length in multihop packet radio networks. Almost all existing algorithms assume exact network topology information and do not adapt to different traffic requirements. Chlamtac and Farago (1994) proposed a topology-transparent algorithm. Following their approach, but with a different design strategy, we propose another algorithm which is optimal in that it maximizes the minimum throughput. We compare our algorithm with that of Chlamtac and Farago's and with the TDMA algorithm, and find that it gives better performance in terms of minimum throughput and minimum and maximum delay times. Our algorithm requires estimated values of the number of nodes and the maximum nodal degree in the network. However, we show that the performance of our algorithm is insensitive to these design parameters.

Optimal capacity placement for path restoration in STM or ATM mesh-survivable networks

Abstract: The total transmission capacity required by a transport network to satisfy demand and protect it from failures contributes significantly to its cost, especially in long-haul networks. Previously, the spare capacity of a network with a given set of working span sizes has been optimized to facilitate span restoration. Path restorable networks can, however, be even more efficient by defining the restoration problem from an end to end rerouting viewpoint. We provide a method for capacity optimization of path restorable networks which is applicable to both synchronous transfer mode (STM) and asynchronous transfer mode (ATM) virtual path (VP)-based restoration. Lower bounds on spare capacity requirements in span and path restorable networks are first compared, followed by an integer program formulation based on flow constraints which solves the spare and/or working capacity placement problem in either span or path restorable networks. The benefits of path and span restoration, and of jointly optimizing working path routing and spare capacity placement, are then analyzed.

Dynamic analysis of current regulators for AC motors using complex vectors

Abstract: The analysis and design of current regulators for polyphase AC loads is presented using complex vector notation. The AC motor current regulation problem is analyzed by studying both the command tracking and disturbance rejection capability of the current regulator. The use of complex vector notation and the generalization of classical control tools like root locus, frequency-response functions and dynamic stiffness functions to complex vectors provide a way of comparing the performance of different controller topologies. Limitations in the performance of the synchronous frame proportional and integral current regulator are outlined, and several ways of improving its performance are suggested and investigated.

Multiwavelength optical networks with limited wavelength conversion

Abstract: This paper proposes optical wavelength division multiplexed (WDM) networks with limited wavelength conversion that can efficiently support lightpaths (connections) between nodes. Each lightpath follows a route in a network and must be assigned a channel on each link along the route. The load /spl lambda//sub max/ of a set of lightpaths is the maximum over all links of the number of lightpaths that use the link. At least /spl lambda//sub max/ wavelengths will be needed to assign channels to the lightpaths. If the network has full wavelength conversion capabilities, then /spl lambda//sub max/ wavelengths are sufficient to perform the channel assignment. Ring networks with fixed wavelength conversion capability within the nodes are proposed that can support all lightpath sets with load /spl lambda//sub max/ at most W-1, where W is the number of wavelengths in each link. Ring networks with a small additional amount of wavelength conversion capability within the nodes are also proposed that allow the support of any set of lightpaths with load /spl lambda//sub max/ at most W. A star network is also proposed with fixed wavelength conversion capability at its hub node that can support all lightpath sets with load /spl lambda//sub max/ at most W. These results are extended to tree networks and networks with arbitrary topologies. This provides evidence that significant improvements in traffic-carrying capacity can be obtained in WDM networks by providing very limited wavelength conversion capability within the network.

FSA-based link assignment and routing in low-earth orbit satellite networks

Abstract: We propose a new framework for the link assignment (i.e., topological design) problem that arises from the use of intersatellite links (ISL's) in low-earth orbit (LEO) satellite networks. In the proposed framework, we model an LEO satellite network as a finite state automaton (FSA), where each state corresponds to an equal-length interval in the system period of the LEO satellite network. This FSA-based framework allows the link assignment problem in LEO satellite networks to be treated as a set of link assignment problems in fixed topology networks. Within this framework, we study various link assignment and routing schemes. In particular, both regular link assignment and link assignment optimized by simulated annealing are considered. For each link assignment, both static and dynamic routing schemes are considered. Our simulation results show that the optimized link assignment combined with static routing achieves the best performance in terms of both newly initiated call blocking probability and ongoing call blocking probability. The results also show that when the link assignment is the same, static routing gives better performance than dynamic routing since the latter requires a substantial amount of time to stabilize its routing table after a state transition.

A feedback based scheme for improving TCP performance in ad-hoc wireless networks

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 may lead to sudden packet losses and delays. Transport protocols like TCP have been built mainly for reliable, fixed networks. Hence, when used in ad-hoc networks, TCP misinterprets this loss as congestion and invokes congestion control. This leads to unnecessary retransmissions and loss of throughput. To overcome this problem, a feedback scheme is proposed, so that the source can distinguish between route failure and network congestion. When a route is disrupted, the source is sent a route failure notification (RFN) packet, allowing it to freeze its timers and stop sending packets. When the route is re-established, the source is informed through a route re-establishment notification (RRN) packet, upon which it resumes by unfreezing timers and continuing packet transmissions. The simulated performance of TCP on ad-hoc networks with and without feedback is compared and reported. It is observed that in the event of route failures, as the route re-establishment time increases, the use of feedback provides significant gains in throughput as well as savings in unnecessary packet transmissions. Several further enhancements and directions for future work are also sketched.

A new approach for distribution feeder reconfiguration for loss reduction and service restoration

Abstract: This paper presents a new approach to solve the distribution feeder reconfiguration problem for loss reduction and service restoration. By using the proposed algorithm, a more efficient network configuration can be obtained to reduce loss. Three switching indices were defined in this paper. Branch voltage-drops and line constants were used with all the electrical constraints. Meshed networks were considered instead of the radial topology by closing all the tie switches. By considering only the largest switching index in each loop, this algorithm can reduce the number of feasible states drastically. The switching index can also be used for service restoration. Many tests have been run to show its effectiveness.

IP switching—ATM under IP

Abstract: Internet protocol (IP) traffic on the Internet and private enterprise networks has been growing exponentially for some time. This growth is beginning to stress the traditional processor-based design of current-day routers. Switching technology offers much higher aggregate bandwidth, but presently only offers a layer-2 bridging solution. Various proposals are under way to support IP routing over an asynchronous transfer mode (ATM) network. However, these proposals hide the real network topology from the IP layer by treating the data-link layer as a large opaque network cloud. We argue that this leads to complexity, inefficiency, and duplication of functionality in the resulting network. We propose an alternative in which we discard the end-to-end ATM connection and integrate fast ATM hardware directly with IP, preserving the connectionless nature of IP. We use the soft-state in the ATM hardware to cache the IP forwarding decision. This enables further traffic on the same IP flow to be switched by the ATM hardware rather than forwarded by IP software. We claim that this approach combines the simplicity, scalability, and robustness of IP, with the speed, capacity, and multiservice traffic capabilities of ATM.

A lightweight adaptive multicast algorithm

Abstract: In this paper, we present a multicast protocol which is built upon the temporally-ordered routing algorithm (TORA). The protocol-termed the lightweight adaptive multicast (LAM) routing algorithm-is designed for use in a Mobile Ad hoc NETwork (MANET) and, conceptually, can be thought of as an integration of the CORE based tree (CBT) multicast routing protocol and TORA. The direct coupling of LAM and TORA increases reaction efficiency (lowering protocol control overhead) as the new protocol can benefit from TORA's mechanisms while reacting to topological changes. Also during periods of stable topology and constant group membership, the LAM protocol does not introduce any additional overhead because it does not require timer-based messaging during its execution.

A performance comparison of the temporally-ordered routing algorithm and ideal link-state routing

Abstract: We present a relative performance comparison of the temporally-ordered routing algorithm (TORA) with an ideal link state (ILS) routing algorithm. The performance metrics evaluated include bandwidth efficiency for both control and data, as well as end-to-end message packet delay and throughput. The routing algorithms are compared in the context of a dynamic, multihop, wireless network employing broadcast transmissions. The network parameters varied include network size, average rate of topological changes and average network connectivity. While the average network connectivity was found not to be a significant factor, the relative performance of TORA and ILS was found to be critically dependent on the network size, and the average rate of topological changes. The results further indicate that for a given available bandwidth-as either the size of network increases or the rate of network topological change increases, the performance of TORA eventually exceeds that of ILS.

Multilevel Power Conversion - An Overview Of Topologies And Modulation Strategies

Abstract: Multilevel power converters represent a potential breakthrou,qh in employing swirching converters to medium voltage applications (2-13 k w . This pciper summarizes the stat e of the art in this rapidly a'eveloping Jeld ofpower electronics.

Apparatus and methods for providing fault tolerance of networks and network interface cards

Abstract: Methods and apparatus for implementation of fault-tolerant networks provide a network fault-tolerance manager for detecting failures and manipulating a node to communicate with an active channel. Failure detection incorporates one or more methods, including message pair and link pulse detection. Failure recovery includes switching all node data communications to a stand-by channel or switching just those nodes detecting a failure. Communication between nodes provides the distributed detection, with detecting nodes reporting failures to their network fault-tolerance manager and the network fault-tolerance manager broadcasting the failure recovery to all nodes. The network fault-tolerance manager is middleware, residing with each node in a logical hierarchy above a physical layer of a network and below a transport layer of the network. The approach is particularly suited to Ethernet LANs and capable of using commercial off-the-shelf network components.

Topology error identification using normalized Lagrange multipliers

Abstract: This paper introduces a method for topology error identification based on the use of normalized Lagrange multipliers. The proposed methodology models circuit breakers as network switching branches whose statuses are treated as operational constraints in the state estimation problem. The corresponding Lagrange multipliers are then normalized and used as a tool for topology error identification, in the same fashion as measurement normalized residuals are conventionally employed for analog bad data processing. Results of tests performed with the proposed algorithm for different types of topology errors are reported.

Communication synthesis for distributed embedded systems

Abstract: Designers of distributed embedded systems face many challenges in determining the tradeoffs when defining a system architecture or retargeting an existing design. Communication synthesis, the automatic generation of the necessary software and hardware for system components to exchange data, is required to more effectively explore the design space and automate very error prone tasks. The paper examines the problem of mapping a high level specification to an arbitrary architecture that uses specific, common bus protocols for interprocessor communication. The communication model presented allows for easy retargeting to different bus topologies, protocols, and illustrates that global considerations are required to achieve a correct implementation. An algorithm is presented that partitions multihop communication timing constraints to effectively utilize the bus bandwidth along a message path. The communication synthesis tool is integrated with a system co-simulator to provide performance data for a given mapping.

Method for displaying a network topology for a task deployment service

Abstract: A method for displaying a network topology begins by presenting a user a set of attributes for network objects in the network. The user the selects a given attribute and an attribute value. A mapper routine of the invention then builds a topology map that includes at least one icon representing network objects that have the user-selected attribute value for the attribute. This “group” icon preferably includes a numeric, textual or graphical representation to indicate the number of objects (or perhaps some given characteristic thereof) represented by the group icon.

Improving wireless LAN performance via adaptive local error control

Abstract: Wireless links can exhibit high error rates due to attenuation, fading, or interfering active radiation sources. To make matters worse, error rates can be highly variable due to changes in the wireless environment. Researchers and developers have explored a wide range of solutions to optimize communication in this difficult error environment, including traditional end-to-end solutions, link-layer solutions, and solutions involving layer four processing inside the network. A significant challenge is ensuring that systems with multiple layers of error control avoid compromising performance by duplication of effort. We argue and demonstrate that protocol-independent link-level local error control can achieve high communication efficiency even in a highly variable error environment, that adaptation is important to achieve this efficiency, and that inter-layer coexistence is achievable. The logical link control layer of our WaveLAN-based experimental LAN includes three error control mechanisms: local retransmission, adaptive packet shrinking, and adaptive error coding. Measurements generated on a variety of network topologies and trace-based error environments demonstrate the TCP performance improvements and good coexistence with TCP's end-to-end retransmission strategy.

Spine routing in ad hoc networks

Abstract: An ad hoc network is a multihop wireless network in which mobile hosts communicate without the support of a wired backbone for routing messages. We introduce a self organizing network structure called a spine and propose a spine-based routing infrastructure for routing in ad hoc networks. We propose two spine routing algorithms: (a) Optimal Spine Routing (OSR), which uses full and up-to-date knowledge of the network topology, and (b) Partial-knowledge Spine Routing (PSR), which uses partial knowledge of the network topology. We analyze the two algorithms and identify the optimality-overhead trade-offs involved in these algorithms.

A resource query interface for network-aware applications

Abstract: Development of portable network-aware applications demands an interface to the network that allows an application to obtain information about its execution environment. The paper motivates and describes the design of Remos, an API that allows network-aware applications to obtain relevant information. The major challenges in defining a uniform interface are network heterogeneity, diversity in traffic requirements, variability of the information, and resource sharing in the network. Remos addresses these issues with two abstraction levels, explicit management of resource sharing, and statistical measurements. The flows abstraction captures the communication between nodes, and the topologies abstraction provides a logical view of network connectivity. Remos measurements are made at network level, and therefore information to manage sharing of resources is available. Remos is designed to deliver best effort information to applications, and it explicitly adds statistical reliability and variability measures to the core information. The paper also presents preliminary results and experience with a prototype Remos implementation for a high speed IP based network testbed.

A genetic algorithm for designing distributed computer network topologies

Abstract: The topological design of distributed packet switched networks consists of finding a topology that minimizes the communication costs by taking into account a certain number of constraints such as the delay and the reliability. This paper proposes a genetic algorithm (GA) for generating low-cost feasible computer network topologies subject to these constraints. The implementation of this algorithm has been subjected to extensive tests in order to measure the quality of solutions. Computational results confirm the efficiency of the GA to provide good solutions for medium-sized computer networks, in comparison with well-tried conventional methods.

Topological optimization of a reliable communication network

Abstract: This paper considers backbone network design under the constraints: minimal total link cost, and 1-FT (fault-tolerant to 1 link-failure). As networks become huge, the backbone layout design is essential to network performance and reliability. A 1-FT backbone can survive any 1-link failure. On the other hand, the total cost of the links in backbone layout is a practical concern. Therefore, the problem is to find a network topology for a set of nodes whose total link cost is minimized, subject to the condition that the backbone network can accommodate 1 link failure. The problem is NP-hard, and methods based on heuristic search are desired to obtain optimal or sub-optimal solutions. This paper proposes an efficient method based on genetic algorithms to solve the problem. The representation of a backbone layout is based on a list of ordered links. The genetic operators attempt to generate a more cost-effective or reliable layout. Simulation shows that the proposed algorithm can efficiently find a sub-optimal solution for most cases.

Adaptive packet routing for bursty adversarial traffic

Abstract: One of the central tasks of networking is packet routing when edge bandwidth is limited. Tremendous progress has been achieved by separating the issue of routing into two conceptual subproblems: path selection and congestion resolution along the selected paths. However, this conceptual separation has a serious drawback: each packet's path is fixed at the source and cannot be modified adaptively en-route. The problem is especially severe when packet injections are modeled by an adversary, whose goal is to cause traffic-jams. In this paper, we consider this adversarial setting, motivated by the adversarial queuing theory model of Borodin et al. (1996, in “Proc. of 28th STOC,” pp. 376?385). More precisely, we consider an adversary who injects packets, with only their destinations specified, into network nodes in a continuous manner subject to certain limitations on the injection rate. The question whether it is possible to deal with such an adversary and to design protocols that would discover routes which avoid traffic jams so that nodes only store a bounded number of packets was left as an open problem by Andrews et al. (1997, in “Proc. of 38th FOCS,” pp. 294?302) (who deal with the nonadaptive case where the adversary provides routes for the packets). In the present paper, we resolve this open problem. In particular, we present a simple, deterministic, local-control protocol that applies to any network topology. Our protocol guarantees that, for any injection sequence generated by the adversary, the buffers at the nodes are polynomially bounded and that each packet has a polynomially bounded delivery time.

Optimal capacity and flow assignment for self-healing ATM networks based on line and end-to-end restoration

Abstract: This paper addresses an optimal link capacity design problem for self-healing asynchronous transfer mode (ATM) networks based on two different restoration schemes: line restoration and end-to-end restoration. Given a projected traffic demand, capacity and flow assignment is jointly optimized to find an optimal capacity placement. The problem can be formulated as a large-scale linear programming. The basis matrix can be readily factorized into an LU form by taking advantage of its special structure, which results in a substantial reduction on the computation time of the revised simplex method. A row generation and deletion mechanism is developed to cope with the explosive number of constraints for the end-to-end restoration-based networks. In self-healing networks, end-to-end restoration schemes have been considered more advantageous than line restoration schemes because of a possible reduction of the redundant capacity to construct a fully restorable network. A comparative analysis is presented to clarify the benefit of end-to-end restoration schemes quantitatively in terms of the minimum resource installation cost. Several networks with diverse topological characteristics as well as multiple projected traffic demand patterns are employed in the experiments to see the effect of various network parameters. The results indicate that the network topology has a significant impact on the required resource installation cost for each restoration scheme. Contrary to a wide belief in the economic advantage of the end-to-end restoration scheme, this study reveals that the attainable gain could be marginal for a well-connected and/or unbalanced network.