Showing papers on "Network topology published in 1997"

Adaptive clustering for mobile wireless networks

Abstract: This paper describes a self-organizing, multihop, mobile radio network which relies on a code-division access scheme for multimedia support. In the proposed network architecture, nodes are organized into nonoverlapping clusters. The clusters are independently controlled, and are dynamically reconfigured as the nodes move. This network architecture has three main advantages. First, it provides spatial reuse of the bandwidth due to node clustering. Second, bandwidth can be shared or reserved in a controlled fashion in each cluster. Finally, the cluster algorithm is robust in the face of topological changes caused by node motion, node failure, and node insertion/removal. Simulation shows that this architecture provides an efficient, stable infrastructure for the integration of different types of traffic in a dynamic radio network.

On the Design of Compliant Mechanisms Using Topology Optimization

Abstract: This paper presents a method for optimal design of compliant mechanism topologies. The method is based on continuum-type topology optimization techniques and finds the optimal compliant mechanism topology within a given design domain and a given position and direction of input and output forces. By constraining the allowed displacement at the input port, it is possible to control the maximum stress level in the compliant mechanism. The ability of the design method to find a mechanism with complex output behavior is demonstrated by several examples. Some of the optimal mechanism topologies have been manufactured, both in macroscale (hand-size) made in Nylon, and in microscale (<.5mm)) made of micromachined glass.

Modeling Internet topology

Abstract: The topology of a network, or a group of networks such as the Internet, has a strong bearing on many management and performance issues. Good models of the topological structure of a network are essential for developing and analyzing internetworking technology. This article discusses how graph-based models can be used to represent the topology of large networks, particularly aspects of locality and hierarchy present in the Internet. Two implementations that generate networks whose topology resembles that of typical internetworks are described, together with publicly available source code.

Routing in ad-hoc networks using minimum connected dominating sets

Abstract: We impose a virtual backbone structure on the ad-hoc network, in order to support unicast, multicast, and fault-tolerant routing within the ad-hoc network. This virtual backbone differs from the wired backbone of cellular networks in two key ways: (a) it may change as nodes move, and (b) it is not used primarily for routing packets or flows, but only for computing and updating routes. The primary routes for packets and flows are still computed by a shortest-paths computation; the virtual backbone can, if necessary provide backup routes to handle interim failures. Because of the dynamic nature of the virtual backbone, our approach splits the routing problem into two levels: (a) find and update the virtual backbone, and (b) then find and update routes. The key contribution of this paper is to describe several alternatives for the first part of finding and updating the virtual backbone. To keep the virtual backbone as small as possible we use an approximation to the minimum connected dominating set (MCDS) of the ad-hoc network topology as the virtual backbone. The hosts in the MCDS maintain local copies of the global topology of the network, along with shortest paths between all pairs of nodes.

Flexible update propagation for weakly consistent replication

Abstract: Bayou's anti-entropy protocol for update propagation between weakly consistent storage replicas is based on pair-wise communication, the propagation of write operations, and a set of ordering and closure constraints on the propagation of the writes. The simplicity of the design makes the protocol very flexible, thereby providing support for diverse networking environments and usage scenarios. It accommodates a variety of policies for when and where to propagate updates. It operates over diverse network topologies, including low-bandwidth links. It is incremental. It enables replica convergence, and updates can be propagated using floppy disks and similar transportable media. Moreover, the protocol handles replica creation and retirement in a light-weight manner. Each of these features is enabled by only one or two of the protocol's design choices, and can be independently incorporated in other systems. This paper presents the anti-entropy protocol in detail, describing the design decisions and resulting features.

Method and system for minimizing the connection set up time in high speed packet switching networks

Abstract: The present invention is directed to a high speed packet switching network and, in particular to a method and system for minimizing the time to establish a connection between an origin and a destination node. Due to high dynamicity of the traffic on transmission links, it is important to select a routing path according to a fully up-to-date information on all network resources. The simpler approach is to calculate a new path for each new connection request. This solution may be very time consuming because there are as many path selection operations as connection set up operations. On another hand, the calculation of paths based on an exhaustive exploration of the network topology, is a complex operation which may also take an inordinate amount of resources in large networks. Many of connections originated from a network node flow to the same destination network node. It is therefore possible to take a serious benefit in reusing the same already calculated paths for several connections towards the same node. The path calculated at the time the connection is requested is recorded in a Routing Database and updated each time a modification occurs in the network. Furthermore, alternate paths for supporting non-disruptive path switch on failure or preemption, and new paths towards potential destination nodes can be calculated and stored when the connection set up process is idle. These last operations are executed in background with a low processing priority and in absence of connection request.

A cluster-based approach for routing in dynamic networks

Abstract: The design and analysis of routing protocols is an important issue in dynamic networks such as packet radio and ad-hoc wireless networks Most conventional protocols exhibit their least desirable behavior for highly dynamic interconnection topologies We propose a new methodology for routing and topology information maintenance in dynamic networks The basic idea behind the protocol is to divide the graph into a number of overlapping clusters A change in the network topology corresponds to a change in cluster membership We present algorithms for creation of clusters, as well as algorithms to maintain them in the presence of various network events Compared to existing and conventional routing protocols, the proposed cluster-based approach incurs lower overhead during topology updates and also has quicker reconvergence The effectiveness of this approach also lies in the fact that existing routing protocols can be directly applied to the network --- replacing the nodes by clusters

A measurement-based admission control algorithm for integrated service packet networks

Abstract: Many designs for integrated services networks offer a bounded delay packet delivery service to support real-time applications. To provide a bounded delay service, networks must use admission control to regulate their load. Previous work on admission control mainly focused on algorithms that compute the worst case theoretical queueing delay to guarantee an absolute delay bound for all packets. In this paper, we describe a measurement-based admission control algorithm (ACA) for predictive service, which allows occasional delay violations. We have tested our algorithm through simulations on a wide variety of network topologies and driven with various source models, including some that exhibit long-range dependence, both in themselves and in their aggregation. Our simulation results suggest that measurement-based approach combined with the relaxed service commitment of predictive service enables us to achieve a high level of network utilization while still reliably meeting the delay bound.

Routing in ad hoc networks using a spine

Abstract: We present a two-level hierarchical routing architecture for ad hoc networks. Within each lower level cluster, we describe a self-organizing, dynamic spine structure to (a) propagate topology changes, (b) compute updated routes in the background, and (c) provide backup routes in case of transient failures of the primary routes. We analyze and bound the worst case of movements between upper level clusters to show that this hierarchical architecture scales well with network size.

Topology preservation in self-organizing feature maps: exact definition and measurement

Abstract: The neighborhood preservation of self-organizing feature maps like the Kohonen map is an important property which is exploited in many applications. However, if a dimensional conflict arises this property is lost. Various qualitative and quantitative approaches are known for measuring the degree of topology preservation. They are based on using the locations of the synaptic weight vectors. These approaches, however, may fail in case of nonlinear data manifolds. To overcome this problem, in this paper we present an approach which uses what we call the induced receptive fields for determining the degree of topology preservation. We first introduce a precise definition of topology preservation and then propose a tool for measuring it, the topographic function. The topographic function vanishes if and only if the map is topology preserving. We demonstrate the power of this tool for various examples of data manifolds.

Evaluation of multicast routing algorithms for real-time communication on high-speed networks

Abstract: Multicast (MC) routing algorithms capable of satisfying the quality of service (QoS) requirements of real-time applications will be essential for future high-speed networks. We compare the performance of all of the important MC routing algorithms when applied to networks with asymmetric link loads. Each algorithm is judged based on the quality of the MC trees it generates and its efficiency in managing the network resources. Simulation results over random networks show that unconstrained algorithms are not capable of fulfilling the QoS requirements of real-time applications in wide-area networks. Simulations also reveal that one of the unconstrained algorithms, reverse path multicasting (RPM), is quite inefficient when applied to asymmetric networks. We study how combining routing with resource reservation and admission control improves the RPM's efficiency in managing the network resources. The performance of one semiconstrained heuristic, MSC, three constrained Steiner tree (CST) heuristics, Kompella, Pasquale, and Polyzos (1992), constrained adaptive ordering (CAO), and bounded shortest multicast algorithm (BSMA), and one constrained shortest path tree (CSPT) heuristic, the constrained Dijkstra heuristic (CDKS) are also studied. Simulations show that the semiconstrained and constrained heuristics are capable of successfully constructing MC trees which satisfy the QoS requirements of real-time traffic. However, the cost performance of the heuristics varies. The BSMA's MC trees are lower in cost than all other constrained heuristics. Finally, we compare the execution times of all algorithms, unconstrained, semiconstrained, and constrained.

Method and apparatus for automatic protection switching

Abstract: A method and apparatus for generating first and second tree topologies for any source node in a network which can be represented as a node or an edge redundant graph, such that any node in the graph remains connected to the source node via at least one tree even after the failure of a node or an edge. This technique provides a recovery mechanism upon detection of a failure in a network.

An analytical delay model for RLC interconnects

Abstract: Elmore delay has been widely used to estimate interconnect delays in the performance-driven synthesis and layout of very-large-scale-integration (VLSI) routing topologies. For typical RLC interconnections, however, Elmore delay can deviate significantly from SPICE-computed delay, since it is independent of inductance of the interconnect and rise time of the input signal. Here, we develop an analytical delay model based on first and second moments to incorporate inductance effects into the delay estimate for interconnection lines under step input. Delay estimates using our analytical model are within 15% of SPICE-computed delay across a wide range of interconnect parameter values. We also extend our delay model for estimation of source-sink delays in arbitrary interconnect trees. We observe significant improvement in the accuracy of delay estimates for interconnect trees when compared to the Elmore model, yet our estimates are as easy to compute as Elmore delay. Evaluation of our analytical models is several orders of magnitude faster than simulation using SPICE. We also illustrate the application of our model in controlling response undershoot/overshoot and reducing interconnect delay through constraints on the moments.

A dynamic routing concept for ATM-based satellite personal communication networks

Abstract: Satellite systems are going to build a part of the future personal communications infrastructure. The first-generation candidates for satellite personal communication networks (S-PCN) will rely on low Earth orbit (LEO) and medium Earth orbit (MEO) constellations. A noticeable trend in this field is toward broadband services and the use of ATM. For LEO satellite systems employing intersatellite links (ISLs), this paper proposes an overall networking concept that introduces the strengths of ATM to their operation. The core of the paper is the design of a new routing scheme for the periodically time-variant ISL subnetwork, discrete-time dynamic virtual topology routing (DT-DVTR), and its ATM implementation. DT-DVTR works completely off line, i.e., prior to the operational phase of the system. In a first step, a virtual topology is set up for all successive time intervals of the system period, providing instantaneous sets of alternative paths between all source-destination node pairs. In the second step, path sequences over a series of time interval are chosen from that according to certain optimization procedures. An ATM-based implementation of DT-DVTR in LEO satellite ISL networks is presented with some emphasis on the optimization alternatives, and the performance in terms of delay jitter is evaluated for an example ISL topology.

Local search genetic algorithm for optimal design of reliable networks

Abstract: This paper presents a genetic algorithm (GA) with specialized encoding, initialization, and local search operators to optimize the design of communication network topologies. This NP-hard problem is often highly constrained so that random initialization and standard genetic operators usually generate infeasible networks. Another complication is that the fitness function involves calculating the all-terminal reliability of the network, which is a computationally expensive calculation. Therefore, it is imperative that the search balances the need to thoroughly explore the boundary between feasible and infeasible networks, along with calculating fitness on only the most promising candidate networks. The algorithm results are compared to optimum results found by branch and bound and also to GA results without local search operators on a suite of 79 test problems. This strategy of employing bounds, simple heuristic checks, and problem-specific repair and local search operators can be used on other highly constrained combinatorial applications where numerous fitness calculations are prohibitive.

Wavelength requirements in arbitrarily connected wavelength-routed optical networks

Abstract: Wavelength division multiplexed optical networks using wavelength routing (WRONs) represent the most promising solution for future high-capacity wide-area network applications. One of the crucial factors which will determine their feasibility is the number of wavelengths required to satisfy the network traffic demand. In this paper, we consider arbitrarily connected networks as physical topologies for WRONs. By analysing a large number of randomly generated networks, bounds on the network wavelength requirements are first evaluated as a function of the physical connectivity. The advantages achievable by multifiber connections and the consequence of single link failure restoration are then assessed for several existing or planned network topologies. The results can be used in the analysis and optimization of the WRON design.

ATM-based routing in LEO/MEO satellite networks with intersatellite links

Abstract: An asynchronous transfer mode (ATM)-based concept for the routing of information in a low Earth orbit/medium Earth orbit (LEO/MEO) satellite system including intersatellite links (ISLs) is proposed. Specific emphasis is laid on the design of an ATM-based routing scheme for the ISL part of the system. The approach is to prepare a virtual topology by means of virtual path connections (VPCs) connecting all pairs of end nodes in the ISL subnetwork for a complete period in advance, similar to implementing a set of (time dependent) routing tables. The search for available end-to-end routes within the ISL network is based on a modified Dijkstra (1959) shortest path algorithm (M-DSPA) capable of coping with the time-variant topology. With respect to the deterministic time variance of the considered ISL topologies, an analysis of optimization aspects for the selection of a path at call setup time is presented. The performance of the path search in combination with a specific optimization procedure is-by means of extensive simulations-evaluated for example LEO and MEO ISL topologies, respectively.

k-ary n-trees: high performance networks for massively parallel architectures

Abstract: The past few years have seen a rise in popularity of massively parallel architectures that use fat-trees as their interconnection networks. In this paper we study the communication performance of a parametric family of fat-trees, the k-ary n-trees, built with constant arity switches interconnected in a regular topology. Through simulation on a 4-ary 4-tree with 256 nodes, we analyze some variants of an adaptive algorithm that utilize wormhole routing with one, two and four virtual channels. The experimental results show that the uniform, bit reversal and transpose traffic patterns are very sensitive to the flow control strategy. In all these cases, the saturation points are between 35-40% of the network capacity with one virtual channel, 55-60% with two virtual channels and around 75% with four virtual channels. The complement traffic, a representative of the class of the congestion-free communication patterns, reaches an optimal performance, with a saturation point at 97% of the capacity for all flow control strategies.

Phylogenetic Reconstruction Using an Unsupervised Growing Neural Network That Adopts the Topology of a Phylogenetic Tree

Abstract: We propose a new type of unsupervised, growing, self-organizing neural network that expands itself by following the taxonomic relationships that exist among the sequences being classified. The binary tree topology of this neutral network, contrary to other more classical neural network topologies, permits an efficient classification of sequences. The growing nature of this procedure allows to stop it at the desired taxonomic level without the necessity of waiting until a complete phylogenetic tree is produced. This novel approach presents a number of other interesting properties, such as a time for convergence which is, approximately, a lineal function of the number of sequences. Computer simulation and a real example show that the algorithm accurately finds the phylogenetic tree that relates the data. All this makes the neural network presented here an excellent tool for phylogenetic analysis of a large number of sequences.

Scheduling and performance limits of networks with constantly changing topology

Abstract: A communication network with tine-varying topology is considered. The network consists of M receivers and N transmitters that, in principle, may access every receiver. An underlying network state process with Markovian statistics is considered that reflects the physical characteristics of the network affecting the link service capacity. The transmissions are scheduled dynamically, based on information about the link capacities and the backlog in the network. The region of achievable throughputs is characterized. A transmission scheduling policy is proposed that utilizes current topology state information and achieves all throughput vectors achievable by any anticipative policy. The changing topology model applies to networks of low-Earth orbit (LEO) satellites, meteor-burst communication networks, and networks with mobile users.

Computer system and computer-implemented process for simultaneous configuration and monitoring of a computer network

Abstract: A centralized computer network management system that simultaneously configures and monitors computers in a computer network. The computer network management system allows a network administrator to describe the computer network according to both its physical topology and logical groups of machines and users in the computer network. Each network device, computer, or groups of network devices and computers can be associated with one or more rules which define configuration parameters. These rules may be inherited by machines through both the physical and logical relationships of the machines in the computer network. This centralized administration of the computer network simplifies the task of configuring and monitoring the network. All machines in the computer network also can be configured or monitored simultaneously.

Filtering postures: local enforcement for global policies

Abstract: When packet filtering is used as a security mechanism, different routers may need to cooperate to enforce the desired security policy It is difficult to ensure that they will do so correctly We introduce a simple language for expressing global network access control policies of a kind that filtering routers are capable of enforcing We then introduce an algorithm that, given the network topology, will compute a set of filters for the individual routers; these filters are guaranteed to enforce the policy correctly Since these filters may not provide optimal service, a human must sometimes alter them A second algorithm compares a resulting set of filters to the global network access control policy to determine all policy violations, or to report that none exist A prototype implementation demonstrates that the algorithms are efficient enough to give quick answers to questions of realistic scale

Interconnect design for deep submicron ICs

Abstract: In this paper, we study the interconnect layout optimization problem under a higher-order RLC model to optimize not just delay, but also waveform for RLC circuits with non-monotone signal response. We propose a unified approach that considers topology optimization, wiresizing optimization, and waveform optimization simultaneously. Our algorithm considers a large class of routing topologies, ranging from shortest-path Steiner trees to bounded-radius Steiner trees and Steiner routings. We construct a set of required-arrival-time Steiner trees or RATS-trees, providing a smooth trade-off among signal delay, waveform, and routing area. Using a new incremental moment computation algorithm, we interleave topology construction with moment computation to facilitate accurate delay calculation and evaluation of waveform quality. Experimental results show that our algorithm is able to construct a set of topologies providing a smooth trade-off among signal delay, signal settling time, voltage overshoot, and routing cost.

A protocol for scalable loop-free multicast routing

Abstract: In network multimedia applications such as multiparty teleconferencing, users often need to send the same information to several (but not necessarily all) other users. To manage such one-to-many or many-to-many communication efficiently in wide-area internetworks, it is imperative to support and perform multicast routing. Multicast routing sends a single copy of a message from a source to multiple receivers over a communication link that is shared by the paths to the receivers. Loop-freedom is an especially important consideration in multicasting because applications using multicasting tend to be multimedia and bandwidth intensive, and loops in multicast routing duplicate looping packets. We present and verify a new multicast routing protocol, called multicast Internet protocol (MIP), which offers a simple and flexible approach to constructing both group-shared and shortest-paths multicast trees. MIP can be sender-initiated or receiver-initiated or both; therefore, it can be tailored to the particular nature of an application's group dynamics and size. MIP is independent of the underlying unicast routing algorithms used. MIP is robust and adapts under dynamic network conditions (topology or link cost changes) to maintain loop-free multicast routing. Under stable network conditions, MIP has no maintenance or control message overhead. We prove that MIP is loop-free at every instant, and that it is deadlock-free and obtains multicast routing trees within a finite time after the occurrence of an arbitrary sequence of topology or unicast changes.

Distributed route server

Abstract: A design for a network route server in which network routing functions are distributed throughout the processing elements that constitute a switching node, while maintaining the global identity and routing information exchange functions of a route server element (RSE). Intelligent line-cards are provided having the ability to route independently of the RSE. This removes the RSE as a bottleneck resource and ensures that the capacity of the switching node is limited only by the switching capacity of its switch fabric. The RSE serves the functions of network topology discovery and routing table construction using a network topology database and an optimal routing algorithm. Copies of the dynamically maintained routing tables are distributed to the intelligent line-cards on a periodic basis governed by predetermined criteria. Wider geographical distribution of the RSE is enabled and most efficient utilization of the switch fabric is ensured. Scaling of distributed switching architectures is also enabled. The advantage is a significant increase in switching capacity as well as an increased degree of network connectivity.

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 the network and must be assigned a channel along each link in its route. The load /spl lambda//sub max/ of a set of lightpath requests 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. We propose ring networks with a fixed wavelength conversion capability within the nodes that can support all lightpath request sets with load /spl lambda//sub max/ at most W-1, where W is the number of wavelengths in each link. We also propose ring networks with selective pairwise wavelength conversion capability within the nodes that can support all lightpath request sets with load /spl lambda//sub max/ at most W. We also propose a star network with fixed wavelength conversion capability at its hub node that can support all lightpath request sets with load /spl lambda//sub max/ at most W. We extend this result to tree networks and networks with arbitrary topologies. These results show that significant improvements in traffic-carrying capacity can be obtained in WDM network by providing a very limited wavelength conversion capability within the network.

Efficient optimization of all-terminal reliable networks, using an evolutionary approach

Abstract: The use of computer communication networks has been rapidly increasing in order to: (1) share expensive hardware and software resources, and (2) provide access to main system from distant locations. The reliability and cost of these systems are important and are largely determined by network topology. Network topology consists of nodes and the links between nodes. The selection of optimal network topology is an NP-hard combinatorial problem so that the classical enumeration-based methods grow exponentially with network size. In this study, a heuristic search algorithm inspired by evolutionary methods is presented to solve the all-terminal network design problem when considering cost and reliability. The genetic algorithm heuristic is considerably enhanced over conventional implementations to improve effectiveness and efficiency. This general optimization approach is computationally efficient and highly effective on a large suite of test problems with search spaces up to 2/spl middot/10/sup 90/.

Lower bounds for the broadcast problem in mobile radio networks

Abstract: In this paper, we prove a lower bound on the number of rounds required by a deterministic distributed protocol for broadcasting a message in radio networks whose processors do not know the identities of their neighbors. Such an assumption captures the main characteristic of mobile and wireless environments [3], i.e., the instability of the network topology. For any distributed broadcast protocol II, for any n and for any D ≤ n/2, we exhibit a network G with n nodes and diameter D such that the number of rounds needed by H for broadcasting a message in G is Ω(D log n). The result still holds even if the processors in the network use a different program and know n and D. We also consider the version of the broadcast problem in which an arbitrary number of processors issue at the same time an identical message that has to be delivered to the other processors. In such a case we prove that, even assuming that the processors know the network topology, Ω(n) rounds are required for solving the problem on a complete network (D=1) with n processors.

Optimal coordination of directional overcurrent relays considering dynamic changes in the network topology

Abstract: This paper presents a method to consider the dynamic changes in the network's topology for the coordination of directional overcurrent relays using linear programming The proper coordination constraints are included by using linear approximations for the relay dynamics The application of the methodology as well as the importance of considering the transient configuration changes are illustrated with a practical example and a test case consisting of a real industrial power system

Scheduling nonuniform traffic in a packet-switching system with small propagation delay

Abstract: A new model of nonuniform traffic is introduced for a single-hop packet-switching system. This traffic model allows arbitrary traffic streams subject only to a constraint on the number of data packets which can arrive at any, individual source in the system or for any individual destination in the system over time periods of specified length. The nonuniform traffic model is flexible enough to cover integrated data networks carrying diverse classes of data. The system model is rather general, and includes passive optical star wavelength-division networks. Transmission algorithms are introduced for a single-hop packet-switching system with such nonuniform traffic and with propagation delay that is negligible relative to the packet length. The algorithms are based on collision-free scheduling of packets using graph-matching algorithms since the global state of the system is known to all stations at any time.