Algebraic Coding and Digital Redundancy

The techniques of coding theory are used to improve the reliability of digital devices. Redundancy is added to the device by the addition of extra digits which are independently computed from the input digits. A decoding device examines the original outputs along with the redundant outputs. The decoder may correct any errors it detects, not correct but locate the defective logic gate or subsystem, or only issue a general error warning. Majority voting and parity bit checking are introduced, and computations are made for several binary addition circuits. A detailed summary of coding theory is presented. This includes a discussion of algebraic codes, binary group codes, nonbinary linear codes, and error locating codes.     

Second-Order Greedy Algorithms for Centralized Teleprocessing Network Design

We consider the problem of designing a centralized telecommunication network comprised of multipoint lines given a set of terminal locations, traffic requirements, and a common central site. The optimal solution to this problem is a capacitated minimal spanning tree. We develop a class of heuristic algorithms for the solution of this problem by imbedding existing heuristics, referred to as first-order greedy algorithms, inside a loop where small, carefully chosen sets of arcs are alternately forced in and out of the solution. The resultant procedure is shown to be superior to existing techniques, producing solutions typically 2 percent better, while requiring only a modest amount of additional computer time.     

Large-Scale Network Topological Optimization

A cost-effective structure for a large network is a multilevel hierarchy consisting of a backbone network and a family of local access networks. The backbone network is generally a distributed network, while the local access networks are typically centralized systems. In special cases, the network may consist primarily of either centralized or distributed portions. This paper discusses topological design problems for such systems, including the concentrator location problem, the terminal assignment problem, the terminal layout problem (the constrained minimum spanning tree problem), the distributed network topological layout problem, and the backbone node location problem. Recent algorithm research, including exact and heuristic problem solutions, are described and computational experience is given. Finally, open problems in large-scale topological design are reported.     

Statistical service assurances for traffic scheduling algorithms

Network services for the most demanding advanced networked applications which require absolute, per-flow service assurances can be deterministic or statistical. By exploiting the statistical properties of traffic, statistical assurances can extract more capacity from a network than deterministic assurances. We consider statistical service assurances for traffic scheduling algorithms. We present functions, so-called effective envelopes, which are, with high certainty, upper bounds of multiplexed traffic. Effective envelopes can be used to obtain bounds on the amount of traffic on a link that can be provisioned with statistical service assurances. We show that our bounds can be applied to a variety of traffic scheduling algorithms. In fact, one can reuse existing admission control functions for scheduling algorithms with deterministic assurances. We present numerical examples which compare the number of flows with statistical assurances that can be admitted with our effective envelope approach to those achieved with existing methods     

Delay and Overhead in the Encoding of Data Sources

A basic property of data sources in interactive applications is burstiness, i.e., short periods of activity followed by long idle periods. In these same applications message delay is the primary performance criterion. The combination of bursty flow and a delay criterion leads to a source encoding problem in which delay plays a central role. A salient feature of this problem is that there is a tradeoff between delay and the number of protocol bits required to represent the state of the source. A simple model of a bursty source is studied with the objective of understanding the relationship between coding efficiency and delay. Two encoding schemes, a block encoding technique and a technique employing flags, are examined in some detail. For both the block encoding and the flag schemes, a significant result is that as the source becomes less bursty, delay grows without bound. This result is obtained in spite of the fact that both schemes are reasonable and in the limit the encoding problem disappears. It also appears that the flag encoding technique has much smaller delay than block encoding.     

Throughput Analysis in Multihop CSMA Packet Radio Networks

In this paper, we use a Markov model to develop a product form solution to efficiently analyze the throughput of arbitrary topology multihop packet radio networks that employ a carrier sensing multiple access (CSMA) protocol with perfect capture. We consider both exponential and nonexponential packet length distributions. Our method preserves the dependence between nodes, characteristic of CSMA, and determines the joint probability that nodes are transmitting. The product form analysis provides the basis for an automated algorithm that determines the maximum throughput in networks of size up to 100 radio nodes. Numerical examples for several networks are presented. This model has led to many theoretical and practical extensions. These include determination of conditions for product form analysis to hold, extension to other access protocols, and consideration of acknowledgments.     

Routing in multidomain networks

Investigates the problem of management and control in a large and, for simplicity, homogeneous packet-switched network. Specifically the authors focus on routing, an important function of network management. The network consists of several individually controlled domains. Domains are interconnected via gateway links. Each domain is controlled by its own network control center, while the overall network performance is managed by an integrated network control center. Each center has only a portion of the information required for global routing. The authors investigate the impact of the reduced information available at each center on network performance (average delay in the present case). They present a general approach to designing a hierarchical algorithm for routing in multidomain networks. They propose a heuristic procedure suitable for packet-switched networks. Several numerical examples illustrate the impact of incomplete information on the network. Performance is compared with a lower bound obtained, which is not differentiating destinations in other domains. Therefore, for this bound, each domain is perceived as a single node in a simplified model of the network     

A Technique for Adaptive Routing in Networks

A two-level adaptive routing scheme for packet-switched computer communication networks is proposed and investigated. The first level is quasi-static and based on the global network status. The second level is dynamic with decisions being made at each node in an attempt to obtain the savings in average delay predicted by a multiserver model of the node. Simulations confirm the predicted improvement.     

Routing of voice and data in burst-switched networks

The static and centralized routing of voice and data traffic in burst switched networks is addressed. It is assumed that the routing allows random bifurcation in voice and data paths and preemptive priorities for voice requirements. A study is made of routing of voice only, by using a multicommodity flow model with linearized link losses and average network loss as a minimization objective. Solving the resulting linear program, it is observed that optimal routing strategies prefer to freeze a requirement at an early stage of its path rather than those requirements that are close to their destinations. A study is made of the voice-data interaction at the link level using an available fluid-flow model, and the combined link performance is translated as a maximum flow constraint on a link. This constraint may have undesirable effects on the voice, such as introducing routes with flow absorbing loops, and unfair freezing of some requirements. All conflicting multiple objectives and constraints are included in a linear programming formulation, and it is shown how parameters can be tuned to produce desirable voice and data paths     

An Algorithm for Evaluation of Throughput in Multihop Packet Radio Networks with Complex Topologies

The problem of analyzing the thoughput of packet radio networks with realistic topologies is considered. We present an algorithm for the solution of this problem and show that both the memory requirements and running time of this algorithm in practice grow polynomially with the size of the problem. Although in theory both can grow exponentially in the worst case, we offer computational experience with the procedure and show that for realistic topologies where connectivity is related to distance, the rate of growth is quadratic in the number of links. Even for regular grids, which are pathological in their symmetry, the rate of growth is only cubic in the number of links. We thus conclude that the procedure is effective for realistic topologies with up to several hundred nodes.