9905546
Mean eccentricities of de Bruijn networks
Given a graph G = (V; E) we deene e(X), the mean eccentricity of a vertex X, as the average distance from X to all the other vertices of the graph. The computation of this parameter appears to be nontrivial in the case of the de Bruijn networks. In this paper we consider upper and lower bounds for e(X). For the directed de Bruijn network, we provide tight bounds as well as the extremal vertices which reach these bounds. These bounds are expressed as the diameter minus some constants. In the case of undirected networks, the computation turns out to be more diicult. We provide lower and upper bounds which diier from the diameter by some small constants. We conjecture that the vertices of the form a a have the largest mean eccentricity. Numerical computations indicate that the conjecture holds for binary de Bruijn networks with diameters up to 18. We prove that the asymptotic diierence, when the diameter goes to innnity, between the mean eccentricities of an arbitrary vertex and that of a a is smaller than a small constant tending to zero with the degree. We also provide a simple recursive scheme for the computation of the asymptotic mean eccentricity of the vertices a a. A by-product of our analysis is that in both directed and undirected de Bruijn networks, most of the vertices are at distance near from the diameter and that all of the mean eccentricities tend to the diameter when the degree goes to innnity.
14587701
Routing and Transmitting Problems in de Bruijn Networks
De Bruijn graphs, both directed and undirected, have received considerable attention as architecture for interconnection networks. In this paper, we focus on undirected de Bruijn networks of radix d and dimension 0, denoted by UB(d, 0). We first discuss the shortest-path routing problem. We present properties of the shortest paths between any two vertices of UB(d, 0) and propose two shortest-path routing algorithms, one of which has linear time complexity. Secondly, we study the transmitting problem. We establish a lower bound for the optimal transmitting time which implies in particular that the optimal transmitting problem is trivial for UB(d, 0) when d/spl ges/5. We present a transmitting scheme on undirected binary de Bruijn networks UB(2, n) with transmitting time n-1 for n/spl ges/5, and conjecture that the optimal transmitting time is n-1 for UB(2, n), and n for U8(3, n) and UB(4, n).
12816052
Worst-Case Analysis of Scheduling Heuristics of Parallel Systems
Abstract It is well-known that most scheduling problems arising from parallel systems are NP -hard, even under very special assumptions. Thus, various suboptimal algorithms, in particular heuristics, were proposed in the literature. Worst-case error bounds are established in this paper for heuristics of makespan minimization of parallel computations. Different parallel computation models are investigated, including interprocessor communication, task duplication, multiprocessor tasks and parallel tasks. Due to the heterogeneity of these systems, scheduling heuristics can be far away from the optimal solutions. These bounds provide insights to the design of scheduling heuristics in order to obtain good performance guarantee than arbitrary heuristics. Moreover, the performance guarantee of these special heuristics is independent of model parameters such as the maximal duplication degree, processor requirement and parallelization degree.
17293889
Complexity of Task Graph Scheduling with Fixed Communication Capacity
Consider a scheduling problem of parallel computations in multiprocessor systems. Let a parallel program be modeled by a task graph, where vertices represent tasks and arcs the communications between tasks. An interprocessor communication time incurs when two tasks assigned to two different processors have to communicate. Such a scheduling problem has recently been studied in the literature, mostly for the case where interprocessor communication times are fully determined. In this paper, we consider the scheduling problem with communication resource constraints. More specifically, we consider the case where all interprocessor communications take place on a network of bounded capacity. We consider two variants of the problem: communications with independent-data semantics and common-data semantics. We show that even for very specific subproblems, viz. scheduling of general graphs on two processors and scheduling of binary trees on an infinite number of processors, the minimization of the makespan of parallel programs in such a multiprocessor system is strongly -hard. We first establish the results for the case of capacity 1, referred to as the single-bus system. We then extend the results to the more general case of fixed communication capacities. As a consequence, the general scheduling problem of parallel programs with communication resource constraints is strongly -hard. These results are to be contrasted with the corresponding scheduling problems without contraint on the communication capacity, where the two-processor case has unknown time complexity and the infinite-processor case is polynomial. Our results are also extended to the case of broadcasting communications, and can be applied to multiprocessor systems with shared memory.
17231470
Dynamic scheduling of parallel computations
Structures of parallel programs are usually represented by task graphs in the scheduling literature. Such graphs are sometimes obtained at compile time. In many other cases, however, they can be determined only at run time. In this paper, we consider the scheduling of parallel computations whose task graphs are generated at run time. We analyze the case where the task graph is a random out-tree. When the number of offspring of a task has a geometric distribution whose parameter is decreasing and convex in the level, then the breadth-first policy stochastically minimizes the makespan. If, however, this parameter is increasing and concave, then the depth-first policy stochastically minimizes the makespan.
7929531
Closed-Form Deterministic End-to-End Performance Bounds for the Generalized Processor Sharing Scheduling Discipline
The Generalized Processor Sharing (GPS) schedulingdiscipline is an important scheduling mechanism that can support both class isolation and bandwidth sharing among different service classes, thus making itan appealing choice for networks providing multiple services with Quality-of-Service guarantees. In this paper, we study a broad classof GPS networks known as Consistent Relative Session Treatment}(CRST) GPS networks and establish closed-form end-to-end performance boundsfor CRST GPS networks. This result generalizes the results of Parekhand Gallager (1994) where simple, closed-form end-to-end performancebounds are derived for a special sub-class of CRST GPS networks, theso-called Rate Proportional Processor Sharing (RPPS) GPS networks, but performance bounds for the general CRST GPS networks do not haveclosed-form. Our result is obtained through the notion of CRSTpartition, which in fact yields a broader class of CRST GPS networksthan the one originally defined in (Parekh and Gallager, 1993). Moreover,our approach is quite general. It not only applies to the deterministicanalysis of GPS networks, but can also be employed in the study of GPSnetworks in a stochastic setting.
12789585
A Secure Agent Architecture for Sensor Networks
Advances in sensor and wireless technologies will enable thousands, or even millions of small but smart sensors to be deployed for a wide range of monitoring applications. The sensor networks must be dynamically configurable and adaptive in order to response actively to events. Security must also be built into sensor networks at the very initial design stage to prevent any potential threats. This paper presents a design of a generic software architecture based on tiny active agents for sensor networks. The architecture has built-in security components. This architecture will significantly benefit the dynamic and secure deployment and configuration tasks of large sensor networks for various applications.
4908568
Insignificant shadow detection for video segmentation
To prevent moving cast shadows from being misunderstood as part of moving objects in change detection based video segmentation, this paper proposes a novel approach to the cast shadow detection based on the edge and region information in multiple frames. First, an initial change detection mask containing moving objects and cast shadows is obtained. Then a Canny edge map is generated. After that, the shadow region is detected and removed through multiframe integration, edge matching, and region growing. Finally, a post processing procedure is used to eliminate noise and tune the boundaries of the objects. Our approach can be used for video segmentation in indoor environment. The experimental results demonstrate its good performance.
9219349
Flexible Secure Multicasting in Active Networks
In this paper we describe an alternative, flexible approach to multicast security in active networks. Traditional schemes for securing multicast communication have key management and scalability problems for many typical applications. In addition, traditional mechanisms are not capable of expressing flexible, situational security policies for multicast sessions and participants. Our scheme exploits the computational power of active networks to provide dynamic, flexible security for multicast applications. One of the main advantages of our scheme is the low communication and key distribution overhead associated with multicast group management. Our approach is based on the Seraphim security architecture implementation, which uses active capabilities for access control. Seraphim is an extensible, reconfigurable security architecture that is flexible and accommodates a wide variety of security policies and mechanisms. It also provides applications and users the ability to create dynamically and enforce highly customized and situational policies. Using these policies we have developed several secure multicast applications that demonstrate the flexible nature and low overheads associated with our architecture.
15783652
Low bit-rate video streaming for face-to-face teleconference
Face-to-face video teleconferencing is very important for real time communication. Current teleconferencing applications use standard video codecs, such as MPEG 1/2/4, for the compression of face video. Either a high bandwidth is required for high quality video transmission, or the transmitted face video is blurred at low bitrates. We present a system for real-time coding of face video at low bit-rate. There are two main contributions. First, we improve the technique of long term memory prediction by selecting frames into the database in an optimal way. A new frame is selected into the database only when it is significantly different from those frames which are already in the database. In this way, the database can cover a wider range of images. Second, we incorporate prior knowledge about faces into the long term memory prediction framework. The prior knowledge includes: (1) facial motions are repetitive such that most of them can be reconstructed from multiple reference frames; (2) different components of the face and the background can tolerate different levels of error because of different perceptual importance. Experiments show that, at similar PSNR, the proposed system works much faster and achieves better visual quality than the standard H.264/JVT codec.
20897828
A Formal Definition for Trust in Distributed Systems
Trust is an important concept in distributed computing environments and plays a critical role in ensuring and enhancing system security. Although various trust models have been proposed for distributed or pervasive computing systems, little research has been conducted to describe trust in precise and formal way. In this paper, we present a formal definition to express the meaning of trust in distributed computing systems. With this definition, we rigorously analyze the important properties of trust relation, such as reflexivity and conditional transitivity, and describe the trust relations in Role-Based Access Control. Our definition is comprehensive in that the semantic meaning of the trust definition is feasible to describe trust relations in both traditional distributed systems and dynamic pervasive computing environments. This research provides a solid base for formal trust reasoning and effective trust management implementation in distributed systems and pervasive computing environments.