Distributed Virtual Environment Systems Research Articles

A Fairness Oriented Consistency Control Method in Distributed Virtual Environment Systems

The distributed virtual environment (DVE) system needs to adopt the consistency control method to maintain the state consistency of each node. Existing consistency control methods mainly consider how to increase the consistency of the whole system with a small overhead, and often neglect the important role of fairness in the virtual environment system. In fact, in many DVE systems, the fairness of all users in the virtual world plays an important role in the usability of the system. In this paper, we propose a new fairness-oriented consistency control method, which ensures the fairness of each node in terms of inconsistency degree by adjusting the ahead deviation time of each node, thus improving the usability of the system. Experimental results show that our method can significantly improve the fairness of the distributed virtual environment system.

A Dynamic Load Balancing Method in Hexagonal Cell Based DVE Systems

Load balancing problem is one of the key issues in multi-server architecture Distributed Virtual Environment (DVE) system. Existing load balancing methods are mainly for the one-dimensional cutting based DVE system or square cell based DVE system, and it is difficult to apply them to the hexagonal cell based DVE system. In this paper, we propose a new load balancing method for hexagonal cell virtual environment system, which can effectively reduce the number of overloaded server at run time and reduce the client migrations between servers by selecting appropriate migration cells. The experimental results show that the proposed method can effectively keep the load balance among servers in the virtual environment system based on hexagon partitioning and reduce the probability of client migrations, thus improving the usability of the system.

On designing and testing distributed virtual environments

SummaryDistributed real‐time (DRT) systems are among the most complex software systems to design, test, maintain, and evolve. The existence of components distributed over a network often conflicts with real‐time requirements, leading to design strategies that depend on domain‐specific and even application‐specific knowledge. Distributed virtual environment (DVE) systems are DRT systems that connect multiple users instantly with each other and with a shared virtual space over a network. DVE systems deviate from traditional DRT systemsx in the importance of the quality of the end user experience. We present an analysis of important, but challenging, issues in the design, testing, and evaluation of DVE systems through the lens of experiments with a concrete DVE, OpenSimulator. We frame our observations within six dimensions of well‐known design concerns: correctness, fault tolerance/prevention, scalability, time sensitivity, consistency, and overhead of distribution. Furthermore, we place our experimental work in a broader historical context, showing that these challenges are intrinsic to DVEs and suggesting lines of future research. Copyright © 2016 John Wiley & Sons, Ltd.

Reducing view inconsistency by predicting avatars' motion in multi-server distributed virtual environments

Multi-server distributed virtual environment (DVE) system contains a set of servers to support a huge number of geographically distributed users. It provides a shared virtual scenario where users represented by avatars participate in and interact with each other through networks. However, there are two technical challenges in designing an efficient DVE system with this architecture: view inconsistency caused by network delays and server overloading caused by uneven distribution of users. While the first problem affects users' perception of the VE and causes user disputes, the second problem affects the system response time. We present a new partitioning framework to simultaneously address the two problems, in which the DVE scenario is divided into a set of disjoint grids called virtual cells. The virtual cells are then indexed by a region quadtree and an algorithm is proposed to search the region quadtree so as to equally partition avatars among servers. By using probabilistic models to predict each avatar's future motion and the virtual region its area of interest (AOI) will cover, a density-based method is proposed to help each server construct avatar clusters from its local avatars in parallel. By estimating the possible distances of avatar's future position to the geometry center of each cluster, an algorithm is proposed to repartition non-assigned avatars among servers so as to reduce view inconsistency and preserve workload balancing among the servers. We conducted comprehensive experiments to evaluate our method by comparing it with other approaches.

On Delay Adjustment for Dynamic Load Balancing in Distributed Virtual Environments

Distributed virtual environments (DVEs) are becoming very popular in recent years, due to the rapid growing of applications, such as massive multiplayer online games (MMOGs). As the number of concurrent users increases, scalability becomes one of the major challenges in designing an interactive DVE system. One solution to address this scalability problem is to adopt a multi-server architecture. While some methods focus on the quality of partitioning the load among the servers, others focus on the efficiency of the partitioning process itself. However, all these methods neglect the effect of network delay among the servers on the accuracy of the load balancing solutions. As we show in this paper, the change in the load of the servers due to network delay would affect the performance of the load balancing algorithm. In this work, we conduct a formal analysis of this problem and discuss two efficient delay adjustment schemes to address the problem. Our experimental results show that our proposed schemes can significantly improve the performance of the load balancing algorithm with neglectable computation overhead.

An Effective Aggregation Method in Distributed Virtual Environments

The aggregation problem of the interest expressions is an important problem in distributed virtual environment (DVE). According to the characteristic of DVE system, a new aggregation method is proposed in this paper, which uses hierarchical bounding volume to aggregate threshold predicate and TAG-ID/MASK to aggregate equivalent predicate separately. Experimental results show that this method can effectively reduce storage of interest expressions and highly improve the system’s scalability.

M-GRASP: A GRASP With Memory for Latency-Aware Partitioning Methods in DVE Systems

A necessary condition for providing quality of service to distributed virtual environments (DVEs) is to provide a system response below a maximum threshold to the client computers. In this sense, latency-aware partitioning methods try to provide response times below the threshold to the maximum number of client computers as possible. These partitioning methods should find an assignment of clients to servers that optimizes system throughput, system latency, and partitioning efficiency. In this paper, we present a new algorithm based on greedy randomized adaptive search procedure with memory for finding the best solutions as possible to this problem. We take into account several different alternatives in order to design both the constructive phase and the local search phase of this multistart metaheuristic for combinatorial problems. Additionally, we enhance this basic approach with some intensification strategies that improve the efficiency of the basic search method. Performance evaluation results show that the new algorithm increases the performance provided by other metaheuristics when applied to solve the latency-aware partitioning problem in DVE systems.

A Task-Based Load Distribution Scheme for Multi-Server-Based Distributed Virtual Environment Systems

Multi-server-based distributed virtual environment (MSDVE) systems have become prevalent, supporting a large number of internet users. In MSDVEs, the load balancing among servers is an important issue to achieve system scalability. However, existing approaches must pay high migration overhead for the state transition of users or regions, thus the excessive holding time during load distribution makes it difficult for the system to keep the interactive performance acceptable. This paper aims to provide an efficient load distribution mechanism in which a group of servers takes charge of regions and shares region information among servers. The proposed mechanism dynamically classifies task types based on features of requested messages, and distributes each task fairly to neighboring servers. We have implemented the proposed mechanism extending our network framework for DVE, ATLAS, and our experiments show that the task distribution reduces both communication and processing overhead during load distribution without significant classification overhead.

A Latency-Aware Partitioning Method for Distributed Virtual Environment Systems

Distributed virtual environment (DVE) systems allow multiple users working on different client computer's interconnected through different networks to interact in a shared virtual world. In these systems, latency is crucial for providing an acceptable quality of service (QoS), since it determines how fast client computers are reported about changes in the shared virtual scene produced by other client computers. This paper presents in a unified manner a partitioning approach for providing a latency below a threshold to the maximum number of users as possible in DVE systems. This partitioning approach searches the assignment of avatars, which represents the best trade-off among system latency, system throughput, and partitioning efficiency when solving the partitioning problem. Evaluation results show that the proposed approach not only maximizes system throughput, but also allows the system to satisfy, if possible, any specific latency requirement needed for providing QoS. This improvement is achieved without decreasing either image resolution or quality of animation, and it can be used together with other techniques already proposed. Therefore, it can contribute to provide QoS in DVEs.

ATLAS: A Scalable Network Framework for Distributed Virtual Environments

A distributed virtual environment (DVE) is a software system that allows users in a network to interact with each other by sharing a common view of their states. As users are geographically distributed over large networks like the internet and the number of users increases, scalability is a key aspect to consider for real-time interaction. Various solutions have been proposed to improve the scalability in DVE systems but they are either focused on only specific aspects or customized to a target application. In this paper, we classify the approaches for improving scalability of DVE into five categories: communication architecture, interest management, concurrency control, data replication, and load distribution. We then propose a scalable network framework for DVEs, ATLAS. Incorporated with our various scalable schemes, ATLAS meets the scalability of a system as a whole. The integration experiences of ATLAS with several virtual reality systems ensure the versatility of the proposed solution.

A genetic approach for adding QoS to distributed virtual environments

Distributed virtual environment (DVE) systems have been designed last years as a set of distributed servers. These systems allow a large number of remote users to share a single 3D virtual scene. In order to provide quality of service in a DVE system, clients should be properly assigned to servers taking into account system throughput and system latency. The latter one is composed of both network and computational delays. This highly complex problem is known as the quality of service (QoS) problem. In this paper, we study the implementation of a genetic algorithm (GA) for solving the QoS problem in DVE systems. Performance evaluation results show that, due to its ability of both finding good search paths and keeping diversity, this nature inspired technique can provide significantly better solutions than other heuristic methods while requiring shorter execution times. Therefore, the proposed implementation of GA search method can actually improve the QoS offered by DVE systems.

An architecture to support scalable distributed virtual environment systems on grid

A Distributed Virtual Environment (DVE) system offers a computer-generated virtual world in which individuals located at different places in the physical world can interact with one another. In order to achieve real-time response for a large user base, DVE systems need to have a scalable architecture. In this paper, we present the design of a grid-enabled service oriented framework for facilitating the construction of scalable DVE systems on computing grids. A service component called "gamelet" is proposed, whose distinctive mark is its high mobility for supporting dynamic load sharing. We propose a gamelet migration protocol which can ensure the transparency and efficiency of gamelet migration, and an adaptive gamelet load-balancing (AGLB) algorithm for making gamelet redistribution decisions at runtime. The algorithm considers both the synchronization costs of the DVE system and network latencies inherent in the grid nodes. The activities of the users and the heterogeneity of grid resources are also considered in order to carry out load sharing more effectively. We evaluate the performance of the proposed mechanisms through a multiplayer online game prototype implemented using the Globus toolkit. The results show that our approach can achieve faster response times and higher throughputs than some existing approaches.

A comparison study of evolutive algorithms for solving the partitioning problem in distributed virtual environment systems

Fast Internet connections and the widespread use of high performance graphic cards are making Distributed Virtual Environment (DVE) systems very common nowadays. However, there are several key issues in these systems that should still be improved in order to design a scalable and cost-effective system. One of these key issues is the partitioning problem. This problem consists of efficiently assigning clients (3D avatars) to the servers in the system. In this paper, we present a comparison study of different modern heuristics for solving the partitioning problem in DVE systems, as an alternative to the ad hoc heuristic proposed in the literature. Performance evaluation results show that some of the heuristic methods can greatly improve the performance of the partitioning method, particularly for large DVE systems. In this way, efficiency and scalability of DVE systems can be significantly improved.

A comparison study of evolutive algorithms for solving the partitioning problem in distributed virtual environment systems

Fast Internet connections and the widespread use of high performance graphic cards are making Distributed Virtual Environment (DVE) systems very common nowadays. However, there are several key issues in these systems that should still be improved in order to design a scalable and cost-effective system. One of these key issues is the partitioning problem. This problem consists of efficiently assigning clients (3D avatars) to the servers in the system. In this paper, we present a comparison study of different modern heuristics for solving the partitioning problem in DVE systems, as an alternative to the ad hoc heuristic proposed in the literature. Performance evaluation results show that some of the heuristic methods can greatly improve the performance of the partitioning method, particularly for large DVE systems. In this way, efficiency and scalability of DVE systems can be significantly improved.

A performance study on multi-server DVE systems

The advance in computer graphics and networking technologies leads to the development of distributed virtual environment (DVE), which allow remote users to share information and interact within a common virtual environment via a local area network or through the Internet. A DVE system needs to perform many tasks, such as processing of clients’ requests, synchronization, object transmission and interaction handling. However, the workload induced by these tasks may become significant in a large scale DVE, where there may be a large number of concurrent users. In such situation, a single server based DVE system may not have enough processing power to maintain the system interactivity. To address this problem, a number of multi-server based DVE systems have been developed. In this paper, we explore how existing multi-server based DVE systems manage large scale DVEs and discuss various issues for implementing a multi-server based DVE system. We study the performance of such a system under various conditions via several simulated experiments.

An efficient partitioning algorithm for distributed virtual environment systems

Distributed virtual environment (DVE) systems model and simulate the activities of thousands of entities interacting in a virtual world over a wide area network. Possible applications for DVE systems are multiplayer video games, military and industrial trainings, and collaborative engineering. In general, a DVE system is composed of many servers and each server is responsible to manage multiple clients who want to participate in the virtual world. Each server receives updates from different clients (such as the current position and orientation of each client) and then delivers this information to other clients in the virtual world. The server also needs to perform other tasks, such as object collision detection and synchronization control. A large scale DVE system needs to support many clients and this imposes a heavy requirement on networking resources and computational resources. Therefore, how to meet the growing requirement of bandwidth and computational resources is one of the major challenges in designing a scalable and cost-effective DVE system. In this paper, we propose an efficient partitioning algorithm that addresses the scalability issue of designing a large scale DVE system. The main idea is to dynamically divide the virtual world into different partitions and then efficiently assign these partitions to different servers. This way, each server will process approximately the same amount of workload. Another objective of the partitioning algorithm is to reduce the server-to-server communication overhead. The theoretical foundation of our dynamic partitioning algorithm is based on the linear optimization principle. We also illustrate how one can parallelize the proposed partitioning algorithm so that it can efficiently partition a very large scale DVE system. Lastly, experiments are carried out to illustrate the effectiveness of the proposed partitioning algorithm under various settings of the virtual world.

Constructing communication subgraphs and deriving an optimal synchronization interval for distributed virtual environment systems

The author considers problems of constructing a communication subgraph and deriving an optimal synchronization interval for a distributed virtual environment system (DVE). In general, a DVE system is a distributed system which allows many clients who are located in different parts of the network to concurrently explore and interact with each other under a high resolution, 3D, graphical virtual environment. Each client in a DVE system is represented by an avatar in the virtual environment, and each avatar can move and interact freely in the virtual environment. There are many challenging issues in designing a cost-effective DVE system. The article addresses two important design issues, namely: 1) how to construct a communication subgraph which can efficiently carry traffic generated by all clients in a DVE system, and 2) how to guarantee that each participating client has the same consistent view of the virtual world. In other words, if there is an action taken by an avatar or if there is any change in the state of an object in the virtual world, every participating client will be able to view the change. To provide this consistent view, a DVE system needs to perform synchronization actions periodically, We present several algorithms for constructing a communication subgraph. In the subgraph construction, we try to reduce the consumption of network bandwidth resources or reduce the maximum delay between any two clients in a DVE system. Based on a given communication subgraph, we then derive the optimal synchronization interval so as to guarantee the view consistency among all participating clients. The derivation of the optimal synchronization interval is based on the theory of Markov chains and the fundamental matrix.

The sound dimension

Although the spotlight of virtual reality research has been on providing views of simulated scenes and objects, some researchers have chosen to study how to fool other senses: hearing, touch, and even smell, into perceiving what is not there. They have good reason: the virtual environments that are best at stimulating multiple senses are also best at evoking a feeling of presence and immersion. Next to sight, hearing is the sense on which people rely the most. So sounds, too, can play an extremely critical role in a distributed virtual environment (DVE). The virtual reality (VR) experience is more satisfying when sound adds to or reinforces other DVE information. The paper discusses the variety of sound in VR systems and considers the selection of software and hardware for these uses of audio in DVE systems.

Channeling the data flood

The distributed virtual environment (DVE) can provide a simulated environment in which two or more participants interact with each other and with their surroundings. In working with this software, users face great technological problems, of which the biggest are: managing the enormous data loads involved; negotiating the interplay of all the virtual objects, including the virtual people; and creating the most efficient network topologies possible for users' machines. At the network level, each occupant of a DVE system uses a computer that is connected to a local- or wide-area network (LAN or WAN), which provides information on the changing state of the virtual environment and its contents. The paper discusses the design of a distributed virtual environment and the use of the Internet.