Large-scale Distributed Virtual Environments Research Articles

Object-Based Viewpoint For Large-Scale Distributed Virtual Environment

Distributed Virtual Environment (DVE) is a shared application consisting many objects, which can be accessed by many users. There have been many methods used to scale the DVE such as dividing simulation workload, dynamic load balancing among servers, and creating alternative architectures. However, they may not accommodate many objects and users. In this paper, we explore all approaches used to scale the DVE and then determine the characteristics of the existing approaches. With those characteristics, we compared existing approaches based on three parameters: the number of simulation per region, implementation, and the number of objects managed by simulator. The results show that all approaches use the same viewpoint, called present viewpoint, in developing the DVE. It views DVE as a world where all objects and activities are managed by a simulator. The results also show that this viewpoint contributes in terms of limitations of the current DVEs performance. In response to these results, we further propose a new viewpoint, called object-based viewpoint, to generate object-based simulators architecture. The experiment results show that our proposed architecture can provide a large scale DVE with better performances than the previous architectures.

Feasibility Study on Building Distributed Virtual Environment Using Mobile Agent Technology

Peer-to-peer (P2P) architectures have been proposed as an efficient and truly scalable solution for distributed virtual environments (DVEs). However, heavy and unbalanced network load has restricted the development of large scale DVEs. To solve this problem, this paper attempts to apply the mobile agent technology in DVEs. First, the virtual environment space was divided into a number of adjacent sub-spaces. Then, using the agent mobility, entities models moved themselves to the adjacent sub-space, and completed interactions with other entities in the sub-space. As a result, a significant part network load is transformed into local calculation load. The theoretical analysis results show that it is feasible and effective to ease the network communications bottleneck in the expansion of the DVEs.

Multi-objective zone mapping in large-scale distributed virtual environments

In large-scale distributed virtual environments (DVEs), the NP-hard zone mapping problem concerns how to assign distinct zones of the virtual world to a number of distributed servers to improve overall interactivity. Previously, this problem has been formulated as a single-objective optimization problem, in which the objective is to minimize the total number of clients that are without QoS. This approach may cause considerable network traffic and processing overhead, as a large number of zones may need to be migrated across servers. In this paper, we introduce a multi-objective approach to the zone mapping problem, in which both the total number of clients without QoS and the migration overhead are considered. To this end, we have proposed several new algorithms based on meta-heuristics such as local search and multi-objective evolutionary optimization techniques. Extensive simulation studies have been conducted with realistic network latency data modeled after actual Internet measurements, and different workload distribution settings. Simulation results demonstrate the effectiveness of the newly proposed algorithms.

An Interest Management Architecture by ALM and Region Partition for Large-Scale Distributed Virtual Environment

Large-Scale Distributed Virtual Environments (LSDVEs) must deal with the challenge of supporting a large number of users interacting on the Internet while keeping the communication among the parties synchronous and highly reactive. Interest Management is widely used to filter unnecessary messages in such systems. We present an interest management architecture that supports a large number of users on the Internet by dividing the virtual environment into multiple adjacent hexagonal regions. In the architecture, messaging among entities is based on a multi-server communication infrastructure by the Application Layer Multicasting (ALM). And each region is mapped to an ALM tree with a master node constructing the overlay tree and managing nodes that lie in its region. The interest area of each entity is composed of two parts: the inner hexagon and outer hexagon. By the two parts, inter-region as well as intra-region interaction is supported to maintain a continuous view without increasing too much traffic. In addition, the buffer borderlines near the boundaries are introduced to reduce the number of connections and disconnections that occur when a node frequently moves at the boundary of the regions without affecting the messaging of the entities and modifying the partition structure.

Greedy Algorithms for Client Assignment in Large-Scale Distributed Virtual Environments

Distributed Virtual Environments (DVEs) such as online games, military simulations or collaborative design are currently very popular. To support large-scale DVEs, an architecture with multiple, geographically distributed servers is usually employed, and the virtual world is partitioned into multiple zones for load distribution. The client assignment problem arises when assigning the participating clients to the servers. Current approaches usually assign clients to servers based on a resource-driven approach, i.e. the only concern is not to overload the servers. This approach may degrade the interactivity of DVEs if the network delay from a client to its assigned server is large. In this paper, we formulate the client assignment problem, and propose three new algorithms to assign clients to servers in a more efficient way. The proposed algorithms are based on the greedy heuristics developed for the well-known Terminal Assignment problem. From extensive simulation studies with realistic settings, we find that even under heterogeneous environments such as the Internet where accurate input data for the assignment algorithms are usually impractical to obtain, some of the proposed algorithms are still beneficial to the performances of DVEs.

A two-phase approach to interactivity enhancement for large-scale distributed virtual environments

Distributed virtual environments (DVEs) are distributed systems that allow multiple geographically distributed clients (users) to interact simultaneously in a computer-generated, shared virtual world. Applications of DVEs can be seen in many areas nowadays, such as online games, military simulations, collaborative designs, etc. To support large-scale DVEs with real-time interactions among thousands or even more distributed clients, a geographically distributed server architecture (GDSA) is generally needed, and the virtual world can be partitioned into many distinct zones to distribute the load among the servers. Due to the geographic distributions of clients and servers in such architectures, it is essential to efficiently assign the participating clients to servers to enhance users’ experience in interacting within the DVE. This problem is termed the client assignment problem (CAP) in this paper. We propose a two-phase approach, consisting of an initial assignment phase and a refined assignment phase to address the CAP. Both phases are shown to be NP-hard. Several heuristic assignment algorithms are then devised and evaluated via extensive simulations with realistic settings. We find that, even under heterogeneous environments like the Internet where accurate input data for the assignment algorithms are usually impractical to obtain, the proposed algorithms are still beneficial to the performances of DVE.

A network-centric approach to enhancing the interactivity of large-scale distributed virtual environments

In Distributed Virtual Environments (DVEs), where many simultaneous human users interact with each other in a shared, 3D virtual world, enhancing interactivity is of crucial importance, since the success of a DVE greatly depends on users’ perceptions. In general, enhancing interactivity for large-scale DVEs with thousands of geographically distributed users faces various challenges such as large Internet latencies, high resource demands and high potential of security threats. In this paper, we propose a network-centric approach to enhance the interactivity of DVEs by directly minimizing the network latencies in client–server communications. We consider two key problems: the server placement problem and the client assignment problem. We have formulated these two problems and proved that they are both NP-hard. We then adapt and develop several degree-based server placement approaches and greedy client assignment algorithms. Extensive simulation studies using realistic models have shown that our approach is very effective in enhancing the interactivity of large-scale DVEs.

Multiserver support for large-scale distributed virtual environments

CyberWalk is a distributed virtual walkthrough system that we have developed. It allows users at different geographical locations to share information and interact within a shared virtual environment (VE) via a local network or through the Internet. In this paper, we illustrate that as the number of users exploring the VE increases, the server will quickly become the bottleneck. To enable good performance, CyberWalk utilizes multiple servers and employs an adaptive region partitioning technique to dynamically partition the whole VE into regions. All objects within each region will be managed by one server. Under normal circumstances, when a viewer is exploring a region, the server of that region will be responsible for serving all requests from the viewer. When a viewer is crossing the boundary of two or more regions, the servers of all the regions involved will be serving requests from the viewer since the viewer might be able to view objects within all these regions. This is analogous to evaluating a database query using a parallel database server, which could improve the performance of serving a viewer's request tremendously. We evaluate the performance of this multiserver architecture of CyberWalk via a detail simulation model.

Time-space consistency in large-scale distributed virtual environments

Maintaining a consistent view of the simulated world among different simulation nodes is a fundamental problem in large-scale distributed virtual environments (DVEs). In this paper, we characterize this problem by quantifying the time-space inconsistency in a DVE. To this end, a metric is defined to measure the time-space inconsistency in a DVE. One major advantage of the metric is that it may be estimated based on some characteristic parameters of a DVE, such as clock asynchrony, message transmission delay, the accuracy of the dead reckoning algorithm, the kinetics of the moving entity, and human factors. Thus the metric can be used to evaluate the time-space consistency property of a DVE without the actual execution of the DVE application, which is especially useful in the design stage of a DVE. Our work also clearly shows how the characteristic parameters of a DVE are interrelated in deciding the time-space inconsistency, so that we may fine-tune the DVE to make it as consistent as possible. To verify the effectiveness of the metric, a Ping-Pong game is developed. Experimental results show that the metric is effective in evaluating the time-space consistency property of the game.

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.

Distributed virtual environments

Putting the reality into virtual environments requires much more than outstanding 3D computer graphics and effective human computer interaction. To be real, elements of the virtual environment must not only look realistic and provide natural interaction, they must move correctly, behave believably, and approximate the complexity of the real world. Individual computer systems cannot provide the necessary degree of complexity, scale, and fidelity, and probably never will. Achieving a representation of an environment that looks accurate and is accurate in its details requires multiple computer systems and distributed virtual environment (DVE) technology. The article provides an introduction to the major components of DVEs and the issues related to assembling large scale DVEs. DVE technology permits real time human interaction with complex environments based on 3D graphical descriptions that interact and move using models of real world physical constraints.