A distributed virtual environment (DVE) is a shared virtual environment where multiple users at their workstations interact with each other over a network. Some of these systems may support a large number of users, for example, multiplayer online games. An important issue is how well the system scales as the number of users increases. In terms of scalability, a promising system architecture is a two-level hierarchical architecture. At the lower level, multiple servers are deployed; each server interacts with its assigned users. At the higher level, the servers ensure that their copies of the virtual environment are as consistent as possible. Although the two-level architecture is believed to have good properties with respect to scalability, not much is known about its performance characteristics. In this paper, we develop a performance model for the two-level architecture and obtain analytic results on the workload experienced by each server. Our results provide valuable insights into the scalability of the architecture. We also investigate the issue of consistency and develop a novel technique to achieve weak consistency among copies of the virtual environment at the various servers. Simulation results on the consistency/scalability trade-off are presented.

Multi-player online games have become very pop- ular in the last few years. Meanwhile, the IEEE 802.11 wireless networks have been in wide use. In this paper, we present an experimental study on the capability of an IEEE 802.11g network in supporting multi-player online games. In particular, we focus on the highly interactive first-person-shooter games. We describe the test bed we set up and the experiments we performed. Factors such as the number of game clients and the amount of background traffic are examined. Our results show that the amount of background traffic has a significant impact on the latency and the loss ratio of the game traffic between the game clients and the game server, which in turn affect the observed game performance greatly. I. INTRODUCTION

virtual environment (DVE) have become increasingly popular. Many DVE implementations use a client-server architecture that requires the server to send the same data to all members of a collaborating or interacting group. This type of group communication operation is often implemented by sending data from the server to each recipient in a unicast fashion. The problem with this approach is that the cost of communication at the server does not scale very well with the number of participants because the application requires significant interaction with the operating system, network stack and drivers for each individual send. In this paper, we first propose a general analytic framework for predicting how group communication performance impacts DVE server capacity. We then conduct an experimental evaluation to determine the extent to which using a kernel-based group communication mechanism reduces the cost of group send operations. Lastly, we use the measurements obtained from these experiments to demonstrate how to apply the analytic framework by determining the extent to which the kernel-based group communication mechanism permits example applications to scale to more users.

A common requirement of many Internet services is to send exactly the same data to a number of hosts at the same time. Without IP-level multicast, this form of group communication is realized by unicasting the data to each desired host. Although this approach is portable and easy to implement, it is extremely inefficient for the sending host. In this paper, we propose a kernel-based technique to efficiently facilitate unicast send operations for group communication with only minimal additions to the sending operating system interface and implementation. We present the design and prototype implementation of our approach and experimentally demonstrate the significant performance improvements it provides. Additionally, we conduct experiments to decompose the processing costs in the network stack and show that the biggest cost reductions are not necessarily due to reduced memory copying.