Optimal assignment of distributed servers to virtual partitions for the provision of immersive voice communication in massively multiplayer games Cong Duc Nguyen * , Farzad Safaei, Paul Boustead Centre for Emerging Networks and Applications, University of Wollongong, Northfields Avenue, Level 1, Building 4, Wollongong, NSW 2522, Australia Available online 15 November 2005 Abstract This paper investigates the assignment of audio mixing operations to a geographically distributed set of servers to provide an immersive voice communication environment for massively multi-player online games. The immersive voice communication service enables each avatar to hear a spatially accurate audio mix of the conversations in its hearing range. There are three primary delivery architectures for this service, namely, peer- to-peer, central server, and distributed servers. We focus on a distributed server architecture, which partitions the virtual world into regions or locales and then assigns the computation associated with the creation of audio scenes for all avatars in each locale to a server. Our aim is to find the optimal way to partition the virtual world into locales and then choose the locale servers in such a way that reduces the total delay perceived by all avatars. We have produced a mathematical formulation for the optimal partitioning and server assignment and developed a heuristics approach based on a graph algorithm. We have developed a simulation environment that creates both the physical world (geographic distribution of participants and the Internet topology model) and the virtual world (distribution of avatars based on different avatar aggregation behaviors). We have solved the problem exactly as well as using the heuristics for a range of simulated virtual and physical worlds. In many cases, the heuristics results were within 5% of the optimal. Our algorithms and simulation study will be of benefit to future immersive voice communication service providers in the design of a cost effective delivery architecture for this service. q 2005 Elsevier B.V. All rights reserved. Keywords: Networked games; Immersive voice communication; Distributed servers; Server location optimization; Heuristics 1. Introduction Massively multi-player online games (MMOG) have become attractive applications over the Internet. Reliable estimates indicate that by 2009 more than 230 million people will be playing multiplayer network games [1]. We are interested in a category of MMOG that simulate a virtual world and each game player is represented as an avatar. Currently, despite the improvements in 3D graphics, com- munication between game participants is still limited and often based on text messages. We believe that these games will be far more attractive if immersive voice communications can be provided to game players. The immersive voice communi- cation service allows all the avatars to hear a realistic mix of voices of other avatars in their area of interest or hearing range. This audio scene is a personalised mix of voices of other avatars, spatially placed and attenuated according to distance to the listener. Although accuracy is desired, there is variation among the avatars about the importance of different audio signals. Let us define the interactive zone as the immediate vicinity of the avatar where active communicative interaction may take place, while the background zone as the region outside the interactive zone stretched to the limits of hearing range. In some cases, the user may only be interested in the interactive zone. The background audio scene, therefore, appears as ‘noise’ and can either be blocked or be simulated by a background chatter of suitable volume. However, there are situations where the multi-person voice communication is either the primary purpose of gathering in the virtual space or at least a very important means for achieving the actual goal. This will be particularly important for the genres of social games and interactive entertainment. Natural multi-person communi- cation is often characterised by the presence of multiple simultaneous conversations among the people gathered in an Computer Communications 29 (2006) 1260–1270 www.elsevier.com/locate/comcom 0140-3664/$ - see front matter q 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.comcom.2005.10.003 * Corresponding author. E-mail addresses: cong@titr.uow.edu.au (C.D. Nguyen), farzad@titr.uow. edu.au (F. Safaei), paul@titr.uow.edu.au (P. Boustead).