Networked virtual environments (NVEs) have become a major trend in distributed computing, mainly due to the enormous popularity of multi-player online games in the entertainment industry. Nowadays, NVE systems are considered as the supporting technology for many networked and virtual organizations (NVO) (Singhal & Zyda, 1999), especially to those classified within the field of computer supported cooperative work (CSCW), where networked computer can be seen as a standard to provide the technological means to support the team design (Ott & Nastansky, 1997). These highly interactive systems simulate a virtual world where multiple users share the same scenario. The system renders the images of the virtual world that each user would see if he was located at that point in the virtual environment. Each user is represented in the shared virtual environment by an entity called avatar, whose state is controlled by the user through the client computer. Hundreds and even thousands of client computers can be simultaneously connected to the NVE system through different networks, and even through the Internet. NVE systems are currently used in many different applications (Singhal & Zyda, 1999) such as civil and military distributed training (Miller & Thorpe, 1995), collaborative design (Salles, Galli, Almeida et al., 1997) and e-learning (Bouras, Fotakis, & Philopoulos, 1998). Nevertheless, the most extended example of NVE systems are commercial multi-player online game (MOG) environments. These systems use the same simulation techniques that NVE systems do, and they are predicted to make up over 25 percent of local area network (LAN) traffic by 2010 (McCreary & Claffy, 2000).
TopIntroduction
Networked virtual environments (NVEs) have become a major trend in distributed computing, mainly due to the enormous popularity of multi-player online games in the entertainment industry. Nowadays, NVE systems are considered as the supporting technology for many networked and virtual organizations (NVO) (Singhal & Zyda, 1999), especially to those classified within the field of computer supported cooperative work (CSCW), where networked computer can be seen as a standard to provide the technological means to support the team design (Ott & Nastansky, 1997). These highly interactive systems simulate a virtual world where multiple users share the same scenario. The system renders the images of the virtual world that each user would see if he was located at that point in the virtual environment. Each user is represented in the shared virtual environment by an entity called avatar, whose state is controlled by the user through the client computer. Hundreds and even thousands of client computers can be simultaneously connected to the NVE system through different networks, and even through the Internet. NVE systems are currently used in many different applications (Singhal & Zyda, 1999) such as civil and military distributed training (Miller & Thorpe, 1995), collaborative design (Salles, Galli, Almeida et al., 1997) and e-learning (Bouras, Fotakis, & Philopoulos, 1998). Nevertheless, the most extended example of NVE systems are commercial multi-player online game (MOG) environments. These systems use the same simulation techniques that NVE systems do, and they are predicted to make up over 25 percent of local area network (LAN) traffic by 2010 (McCreary & Claffy, 2000).
Although centralized server (also client-server) architectures or peer-to-peer (P2P) architectures were also proposed for NVE systems (Singhal & Zyda, 1999), architectures based on multiple servers are becoming a de-facto standard for this type of distributed systems (Greenhalgh, Bullock, Frecon et al., 2001; Lui & Chan, 2002; Steed & Abou-Haidar, 2003). The reason for this trend is that on one hand, architectures based on a single, centralized server are not scalable with the number of connected clients (particularly for the most extended application of NVE systems, multi-player online games). On the other hand, NVE systems based on peer-to-peer architectures seems to be scalable enough, but they must still efficiently solve the awareness problem. This problem consists of ensuring that each avatar is aware of all the avatars in its neighborhood (Smith, Hixon, & Horan, 2001).
In multiple server architectures, servers must contain the current status of different 3D models, perform positional updates of avatars and transfer control information among different clients. Thus, each new avatar represents an increase in both the computational requirements of the application and the amount of network traffic. When the number of connected clients increases, the number of updating messages must be limited in order to avoid a message outburst. In this sense, different approaches have been proposed in order to limit the number of surrounding avatars that a given avatar must communicate with. Concepts like areas of influence (AOI) (Singhal & Zyda, 1999; Zou, Ammar, & Diot, 2001), locales (Anderson, Barrus, Howard et al., 1995) or auras (Greenhalgh et al., 2001) define a neighborhood area for avatars, in such a way that a given avatar must notify its movements (by sending a message) only to those avatars located in its neighborhood. Thus, the AOI of a given avatar determines the amount of network traffic generated by that avatar. Other approaches use three tiered architectures (Abrams, Watsen & Zyda, 1997; Lee & Lee, 2003) data filtering (Trefftz, Marsic, & Zyda, 2003) or distributed cache management (Capps, 2000) in order to minimize the impact of network traffic on the performance of the NVE systems.