A Survey of Routing Techniques for Mobile Collaborative Virtual Environment Applications <s> Client-Server-based CVE <s> Network-based virtual worlds allow multiple virtual worlds connected on a network to share information with one another. The development effort required to produce a network-based virtual world is quite large. The BrickNet toolkit simplifies this development by providing the standard facilities required by a wide range of network-based virtual worlds. It provides support for graphical, behavioral and network modeling of virtual worlds in an object-oriented fashion. BrickNet enables graphic objects to be maintained, managed, and used efficiently, and permits objects to be shared by multiple virtual worlds. In this paper, the architecture and implementation of BrickNet are described. <s> BIB001 </s> A Survey of Routing Techniques for Mobile Collaborative Virtual Environment Applications <s> Client-Server-based CVE <s> This paper describes the client-server design, implementation and experimental results for a system that supports real-time visual interaction between a large number of users in a shared 3D virtual environment. The key feature of the system is that server-based visibility algorithms compute potential visual interactions between entities representing users in order to reduce the number of messages required to maintain consistent state among many workstations distributed across a wide-area network. When an entity changes state, update messages are sent only to workstations with entities that can potentially perceive the change—i.e., ones to which the update is visible. Initial experiments show a 40x decrease in the number of messages processed by client workstations during tests with 1024 entities interacting in a large densely occluded virtual environment. <s> BIB002 </s> A Survey of Routing Techniques for Mobile Collaborative Virtual Environment Applications <s> Client-Server-based CVE <s> In this paper we make use of automatically generated logs of user activity from 6 meetings held using the MASSIVE-1 virtual reality tele-conferencing system to determine a number of characteristics of user movement and world transition. These results are applied to a consideration of four issues for CVE system design and resource requirements: the amount of network bandwidth and computation required to handle movement within worlds; the degree of look-ahead required when moving; whether world transitions by groups of participants could benefit from special handling (e.g. some form of multicast state-transfer); and whether caching of world state would be useful in these contexts. In each case the implications are quantified for the meetings analysed. <s> BIB003 </s> A Survey of Routing Techniques for Mobile Collaborative Virtual Environment Applications <s> Client-Server-based CVE <s> Complex virtual human representation provides more natural interaction and communication among participants in networked virtual environments, hence it is expected to increase the sense of being together within the same virtual world. We present a flexible framework for the integration of virtual humans in networked collaborative virtual environments. A modular architecture allows flexible representation and control of the virtual humans, whether they are controlled by a physical user using all sorts of tracking and other devices, or by an intelligent control program turning them into autonomous actors. The modularity of the system allows for fairly easy extensions and integration with new techniques making it interesting also as a testbed for various domains from “classic” VR to psychological experiments. We present results in terms of functionalities, example applications and measurements of performance and network traffic with an increasing number of participants in the simulation <s> BIB004 </s> A Survey of Routing Techniques for Mobile Collaborative Virtual Environment Applications <s> Client-Server-based CVE <s> We describe NetEffect, a highly-scalable architecture for developing, supporting and managing large, media-rich, 3D virtual worlds used by several thousand geographically dispersed users using low-end computers (PCs) and modems. NetEffect partitions a whole virtual world into communities, allocates these communities among a set of servers, and migrates clients from one server to another as clients move through the communities. It devotes special attention to minimizing the network traffic, in particular, the traftic that must go through servers. HistoryCity, a virtual world for children, has been developed on NetEffect and is currently being beta-tested for deployment in Singapore. <s> BIB005
In this section we present the main CVE systems that fall in the client-server and multi-server categories. Each avatar has to send an update message to the server, no matter how many users are participating in the CVE, and receive an update message from other nodes through the server. The major users of client server model are commercials multiplayer games; EverQuest  , and UltimaOnline BIB003 are such games worthy of mention. Usually, game designers choose to exhaust less effort on network performance in favor of enhancing graphics representation and sounds. Many other works have been proposed in the CVE literature BIB004 , BIB005 , BIB002 , and BIB001 for academic and commercial use. BrickeNet BIB001 provides a filtering technique to minimize the number of messages to be handled by each user. Compared to other existing architectures, BrickeNet introduces an interesting strategy of controlling shared objects. Instead of sharing the virtual environment, each node manages a local copy of the VE, and selects a set of objects to be shared with other users. This strategy offers the user full privacy, but only at the expense of collaboration activities, since users cannot access all the VE resources. NetEffect  was designed to simplify the development of network-based virtual worlds. A server master guarantees the load balancing among the set of available servers by migrating clients from one overloaded server to another one with less workload. In Ring system BIB002 , authors exploit a Potentially Visible Set (PVS) data structure to handle the scalability requirement. Unlike NetEffect, Ring uses a static virtual environment partition, where each user is represented by an entity rendered on every other computer participating in the network. Based on its current position, the user enjoys the leisure of selecting statistically which server to connect to. The key feature of the Ring system is that the serverbased visibility algorithms compute potential visual interactions between users in order to reduce the number of messages required to maintain consistency. Peer-to-peer architecture has been developed as an alternative solution to the Client-Server; next section describes some peer-topeer CVE applications.