Simultaneous Output-Timing Control in Networked Games and Virtual Environments
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In this chapter, we make a survey of techniques for simultaneous output-timing control, which adjusts the output timing of media streams among multiple terminals in networked games and virtual environments. When media units (MUs, each of which is the information unit, such as a video frame and a voice packet for media synchronization) are transmitted over non-guaranteed Quality of Service (QoS) networks like the Internet, the receiving times of each MU at the terminals may be different from each other owing to network delays and delay jitters. Therefore, for example, the fairness among players may be damaged in networked games, and collaborative work may not be done efficiently among users in virtual environments. It is important for multiple players/users to play/do networked games/collaborative work while watching the same displayed images simultaneously. To solve the problems, the simultaneous output-timing control, such as media synchronization control and causality control is needed. In this chapter, as the control, we mainly handle the group (or inter-destination) synchronization control, which is a type of media synchronization control, the adaptive Δ-causality control, and the dynamic local lag control. We also discuss the similarities and differences among the three types of control. Generally, the group synchronization control or adaptive Δ-causality control can be employed to keep the fairness and/or the consistency in good conditions among multiple terminals in networked games and virtual environments, and the dynamic local lag control is used for sound synchronization in networked virtual ensembles. However, the interactivity may seriously be deteriorated under such types of control. Therefore, we introduce prediction control to improve the interactivity. As a result of Quality of Experience (QoE) assessment, we demonstrate that the prediction control improves the interactivity and there is the optimum prediction time according to the network delay. Finally, we discuss the future directions of simultaneous output-timing control in networked games and virtual environments.
KeywordsQoS control Simultaneous output-timing control Networked game Virtual environment
The authors thank Prof. Hitoshi Watanabe of Tokyo University of Science for his valuable discussions.
QoS is the quality of service which is provided from a layer to its upper layer in network layer model and it is defined as how much the service is faithful to the ideal situation.
QoE is also called the user-level QoS. QoE is the quality which is perceived subjectively and/or experienced objectively by end-users.
MOS is a scale of the rating scale method and is obtained by averaging scores of all the subjects for each stimulus. MOS is one of subjective QoE measures. In the rating scale method for example, the five-grade quality scale or impairment scale is employed.
A network which does not guarantee the Quality of Service (QoS).
Therefore when packets are transmitted over the network, large network delays, delay jitter, and packet loss may occur.
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