Declan Delaney

On consistency and network latency in distributed interactive applications: a survey--part I

This paper is the first part of a two-part paper that documents a detailed survey of the research carried out on consistency and latency in distributed interactive applications (DIAs) in recent decades. Part I reviews the terminology associated with DIAs and offers definitions for consistency and latency. Related issues such as jitter and fidelity are also discussed. Furthermore, the various consistency maintenance mechanisms that researchers have used to improve consistency and reduce latency effects are considered. These mechanisms are grouped into one of three categories, namely time management, information management, and system architectural management. This paper presents the techniques associated with the time management category. Examples of such mechanisms include time warp, lock step synchronization, and predictive time management. The remaining two categories are presented in part II of the survey.

On Consistency and Network Latency in Distributed Interactive Applications: A Survey—Part II: Presence 2005: Eighth International Workshop I

This paper is the second part of a two part paper that documents a detailed survey of the research carried out on consistency and latency in distributed interactive applications (DIAs) in recent decades. Part I reviewed the terminology associated with DIAs and offered definitions for consistency and latency. A classification for consistency maintenance mechanisms was given and various mechanisms belonging to the first of three categories, time management, were described. Here, in the second part of the paper, the remaining two categories of mechanisms are examinedinformation management (such as predictive contract techniques, relevance filtering, packet bundling) and system architecture (such as QoS and protocols).

Exploring the use of local consistency measures as thresholds for dead reckoning update packet generation

Human-to-human interaction across distributed applications requires that sufficient consistency be maintained among participants in the face of network characteristics such as latency and limited bandwidth. Techniques and approaches for reducing bandwidth usage can minimize network delays by reducing the network traffic and therefore better exploiting available bandwidth. However, these approaches induce inconsistencies within the level of human perception. Dead reckoning is a well-known technique for reducing the number of update packets transmitted between participating nodes. It employs a distance threshold for deciding when to generate update packets. This paper questions the use of such a distance threshold in the context of absolute consistency and it highlights a major drawback with such a technique. An alternative threshold criterion based on time and distance is examined and it is compared to the distance only threshold. A drawback with this proposed technique is also identified and a hybrid threshold criterion is then proposed. However, the trade-off between spatial and temporal inconsistency remains.

Bounding Inconsistency Using a Novel Threshold Metric for Dead Reckoning Update Packet Generation

Human-to-human interaction across distributed applications requires that sufficient consistency be maintained among participants in the face of network characteristics such as latency and limited bandwidth. The level of inconsistency arising from the network is proportional to the network delay, and thus a function of bandwidth consumption. Distributed simulation has often used a bandwidth reduction technique known as dead reckoning that combines approximation and estimation in the communication of entity movement to reduce network traffic, and thus improve consistency. However, unless carefully tuned to application and network characteristics, such an approach can introduce more inconsistency than it avoids. The key tuning metric is the distance threshold. This paper questions the suitability of the standard distance threshold as a metric for use in the dead reckoning scheme. Using a model relating entity path curvature and inconsistency, a major performance related limitation of the distance threshold technique is highlighted. We then propose an alternative time—space threshold criterion. The time—space threshold is demonstrated, through simulation, to perform better for low curvature movement. However, it too has a limitation. Based on this, we further propose a novel hybrid scheme. Through simulation and live trials, this scheme is shown to perform well across a range of curvature values, and places bounds on both the spatial and absolute inconsistency arising from dead reckoning.

Game-state fidelity across distributed interactive games

Distributed interactive games offer players a three dimensional virtual world experience. Within this virtual world, players interact with each other and with their environment in real-time. They experience the same events, but from different viewpoints. As interactive games have evolved, they have driven the technologies underlying them. The distinguishing feature of any distributed interactive game is the network: the medium by which information is exchanged and shared between participants. The network impacts not only the design and development of distributed interactive games, but also their potential entertainment value. In this article, four of the fundamental networking issues and their effect on the design and operation of distributed interactive games will be discussed. In addition, a description of the different communication architectures used in distributed interactive games will be provided. Finally, as an illustrative example, these issues will be related to Unreal Tournament, a popular distributed interactive game.

Exploring the Spatial Density of Strategy Models in a Realistic Distributed Interactive Application

As Distributed Interactive Applications (DIAs) become increasingly more prominent in the video game industry they must scale to accommodate progressively more users and maintain a globally consistent worldview. However, network constraints, such as bandwidth, limit the amount of communication allowed between users. Several methods of reducing network communication packets, while maintaining consistency, exist. These include dead reckoning and the hybrid strategy-based modelling approach. This latter method combines a short-term model such as dead reckoning with a long-term strategy model of user behaviour. By employing the strategy that most closely represents user behaviour, a reduction in the number of network packets that must be transmitted to maintain consistency has been shown. In this paper a novel method for constructing multiple long-term strategies using dead reckoning and polygons is described. Furthermore the algorithms are implemented in an industry-proven game engine known as Torque. A series of experiments are executed to investigate the effects of varying the spatial density of strategy models on the number of packets that need to be transmitted to maintain the global consistency of the DIA. The results show that increasing the spatial density of strategy models allows a higher consistency to be achieved with fewer packets using the hybrid strategy-based model than with pure dead reckoning. In some cases, the hybrid strategy-based model completely replaces dead reckoning as a means of communicating updates.

Dealing with the Effect of Path Curvature on Consistency of Dead Reckoned Paths in Networked Virtual Environments

Collaboration and competition are important factors of Networked Virtual Environments (NVE). Both require a certain level of consistency in order for the interaction to be fruitful and compelling. However, finite network bandwidth and communication delay are key factors affecting this aspect of interactivity. A popular method in their mitigation for dynamic entities is the IEEE DIS standard dead reckoning mechanism [1].

Exploring the Effect of Curvature on the Consistency of Dead Reckoned Paths for Different Error Threshold Metrics

Dead reckoning is widely employed as an entity update packet reduction technique in distributed interactive applications (DIAs). Such techniques reduce network bandwidth consumption and thus limit the effects of network latency on the consistency of networked simulations. A key component of the dead reckoning method is the underlying error threshold metric, as this directly determines when an entity update packet is to be sent between local and remote users. The most common metric is the spatial threshold, which is simply based on the distance between a local users actual position and their predicted position. Other, recently proposed, metrics include the time-space threshold and the hybrid threshold, both of which are summarised within. This paper investigates the issue of user movement in relation to dead reckoning and each of the threshold metrics. In particular the relationship between the curvature of movement, the various threshold metrics and absolute consistency is studied. Experimental live trials across the Internet allow a comparative analysis of how users behave when different threshold metrics are used with varying degrees of curvature. The presented results provide justification for the use of a hybrid threshold approach when dead reckoning is employed in DIAs

Using Neural-Networks to Reduce Entity State Updates in Distributed Interactive Applications

Dead reckoning is the most commonly used predictive contract mechanism for the reduction of network traffic in Distributed Interactive Applications (DIAs). However, this technique often ignores available contextual information that may be influential to the state of an entity, sacrificing remote predictive accuracy in favour of low computational complexity. In this paper, we present a novel extension of dead reckoning by employing neural- networks to take into account expected future entity behaviour during the transmission of entity state updates (ESUs) for remote entity modeling in DIAs. This proposed method succeeds in reducing network traffic through a decrease in the frequency of ESU transmission required to maintain consistency. Validation is achieved through simulation in a highly interactive DIA, and results indicate significant potential for improved scalability when compared to the use of the IEEE DIS Standard dead reckoning technique. The new method exhibits relatively low computational overhead and seamless integration with current dead reckoning schemes.

Visibility Path-Finding in Relation to Hybrid Strategy-Based Models in Distributed Interactive Applications

The hybrid strategy-based modeling approach is a method for reducing the number of network packets that need to be transmitted to maintain global consistency in Distributed Interactive Applications. It combines a short-term model such as dead reckoning with a long-term strategy model. A key aspect of this approach is to determine strategies that users adopt in navigating the simulated environment to satisfy some objective or goal. Computer-generated artificial entities called BOTS, navigate by employing an Artificial Intelligence technique called path finding. This paper proposes using the A* path finding algorithm to automatically compute strategies that human users might take through the simulated environment. Since the A* algorithm operates on a graph representation of the environment and because of the real-time constraints imposed on Distributed Interactive Applications, the paper also carries out a comparative analysis of two extreme graph representations of the environment — a standard regular grid and a minimal grid representation. The comparison shows that the minimal grid leads to an order of magnitude reduction in real-time computation compared to the regular grid. In addition the paths computed using the minimal grid and the A* algorithm are used to determine strategy models as part of the hybrid strategy-based modeling approach. It is shown that this reduces the network traffic required to maintain global consistency of entity dynamics in two simulated environments.