Cascading Impact of Lag on User Experience in Cooperative Multiplayer Games Eben Howardy, Clint Coopery, Mike P. Wittiey, Stephen Swinford*, Qing Yangy Department of Computer Sciencey, Department of Sociology and Anthropology* Montana State University {ehoward, clint.cooper, mwittie, qing.yang}@cs.montana.edu, [email protected] ABSTRACT player third person shooter (TPS) type game. Our results Playing cooperative games should be fun for everyone in- indicate that the change in QoE of one player due to their volved. Part of having fun in games is being able to perform lag has a cascading effect on the QoE of the other players. well, be immersed, and stay engaged [18, 30, 31]. These in- Understanding a player's QoE as a cascade function that dicators of experience are part of a game player's Quality of includes other users' network conditions provides a more Experience (QoE). complete understanding of the impact of lag in cooperative One of the primary causes of QoE degradation in online multiplayer games. This has the potential to improve lag games is lag, a general term for communication delay and mitigation strategies for multiplayer games and other group loss due to poor network conditions [7, 10, 12, 33, 34, 42]. applications. These strategies may include networking tech- niques such as prioritizing data center traffic for lagged users Current lag mitigation strategies are constructed using the 3 assumption that when evaluating the QoE of a player only using D , DCTCP, or similar protocols [3, 38]. Other strate- that player's network conditions impact their QoE [5]. gies might include improved player behavior prediction, bet- We have designed an experimental framework to study the ter server processing behavior, and improved time warp re- effects of lag on a group of players by measuring a gamut of sponses. subjective and objective QoE metrics. Our results indicate The rest of this paper is organized as follows. Section 2 that the change in QoE of one player due to their lag has a describe previous research related to user QoE in networked cascading effect on the QoE of the other players. applications. In Section 3 we described background on QoE Understanding a player's QoE as a cascade function that and collaborative applications. Section 4 described the method- includes other users' network conditions provides a more ology of our study and Section 5 presents its results. Finally, complete understanding of the impact of lag in cooperative we conclude in Section 6. multiplayer games. This has the potential to improve lag mitigation strategies for multiplayer games and other group 2. RELATED WORK applications. The effect of network latency and packet loss on the ex- perience of a single user is well studied. Previous research 1. INTRODUCTION shows that high latency, high jitter, and moderate loss neg- atively impact the experience of a single user [4, 5, 6, 12, 14, Playing cooperative games should be fun for everyone in- 19, 24, 25, 32]. The effects of high latency and high jitter volved. Part of having fun in games is being able to perform are most apparent when they occur together [6, 27]. well, be immersed, and stay engaged [18, 30, 31]. These in- Some types of online games have been shown to be more dicators of experience are part of a game player's Quality of sensitive to lag than other types [4, 5, 28]. In particular, Experience (QoE). Previous research has shown that a high First (and Third) Person Shooters tend to be the most sensi- QoE in a game makes it more likely that players will play tive. This is because they require the user to respond quickly the game longer which translates into increased profit for to input while directly controlling a single character. the game developer. The QoE of players in a multiplayer games has been pre- One of the primary causes of QoE degradation in online viously tested in four general ways. These ways are simula- games is lag, a general term for communication delay and tions of user interactions through artificial intelligence (AI) loss due to poor network conditions [7, 10, 12, 33, 34, 42]. bots in competitive tasks, observational studies in real-world Game developers and academic researchers both operate applications, in applications developed for the specific re- with the assumption that for any given individual in a group, search, and as by-products of studies interested in single that individual's QoE is affected only by their own network individuals' QoE [8, 9, 11, 16, 20, 21, 26, 27, 33, 41]. These condition and not the network conditions of the other group previous studies focused on the QoE of players with lag, but members [5, 7, 9, 10, 12, 32, 35, 37, 42, 43]. Current lag mit- did not directly examine the QoE of group members with igation strategies are constructed using this assumption [5]. no lag. We show that this assumption is not always correct. We have designed an experimental framework to study the effects of lag on a group of players by measuring a gamut 3. BACKGROUND of subjective and objective QoE metrics. Using this frame- In this section we describe properties of collaborative ap- work, we have studied player interactions in controlled group plications, how QoE can be quantified in such applications, sessions with Mass Effect 3, an online cooperative multi- and an overview of current lag mitigation techniques. 3.1 Cooperative Application Concentration is a measure of a player's ability to focus This study examined QoE during matches of Mass Effect on the game. When a player is focused completely on a 3, a popular video game available on both PCs and console task, then the Concentration measure is high. The subjec- game systems [2]. This game features a team of players tive metric for Concentration is a survey question asking who cooperate to defeat computer-controlled enemies. Mass the player to rate how Engaging the match was. A high Effect 3 uses a peer-to-peer (p2p) matchmaking system so Engaging rating indicates as high level of Concentration. no central server is directly involved in the game matches. The objective metric Concentration is an Attention rating There are several features of Mass Effect 3 which make it gathered by the EEG device [1]. When a player has a high ideal for cooperative gaming research. It encourages coop- level of Attention then their Concentration measure is high. eration between players but does not require it for the game The device used categorizes Attention into five levels: Low, goals to be reached. The game mode choice allows the exact Somewhat Low, Normal, Somewhat High, High [1]. same game character avatars, level map, and enemy distri- Enjoyment is a measure of a player's feelings of happi- bution to be chosen. In addition, Mass Effect 3 provides a ness during a match. A survey question asking the player to scoring system where each player gets an individual score at rate how Enjoyable the match was is used as the subjective the end of a match, allowing for an objective performance metric for Enjoyment, with a high player rating meaning measure. high Enjoyment. The objective metric for Enjoyment was Mass Effect 3 encourages cooperation through a two part Time Dilation which was established through a survey ques- combination attack system. In this system, one player can tion asking the player to indicate if the match felt shorter, use a power which will mark an enemy as the first step. longer, or about the same length of time compared to other When a second player uses a matching power on the marked matches. When a player indicates that a match was shorter enemy, a larger amount of damage is done than without than usual when it was not indicates that the player had a the marking process. This combination attack is a highly high Enjoyment. If the player indicates that a match was effective play strategy, but not the only means of overcoming longer than usual when it was not then they have a low the computer controlled enemies. Enjoyment for that match. Sense of Being is a measure of a player's feeling of being 3.2 QoE Defined part of the actual game world. When the player has a sus- Quality of Experience is a way to represent a player's pension of disbelief and the feeling that the character they emotional, cognitive, and behavioral responses to a specific control is a direct representation of themselves within the system [13, 22, 39]. QoE is multi-dimensional and repre- game world then the Sense of Being is high. The subjec- sents the player's total experience. The dimensions which tive metric for Sense of Being is a survey question asking are most important are dependent on the system studied. the player to rate how Immersive they found the match to In this study the systems analyzed are matches in a popular be. If the player finds the match to be highly Immersive multiplayer game, Mass Effect 3 [2]. then their Sense of Being is high. The objective metric for The QoE dimensions which are important in a multiplayer Sense of Being is a Calmness rating gathered by the EEG cooperative game are Concentration, Enjoyment, Sense of device, called Meditation by the device developers [1]. This Being, Performance Gains, and Exploratory Behaviors. Each Calmness rating is an indicator of what is commonly con- of these categories can be quantified through subjective and sidered a feeling of \oneness" or being \in the zone." When objective metrics associated with them, as described below. the Calmness rating is high then Sense of Being is high.