Data-driven biped control Figure 1: Our data-driven controller allows the physically-simulated biped character to reproduce challenging motor skills captured in motion data.We present a dynamic controller to physically simulate underactuated three-dimensional full-body biped locomotion. Our datadriven controller takes motion capture reference data to reproduce realistic human locomotion through realtime physically based simulation. The key idea is modulating the reference trajectory continuously and seamlessly such that even a simple dynamic tracking controller can follow the reference trajectory while maintaining its balance. In our framework, biped control can be facilitated by a large array of existing data-driven animation techniques because our controller can take a stream of reference data generated on-thefly at runtime. We demonstrate the effectiveness of our approach through examples that allow bipeds to turn, spin, and walk while steering its direction interactively. Physically simulating under-actuated biped locomotion has been a notorious challenge in computer graphics for two decades. Most of early biped controllers were either manually designed and handtuned or relying on optimization with energy-minimizing objectives. Though some of those controllers are very robust, they tend to result in stereotyped gaits often looking robotic and lifeless. SIMBICON is an exemplar of manually-crafted biped controllers, which is simple, easy-to-implement, and remarkably robust [Yin et al. 2007] . Its robustness allowed it to be employed in further challenges such as controller adaptation , composition [Coros et al. 2009 ] and stepping planning . However, it is also true that SIMBICON generates stereotyped, marching-like gaits.. Recently, motion capture data were employed to achieve natural and realistic locomotion from physically based controllers. Reference-tracking controllers pose yet other challenges such as collecting physically-feasible training data and developing robust feedback control algorithms for the tracking of reference trajectories and the maintenance of balance.. Our goal is to build full-body, three-dimensional locomotion controllers those are as simple and robust as SIMBICON, and still can faithfully reproduce natural and realistic locomotion guided by reference motion capture data. Our data-driven controllers can generate a variety of locomotor behaviors, such as turning and spinning. The key challenge of our approach is modulating a continuous stream of reference data in a seamless way while synchronizing with forward dynamic simulation. Reference-tracking controllers often fail when the swing foot lands on the ground earlier/later than the reference data indicates. Because the ground reaction force is the only source of control to balance under-actuated bipeds, unexpected changes in ground contacts could easily drive the controllers to unrecoverable failure states. In this paper, we show that carefully synchronizing the reference trajectory and the simulated biped at contact changes in a feedback loop allows us to achieve both the robustness of feedback controllers and the quality of motion capture data simultaneously.. The biggest advantage of our approach is that physically based control can be facilitated by a large array of existing data-driven animation techniques. Our biped controllers are equipped with a datadriven animation engine at the front end. The data-driven engine generates a sequence of movement patterns by editing [Lee and Shin 1999; Kim et al. 2009 ], blending [Rose et al. 1998 ], retargetting [Gleicher 1998 ], and composing [Lee et al. 2002; Kovar et al. 2002] motion fragments in the database. In this framework, the role of dynamic controllers can be greatly simplified, that is, tracking reference trajectories. We will demonstrate the effectiveness of our approach through examples that allow bipeds to turn, spin, and walk while steering its direction interactively. Biped control requires two essential mechanisms for shaping trajectories and robust balancing/tracking control. Our work is perhaps emphasizing the importance of trajectory shaping. Motion capture reference data allowed our controller to generate realistic human locomotion. Even balance control was achieved in a datadriven manner by modulating reference trajectories. Presumably, advanced control methodologies would improve our work in several directions. Regression-based approaches [Sok et al. 2007 ] would allow us to represent natural variations of locomotion in statistical models, which would cope with variations in environments and simulation conditions. Advanced optimal control methods, such as LQR [da Silva et al. 2008a] and NQR [Muico et al. 2009 ], would allow less stiff systems for tracking control. Even with such expected advantages, we were unable to employ sophisticated control methods because those methods require all reference data be prepared for preprocessing. No reference data generated on-the-fly could be fed into the controller. This restricts the flexibility and versatility of biped control. Designing robust model-free controllers would be an important advance in biped control. We can also think of online model learning that builds a control model incrementally at runtime.. Our controller is more robust if it sacrifices its motion quality by maintaining the direction of the stance foot to match the angle of the ground surface while in contact. It means our controller becomes less robust with more natural stance phases including heel-strike, midstance, and toe-off. In our push experiments, a controller with its stance foot angle fixed with respect to the ground tends to withstand stronger pushes by 10N to 20N. A similar observation was reported by Wang et al. [2009] . The loss of robustness is probably related to inaccurate modeling of the foot. Since our foot model is rigid, it usually have a small contact region on which ground reaction forces are applied. This makes the stance foot to wobble in the simulation. More realistic foot models might improve the robustness of our controller.. Ideally, reference motion data should be physically feasible for best tracking performance, though motion capture data are in general physically imprecise. Some of previous approaches [Sok et al. 2007; Muico et al. 2009 ] preprocessed motion data to make them physically feasible via spacetime optimization. Spacetime optimization of three-dimensional, full-body motion data is notorious for its challenging nature of numerical instability and heavy computational burden. We did not employ such optimization in our ex-129:6 • Y. Lee et al. periments because our feedback rules worked effectively with our test data. However, we suspect that optimized reference data would allow our controller to be more robust and to follow the reference data more precisely. The optimization of reference data would be particularly important if motion data need to be warped, blended, and retargetted.. We have experimented mostly with locomotion data. Controlling and simulating a wider spectrum of human motions will be an exciting avenue for future research. We anticipate that we will see compelling biped characters equipped with a variety of motor skills spanning from low-energy locomotion to highly-dynamic dancing and athletic skills that build on data-driven control techniques such as those presented here. All motion data in our experiments were originally captured using a Vicon optical motion capture system at the rate of 120 frames/second and then down-sampled to 30 frames/second. Motion data include walking in a variety of different speeds, turning angles and styles. We also captured sharp U-turning and spinning. We lifted up the swing foot trajectory slightly in the motion data of turning, spinning and interactive control examples which have low step height. We set dynamics and integrator parameters to achieve robust simulation in a conservative manner while maintaining the performance of realtime simulation at the rate of 30 frames/second. The ground reaction is modeled as a damped spring. Parameters were manually tuned for each motion data. Parameter tuning was not formidable because each parameter has an intuitive meaning and many of gains are simply constant for all motion data. Most of motion data can be stably reproduced for a wide range of parameter choices excepting for several very challenging examples, such as spinning, which requires careful parameter tuning (see the accompanied video). Most of our controllers generated stable cycles and became resilient to mild pushes without position feedbacks, that is, c2 = c4 = 0. Without external perturbation, the velocity feedback alone allows the reference trajectory to be followed closely and thus the position feedback has nothing to do with balancing. However, external pushes would make the simulation to deviate from the reference trajectory and the position feedback would play an important role. Therefore, we first tune c1 and c3 to achieve stable cycles without any external perturbation and then tune c2 and c4 later in the presence of random pushes at the center of mass.. Locomotion Control. Our biped character is able to reproduce various gaits of human walking (see Figure 6 ). Each motion clip recorded a subject standing still, starting to walk, taking 6 to 8 steps, Figure 6 : Data-driven biped simulation from motion capture data including (top to bottom) WalkForwardNormal, WalkBrisk, WalkMarch, WalkBackward, U-turn, and Spin.. and then stopping. Representing motion data as a motion graph allows us to produce an arbitrarily long sequence of locomotion by splicing walking steps. Our biped can track an arbitrary combination of locomotion steps including slow/fast walking, turning of different angles, and different gaits.. Effects of Individual Components. Our controller consists of several components for balancing, tracking, and synchronization. Disabling any of these components would result in either falling over in several steps or the degradation in motion quality. We evaluated the effect of each component by disabling each one at a time for a variety of walking data:. • Disabling synchronization always leads to falling over in 3 to 6 steps.. • Disabling the feedback on a stance hip makes the torso to lean and eventually leads to falling over in 2 to 4 steps.. • Disabling the feedback on a swing hip makes the character to lean to one side and eventually leads to falling over in 6 to 10 steps.. • Disabling the feedback on a stance ankle or swing foot height managed to avoid falling over for some gaits, but the motion looks unnatural.. Robustness under Various Conditions. We tested our walking controller on varied simulation conditions to evaluate its robustness (see Figure 7) . Our controller generated stable cycles of walking for up to 15 Kg of extra weight on one leg, 50% longer legs, 50% shorter legs, one leg 3% shorter than the other, up and down slopes up to 6 and 4 degrees respectively, 60% to 1200% variations of friction coefficients. These numbers were acquired from the same reference data and the same parameters with those used in walking examples. The limits can be significantly improved if we adapt the Interactive Control. Our model-free approach allows us to blend a set of motion data on-the-fly and feed inbetween data to the controller. The feedback gains are also interpolated at the same ratio as motion data. The motion set includes turning in 90, 135, and 180 degrees, straight walking at normal/fast speeds and two different gaits. Motion blending and transitioning are computed in the datadriven animation engine and our controller simply tracks a stream of reference data generated at the animation engine. Our biped character can steer in arbitrary turning angles, change its speed, and make transition between different gaits. The user can control the biped character interactively by specifying walking direction, speed and a type of gaits through simple user interfaces. Our controller allows the biped to respond to external perturbations, such as intentional pushes and collision with stacked boxes (see Figure 8 ). Biped controllers have extensively been explored in computer graphics and robotics. Hodgins and her colleagues presented manually designed biped controllers for highly-dynamic athletic motor skills, such as running, jumping, and bicycling [Hodgins et al. 1995] . Their controllers were equipped with finite state machines for phase transition control and a feedback balancing mechanism based on step placements. van de Panne and his colleagues have extensively studied the design of biped and quadruped controllers. Most notably among them, SIMBICON is a robust, threedimensional walking controller employing a series of key-poses to shape a reference trajectory and a step-based feedback loop to follow the reference trajectory [Yin et al. 2007 ]. Robust walking controllers can also be acquired by approximating bipeds with simplified inverted pendulums, which allow a guaranteed balancing strategy in a closed-form solution Tsai et al. 2009 ]. Those walking controllers tend to raise their swing feet higher and keep them in the air longer than natural human walking. Their robustness partly comes from the tendency of extended swing phases, which allows more flexibility in step placements for balancing.. Optimization has served as a key methodology in biped controller design. Hodgins and Pollard [1997] adapted existing controllers to new characters of different scales by searching control parameters via simulated annealing. This type of optimization is a very challenging problem because each controller has a lot of parameters to tune and the objectives are highly non-linear. The continuation method employed by Yin et al. [2008] addressed a difficult controller-adaptation problem by solving a progressive sequence of problems that trace a path from a solved problem to the target unsolved problem. Wang et al. [2009] optimized SIMBICON controllers to allow more human-like gaits using biomechanicallymotivated objective functions. Control policy searching techniques have also been used to learn walking controllers of physical robots [Morimoto et al. 2003; Tedrake et al. 2004] . It should be noted that our research goal is different than trajectory optimization [Liu and Popović 2002; Fang and Pollard 2003; Safonova et al. 2004; Wampler and Popović 2009] , which attempts to find a specific trajectory of motion minimizing energy consumption subject to user constraints.. Once a collection of robust controllers are acquired, high-level control over biped behaviors is desired. Faloutsos and van de Panne [2001] discussed the precondition and postcondition of individual controllers to make transition between controllers. Integrated controllers equipped with various motor skills have been employed to clear stepping stones and steer through obstacles [Coros et al. 2009 ]. da Silva et al. [2009] studied a combination of controllers to create inbetween controllers and coordinate the operation of multiple controllers.. Several biped controllers have been supplemented with the realism of motion capture data. To circumvent the difficulty of balance control, some controllers allow only the upper-body to be driven by motion capture data while the lower-body is either fixed or controlled by a conventional balance controller [Zordan and Hodgins 2002; Nakaoka et al. 2003 ]. Data-driven control of two-dimensional biped locomotion was first addressed by Sok et al. [2007] . They pointed out that motion capture data are physically inaccurate and rectified motion capture data to make them physically plausible using spacetime optimization. They also demonstrated that even very simple regression and tracking methods can generate stable biped walking, running and jumping when they are combined with physically plausible reference data. da Silva et al. [2008b] developed three-dimensional walking controllers that exhibit improved robustness and stability. Their controller employed short-horizon tracking and quadratic programming to maintain biped balance. This idea In robotics, biped humanoid robots are often driven by oscillatory movement pattern generators described by differential equations. Several researchers have explored a strategy to synchronize reference movement patterns and an actual humanoid via phase resetting [Nakanishi et al. 2004; Nakanishi et al. 2006 ]. Phase resetting is conceptually similar to our synchronization method, though our controller can deal with realistic motion capture references that necessitate coordinated movements of many actuated joints (upto 42 degrees of freedom in our experiments).. Comparing to previous data-driven controllers, our controller requires neither any precomputed control model (such as regression models, LQR, or NQR) nor non-linear optimization (such as quadratic programming). The model-free approach allows our controllers to take any reference trajectory generated on-the-fly at runtime. It also allows us to decouple the data-driven animation engine and physically based controllers. Therefore, any existing datadriven techniques can be used to actuate and drive physically simulated bipeds without any restriction or precomputation. Our controller does not require the derivative evaluation of equations of motion or a non-linear optimization solver. This makes our controller easy-to-implement and computationally efficient.

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Human Perception of Audio-Visual Synthetic Character Emotion Expression in the Presence of Ambiguous and Conflicting Information Abstract-Computer simulated avatars and humanoid robots have an increasingly prominent place in today's world. Acceptance of these synthetic characters depends on their ability to properly and recognizably convey basic emotion states to a user population. This study presents an analysis of the interaction between emotional audio (human voice) and video (simple animation) cues. The emotional relevance of the channels is analyzed with respect to their effect on human perception and through the study of the extracted audio-visual features that contribute most prominently to human perception. As a result of the unequal level of expressivity across the two channels, the audio was shown to bias the perception of the evaluators. However, even in the presence of a strong audio bias, the video data were shown to affect human perception. The feature sets extracted from emotionally matched audio-visual displays contained both audio and video features while feature sets resulting from emotionally mismatched audio-visual displays contained only audio information. This result indicates that observers integrate natural audio cues and synthetic video cues only when the information expressed is in congruence. It is therefore important to properly design the presentation of audio-visual cues as incorrect design may cause observers to ignore the information conveyed in one of the channels. The emotional experience provided by an audio-visual presentation is distinct from that of either an audio-only or videoonly experience. The emotional perception of the evaluators is affected by both the presented audio and video information. This results in a combined audio-visual perception that is often distinct from that of either of the individual channels. The emotional context of the audio-visual experience in our experiments was biased heavily by the natural audio information. However, despite this bias, the synthetic video information contributes to the emotional evaluations of the users. Feature analysis further supports the presence of an audio bias through the selection of mostly audio features.. Audio-visual presentations offer a unique emotional experience. Angry and happy audio-visual presentations are more differentiable than those of either audio-only or video-only presentations. However, the sad and neutral audio-visual presentations were less differentiable. The results with happiness and anger are not surprising as humans generally rely on audio for the activation information and additional modalities, such as video, for the valence differentiation [28] . For example, anger is differentiated from sadness by its activation (anger and sadness have similar valence) and from happiness by its valence (anger and happiness have similar activation). The overlap between the sadness and neutrality is surprising. Sadness is generally described by low valence and low activation, both of which should distinguish such a presentation from a neutral presentation. However, in the final audio-visual presentation, the two clusters are recognized with a lower accuracy rate (using discriminant analysis) than in either the audio-or video-only presentations. In the audio-only presentation, sadness and anger were not jointly misclassified. However, in the audio-visual presentation, 7.7% of the sad presentations were classified as anger and 2.5% of the angry presentations were misclassified as sad. This sadness-neutrality overlap was also observed in [14] .. As stated earlier, the video channel did not provide as much activation information as did the audio channel. It is therefore possible that the increased confusion between sadness and neutrality in the audio-visual presentation was the result of a reliance placed on a channel without the emotionally discriminative power necessary for such a differentiation. The increased confusion in the audio-visual presentations suggests that evaluators were influenced by the emotion conveyed on both channels, reinforcing the need for proper emotional design. If a user is reliant on a channel for information that it cannot provide, the final emotional perception may not be the perception that the designer anticipated.. The final user evaluations are affected by the amount of information conveyed across each channel. The evaluations within this study were biased by the natural audio information. This bias can be seen with respect to the graphical presentation of results (Fig. 3) , the discriminant analysis results (Table II) , and the cluster shift analysis (Table III) .. Feature selection techniques further verified an audio bias. In the conflicting presentations, evaluators had to decide to which information they should attend when making their assessments. The results suggest that, in the presence of conflicting information, evaluators relied on the audio channel for emotional disambiguation. The VAD cluster centers of the conflicting presentations were shifted with respect to the audio-only cluster centers (Table III) but the shift was not large enough to alter the binary class assignment of the dimensions (e.g., positive vs. negative valence). This combined with the feature selection results suggests that in the conflicting presentations, the audio information affects the gross emotional assessment behavior while the video information affects the fine emotional assessment. Within the congruent database, the feature selection results suggest that in the presence of congruent information, evaluators utilize both the audio and video information. The presence of video features in all three dimensions in this database analysis suggests that evaluators strongly attune to video features only when the video features provide information that matches the audio channel.. The results of the SVM analysis support trends observed in audio-visual evaluation. The SVM classification results for the activation dimension do not change significantly between the congruent and conflicting presentation types (Table VIII) . This is expected since humans tend to rely on audio for activation detection. Evaluators relied primarily on the audio channel when presented with conflicting information. Therefore, when asked to determine the activation, there is evidence that the users were well prepared in either presentation type because the variance in the evaluations did not increase enough to decrease the SVM classification performance.. Humans tend to utilize facial information for valence disambiguation. In the audio-visual presentations, the evaluators integrated the audio and the video information when making their valence assessment. However, as previously stated, when observing conflicting emotional presentations, evaluators tended to rely on the audio information. Therefore, when the evaluators attempted to analyze the valence dimension based primarily on the audio rather than the video signal, the variance in the evaluations increased and the performance of the SVM classification algorithm decreased.. The results of the SVM classification on the full and reduced feature sets suggest that it is possible to identify a feature subset with emotional explanatory power in the congruent presentations and in the activation and dominance dimensions of the conflicting presentations. SVM classification performance on this feature subset was not significantly different from that of the full feature set (except for the valence dimension, over which no features were selected). This result indicates that the selected feature subset could adequately explain the variance in the user evaluations.. The VAD ratings of the dominance dimension are affected by both the audio and video channels (Table III) . It is therefore expected that the results of the SVM classification would lie in between that of the activation and valence dimensions with respect to performance decrease. The SVM classification results support the hypothesis that the variance in the evaluation of dominance is affected by both the audio and video channels.. Classification tasks have previously been used for perceptual experiments. In [25] , speech synthesis parameters were selected using classification techniques. The features that were selected in this process were used to synthesize and modify speech. The creation of this feature subset allowed the researchers to minimize the utterances to be rated by evaluators. In future studies, this same technique will be applied to determine which combinations of facial and vocal channel capabilities should be utilized for emotion recognition applications. This presentation framework will allow for the study of how various audio-visual combinations affect human emotional perception.. In [40] , the authors presented an analysis of audio-visual emotional modulation. The data were segmented by utterances and compared across four emotion classes (angry, happy, sad, neutral). The utterances were further segmented by phoneme boundaries. The phonemes of the emotional utterances were compared to a neutral baseline. The data suggested that phonemes that contained little emotion modulation (the feature values of the emotional utterance were not significantly different than those of the neutral utterance) were accompanied by more facial movement than those phonemes that were more strongly emotionally modulated. This recasts the emotion production problem as an emotional bit allocation problem in which the information is transmitted across the two channels based on the channel bandwidth available. In the work presented in the current paper, such emotional subtleties were not included due to the limited nature of the video channel and the nonlinearities inherent in a fusion between audio and video channels. Future experiments will utilize audio-visual data recorded using motion capture and microphone recording devices [41] . This will allow for a closer study of the channel modulation, fusion, and perceptual integration resulting from the natural expression of audio-visual emotional utterances. This work was principally motivated by the McGurk effect [20] . The McGurk effect occurs when mismatched audio and video phonemes (called visemes) are presented to a human observer. Instead of perceiving either of the two presented phonemes, McGurk and MacDonald found that observers perceived a third, distinct phoneme. This finding has motivated many emotion research studies designed to determine if such an effect occurs within the emotion domain. Emotional McGurk evaluations are primarily conducted using either discrete emotion assignment (e.g., happy or angry) [6] - [11] or dimensional evaluation [17] , [18] , [21] . This effect has also been studied using fMRI [21] and EEG measurements [22] . The emotion presentations include congruent and conflicting information from the facial and vocal channels (e.g., [6] ), facial channel and context (e.g., [21] ), and facial and body postural/positional information (e.g., [22] ).. In discrete choice evaluations, users are asked to rate the utterance by selecting the emotional label that best fits the data. Such evaluations allow researchers to determine at which point along an emotional continuum a given face or voice is of a sufficient emotional strength to bias the decision of the evaluators [7] . In a dimensional analysis evaluation, evaluators are asked to rate the presented stimuli according to the properties of those stimuli. Common properties (or dimensions) include valence (positive vs. negative), activation (calm vs. excited), and dominance (passive vs. dominant). One common dimensional evaluation technique utilizes self-assessment manikins (SAM) [23] . This evaluation methodology presents the dimensions of valence, activation, and dominance using a pictorial, text-free display. This display method allows evaluators to ground their assessments using the provided end-points.. In [7] , researchers combined still images with single spoken words in three distinct experiments. The first experiment presented images morphed from two archetypal happy and sad emotional images into a visual emotional continuum. These images were accompanied by either vocally happy or sad human utterances. The evaluators were presented with an audio-only, video-only, or combined audio-visual presentation and were asked to assign the combined presentation into one of the discrete emotional categories of "happy" or "sad." The researchers found that the evaluators were able to correctly recognize the emotion in the audio-clip 100% of the time. In the combined audio-visual presentation, they found that the voice altered the probability that an evaluator would identify the presentation as "sad." The researchers then repeated the experiment, this time asking the users to judge the face and ignore the voice. They found that the emotion presented in the audio channel still had an effect on the discrete emotion assignment. However, in this experiment, they found that the effect was smaller than that seen previously. In the final experiment, the researchers created a vocal continuum ranging from happiness to fear (to allow for a more natural vocal continuum). They asked the users to attune to the voice and to ignore the face. They found that users were still affected by the visually presented emotion.. A similar study was presented in [10] . In that work, evaluators were asked to rate 144 stimuli composed of still images and emotional speech using happy, angry, and neutral emotions. The evaluators were asked to respond as quickly as possible after viewing a stimulus presentation. In the first experiment, the researchers asked the evaluators to attune to the emotion presented in the facial channel. They found that the emotion presented in the vocal channel (the unattended emotion) affected the evaluators with respect to accuracy (discrete classification of facial emotion) and response time (faster for congruent presentations). When the evaluators were instead asked to attune to the facial channel, the researchers found that vocal channel mismatches decreased the facial emotion recognition accuracy and increased the response time. In the final experiment, users were presented with an emotional stimulus (a vocal utterance), a pause, and a second emotional stimulus (the facial emotion) used as a "go-signal." The researchers found that when the emotion presentations were separated by a delay, the channels no longer interacted in the emotion evaluation and the evaluators based their decisions on the vocal signal only.. Interactions between emotional channels have also been studied using emotional faces paired with contextual movies [21] . In that study, the evaluators were presented with four seconds of a movie and were then shown a static image. The contextual emotions included positive, negative, and neutral. The emotional faces included happy, fear, and neutral. These combined presentations were rated using SAMs. The researchers found that faces presented with a positive or negative context were rated significantly differently than faces presented in a neutral context. Furthermore, the fMRI data showed that pairings between faces and emotional movies resulted in enhanced BOLD responses in several brain regions.. The McGurk effect has also been studied with respect to body posture and facial analyses [22] . In this study, researchers paired emotional faces with emotional body positions (fear and anger for both) to analyze the interplay between facial and postural information in emotion evaluation. They found that evaluators were able to assess the emotion state (using a discrete choice evaluation) of the stimulus most quickly and accurately when viewing congruent presentations. The results showed that the analysis time for faces-only was faster than for bodies-only. These results suggest that facial emotional assessment is biased by the emotion embedded in body posture.. The expression of emotion has also been studied in a more localized manner. One such method utilizes a "Bubble" [24] . This method is designed to identify regions of interest that correspond to task-related performance by only permitting users to view certain areas of the stimulus. The stimulus is covered by an opaque mask. Regions are randomly shown to the evaluators by creating Gaussian "bubbles," which allow users to glimpse regions of the masked stimulus. Given an infinite number of trials, all window combinations will be explored. The "Bubble" method allows for a systematic evaluation of stimuli components, but produces results that are difficult to translate into system design suggestions.. These past studies suggest that the video and audio channels interact during human emotion processing when presented synchronously. However, due to the discrete nature of the evaluation frameworks, it is difficult to determine how the perception of the targeted emotions change in the presence of conflicting information. The present work uses the dimensional evaluation method reported in [23] to ascertain the nature of the audio-visual channel interactions. This study also differs from previous work in its use of video clips rather than static photographs. Human-computer and human-robot interactions usually include dynamic faces and voices. The inclusion of dynamic facial stimuli in this study makes the results more transferable to the interactive design domain.. This work was also motivated by the analysis by synthesis approach to emotion research. In this approach, stimuli are synthesized using a broad range of parameters to determine combinations that result in enhanced performance. In [25] , automatic emotion recognizers were used to decrease the input feature space. This reduction allowed for a tractable user-evaluation over an initially large input parameter space. The work presented in this paper represents a first step in the analysis by synthesis approach for audio-visual emotional expressions. The feature selection analysis (Section VI) suggests audio-visual features that should be included in future emotional synthesis presentation experiments. This reduced audio-visual feature subset will decrease the necessary number of synthesized utterances and will allow for a more concentrated focus on the creation of emotional models that represent the relationship between specific feature variation and human emotion perception.

O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O 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Mesh-based content routing using XML We have developed a new approach for reliably multicasting timecritical data to heterogeneous clients over mesh-based overlay networks. To facilitate intelligent content pruning, data streams are comprised of a sequence of XML packets and forwarded by application-level XML routers, XML routers perform contentbased routing of individual XML packets to other touters or clients based upon queries that describe the information needs of downstream nodes. Our PC-based XML router prototype can route an 18 Mbit per second XML stream.Our routers use a novel Diversity Control Protocol (DCP) for router-to-router and router-to-client communication. DCP reassembles a received stream of packets from one or more senders using the first copy of a packet to arrive from any sender. When each node is connected to n parents, the resulting network is resilient to (n -1) router or independent link failures without repair. Associated mesh algorithms permit the system to recover to (n -1) resilience after node and/or link failure. We have deployed a distributed network of XML routers that streams real-time air traffic control data. Experimental results show multiple senders improve reliability and latency when compared to tree-based networks. Our research is motivated by an interest in highly reliable data distribution technologies that can deliver information to end clients with low latency in the presence of both node and link failures. Low latency can be crucial for certain data that are extremely time critical. For example, real-time trading systems rely upon the timely arrival of current security prices, air-traffic control systems require up-to-the-second data on aircraft position and status, and gaps or delay in live network video and audio feeds can be distracting. In. This research was supported in part by DARPA (Grant No. F30602-97-1-0283). Alex C. Snoeren was supported by a National Defense Science and Engineering Graduate (NDSEG) Fellowship.. We observe that the achievable latency of a reliable data stream is bounded by the packet loss-recovery mechanism. Packet losses can be handled by retransmission or redundant coding. Retransmission methods limit recovery time to the round-trip delay between communicating nodes. In order to avoid retransmission in the face of loss redundant data must be sent.. This work is based upon the assumption that, in certain cases, the value of reliable and timely data delivery may justify increased transport costs if the cost increase allows us to meet a desired reliability goal. Systems often try to avoid the delay penalty by using loss-resistant coding schemes which encode redundant information into the data stream. We extend this redundancy to network delivery paths and senders. Recent work in overlay networks has shown that multiple, distinct paths often exist between hosts on the Intemet [2] . We attempt to leverage these redundant network links. While some may consider this additional bandwidth wasteful, we believe the system described herein presents an interesting and elegant method of utilizing additional network resources to achieve levels of reliability and latency previously difficult to obtain.. Our basic approach is to construct a content distribution mesh, where every node is connected to n parents, receiving duplicate packet streams from each of its parents. The value of n is a configuration parameter that is used to select the desired trade-off between latency, reliability, and transport costs. By maintaining an acyclic mesh, this approach guarantees that the minimum cut of the mesh is n nodes or independent links. Thus, a mesh is resilient to (n -1) node or (n -1) independent link failures (we say (n -1) resilien0 without repair. If a mesh failure occurs, recovery algorithms restore the mesh to (n -1) resilience in a few seconds.. Our architecture is based upon an overlay network that transports XML streams. An XML packet is a single independent XML document [7] . An XML stream is a sequence of XML packets, and each XML packet in a stream can have a different document type definition (DTD). When clients join an overlay network they specify an XML query that describes the XML packets they would like to receive. It is the job of the overlay network to configure itself to deliver the desired XML stream to a client at reasonable cost given reliability goals. Queries are expressed in a general language such as XQuery [11] .. Our overlay network is implemented by XML routers. An XML router is a node that receives XML packets on one or more input links and forwards a subset of the XML packets it receives to each output link. Each output link has a query that describes the portion of the router's XML stream that should be sent to the host on that link. XML routers are components in a distributed publishsubscribe network and implement the selective forwarding of XML packets according to subscriptions described by queries.. XML has a number of advantages over a byte stream for multicast delivery. First, XML permits the network to interpret client data needs in terms of well-defned XML queries. Second, XML packets suggest what logical units of data will be processed together by' a client and thus can aid network scheduling. Third, many tools and standards exist for XML making it easy for both the data originator and receiver to build robust applications. Finally, our approach allows applications and databases to push part of their processing into the network fabric. We expect that query languages such as XQuery will become standardized, allowing a single language to be used to describe data requirements. This standardization will permit applications to program our network fabric to deliver the data they need in a simple, consistent fashion.. The primary disadvantage of XML is often thought to be the increased number of bytes required to represent the same information in XML when compared to an application specific encoding. However, our experimental results suggest that conventional data compression eliminates this disadvantage. While an XML stream must be decompressed and recompressed at any router that wishes to do query matching, a router that passes all packets to every client can bypass the XML switch component entirely, and no decompression or compression need be performed. Thus, routers can include a fast-path for clients that subscribe to the unfiltered XML stream. This paper presented three key ideas. First, we introduced the idea of XML touters that switch self-describing XML packets based upon any field. Second, we showed how XML touters can be organized into a resilient overlay network that can tolerate both node and link failures without reconfiguration and without interrupting real-time data transport. Finally, we introduced the Diversity Communication Protocol as a way for peers to use redundant packet transmissions to reduce latency and improve reliability.. A wide variety of extensions can be made to the work presently reported, both in protocol refinements and additional functionality. We arc actively investigating methods of DCP self-tuning, both for adaptive timers and sophisticated flow control. DCP can also can be used for uninterpreted byte streams. Thus, DCP-like ideas may find application in contexts outside of XML routers. For example, contemporary work on reliable overlay networks (RONs) could use DCP as a RON communication protocol to maximize performance and reliability [2] .. Just as secondary storage has become viewed as expendable in pursuit of enhanced functionality and performance [34] , we believe that, for certain tightly-constrained applications, network bandwidth across multiple paths may be similarly viewed as well-spent in return for substantial gains in reliability and latency. It is unlikely that multiple disjoint paths with excess capacity will always exist on the last mile to a client. Hence, many installations may benefit from meshes that change to lower levels of redundancy at critical network points such as points-of-presence before last mile cable.. Within the scope of XML routing, our current XML routers could be extended to support.. • More sophisticated XML mesh building and maintenance algorithms.. • Combiners that integrate multiple XML streams for multicast transport as a single stream.. • Using XML routers for duplex communication.. * Other XML network components, such as stream storage and replay.. • Transcoding XML routers that produce output packets that are derivatives of input packets, based upon client queries.. Even in its current form, however, we believe our architecture demonstrates XML is a viable mechanism for content distribution, providing a natural way to encapsulate related data, and a convenient semantic framing mechanism for intelligent network transport and routing. This section considers the strengths and weaknesses of our approach to content routing using XML. While we believe that many of the techniques we developed for our ATC application are widely applicable, we would like to make our assumptions clear. Our work on XML routers and DCP builds on a large body of past work in reliable multicast and overlay networks. We consider related work in four areas: reliable multicast, overlay networks, redundant coding and transmission schemes, and publish-subscribe networks.

O O B O O O O O O O O O O O O B O O O O O O O O O O O O B B O O O O B O O O O O O O O O O O O O B O O O O O O O O O O O B O O O O O O O O O O O B B O O O O O O O O O O B O O B B O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O B O O O O O O O B O O O O O O O O O B O O O O B O O O O B B O O O O B B B I I B O O O O O O B O B O O O O O O O O O O O O O O O B O O O O O O O O O O O B O O O O O O O O O O B O O O O O B O O O O O O O B O B O O O O O O O O O O B B O O B O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O B O B O O O O O O O O O O O O O O O O O O B O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O B O B O O O O O O O O O O O O O B O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O B B O O O O O O O B O O O O O O O O O B O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O B B O O O O O B O O O B O O O O O O O O O O O B O O O B O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O B O B O O O O O O O O O O O O O B O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O B O O O O O O O O O O O O O B O O O O O O O O O O O B O O O O O B O O O B O O O O O O B O O O O O O O O B O O O O O O O O O O O B O O O O O O O O O O O O O O O B O O O O O O B O O O O B O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O B O O O O O B O O O B O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O B O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O B B O O O O O B B O O O O O O O B O O O O O B O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O B O O O O B O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O B B O O O O O O O O O O O O O O O O B O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O B O O O O O O O O O O O O O B O O O O O O O O O O B O B O O O O O O O O O O O B O O O B O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O B O O B O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O B O O O O O O O O O O


Multicast Capacity of Wireless Ad Hoc Networks Under Gaussian Channel Model Abstract-We study the multicast capacity of large-scale random extended multihop wireless networks, where a number of wireless nodes are randomly located in a square region with side length a = p n, by use of Poisson distribution with density 1. All nodes transmit at a constant power P , and the power decays with attenuation exponent > 2. The data rate of a transmission is determined by the SINR as B log(1 + SINR), where B is the bandwidth. There are n s randomly and independently chosen multicast sessions. Each multicast session has k randomly chosen terminals.We show that when k 1 n (log n) and n s 2 n 1=2+ , the capacity that each multicast session can achieve, with high probability, is at least c 8 p n n p k , where 1 , 2 , and c 8 are some special constants and > 0 is any positive real number. We also show that for k = O( n log n ), the per-flow multicast capacity under Gaussian channel is at most O( p n n p k ) when we have at least n s = (log n) random multicast flows. Our result generalizes the unicast capacity for random networks using percolation theory. I N MANY applications, e.g., wireless sensor networks, we often need an estimation on the (asymptotic) achievable throughput when we randomly deploy wireless nodes in a given region. The main purpose of this paper is to study the asymptotic capacity of large-scale random wireless networks where a large number of nodes are randomly placed in the deployment region, when we choose the best protocols for all A. Protocol Interference Model (PrIM) [7] In this model, a transmission by a node is successfully received by an intended target iff node is sufficiently apart from the source of any other simultaneous transmission, i.e., for any simultaneously transmitting node . Here, is a constant depending on the environment. A number of interesting questions remain open. The first question is to derive tight upper bound and lower bound on the network capacity when could be any arbitrary value from 2 to . The lower bounds presented here only hold when . The second question is to study the capacity when the receiving terminals in a multicast group are within a certain region (e.g., a disk with a radius or a square with a side length ). Finally, we point out that the problem of optimizing the multicast throughput of a given arbitrary network by choosing best routing protocol and optimizing the hidden constant in our formulas remains open. [3] Consider a square lattice with side length . We declare each edge of the square grid open with probability , and closed otherwise, independently of all other edges.. For any given , let us partition into rectangles of sides . We choose as the smallest value such that the number of rectangles in the partition is an integer. It is easy to see that as . We let be the maximal number of edge-disjoint left-to-right crossings of rectangle and let . The result is the following.. Theorem 22 [3]

B B O B I I I O B I O O O O B B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O B O O O O O O O O O O O O O O O O O O O O O O B O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B B O B I O O O O O O O O O O O O O O O O O O O O O B O O O O O B B O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O


A system for peer-to-peer video streaming in resource constrained mobile environments Peer-to-Peer based near-live video streaming is becoming more and more popular with users of fixed-line broadband network access, but it is mostly unavailable to mobile users, as cellular networks, such as GPRS/UMTS, cannot meet the bitrate requirements, while other wireless technologies, such as WLAN, may be fast enough but cover only very limited areas. However, there is a small but important set of scenarios, where several mobile users in close physical proximity are interested in retrieving the same content. We propose a P2P-TV system that enables them to retrieve video chunks in a cooperative way. The coordinated and efficient usage of all wireless resources available to a group of mobile hosts is the key to enable P2P-TV in mobile environments. This paper introduces our general concept. Simulation based studies are presented to assess different resource allocation strategies and to demonstrate the feasibility of our approach for delivering near-live TV in resource constrained mobile environments. The IP-based delivery of live or near-live video content is an emerging application that is quickly gaining momentum. Several distribution approaches for IPTV exist; the spectrum reaches from establishing IP-based networks that are separate from the Internet (e. g., TISPAN [4] ) via classic Permission to make digital or hard copies of all or part of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advantage and that copies bear this notice and the full citation on the first page. To copy otherwise, to republish, to post on servers or to redistribute to lists, requires prior specific permission and/or a fee. client/server applications to P2P based video distribution in the Internet (e. g., PPLive [9] ). However, these efforts currently focus mainly on users with fixed (e.g., DSL) Internet access, since live video streaming requires -depending on content type and image quality -a sustained bitrate in a range from 300 kbps to 16 Mbps, which can be delivered easily and reliably only via wireline access networks.. Considering wireless Internet access for mobile devices, users basically have the choice between high speed and good coverage area, but they cannot have both at the same time: Cellular networks such as GPRS or UMTS, cover large areas of the country (at least in many densly populated and highly developed nations), but even there may be badly covered spots and connections may be aborted, e. g., if travelling on a high-speed train through a tunnel. And more importantly, these networks usually cannot deliver bitrates suitable for video streaming to every single user. Although it is foreseeable that technological advancements such as LTE [2] will increase the maximum bitrate, the average effective bitrate per user will be unsatisfactory for video streaming due to the volatile nature of the wireless channels. In contrast, other wireless access technologies such as WiFi or WiMAX, that provide significantly higher bitrates, cover only rather small areas, e. g., at home or at train stations.. The basic idea behind our approach is very simple: use several wireless Internet access links at the same time and distribute the load among them ("channel bonding"). These links do not have to use the same technology or connect to the same network operator. Actually, diversity reduces the risk of "fate sharing", i. e., the likelihood that several links become unavailable (or very slow) at the same time. One approach would be to terminate all these connections in one mobile device with several wireless interfaces, and in fact we analyze this configuration to demonstrate the feasibility of the general concept. However, having several subscriptions to network operators at the same time would cause significant costs for a user. Therefore, we propose a scheme that allows a group of users in physical proximity to jointly access a video stream, by efficiently using the Internet access ressources each user contributes to the group. This paper is structured as follows: Section 2 presents one use case, for illustrating the basic idea of cooperative P2P streaming and for introducing additional constraints. A systematic description of the system components in Section 2 is followed by a performance analysis in Section 3. There, simulation results demonstrate the feasibility of our approach, which is compared with related work in Section 4, before Section 5 summarizes our findings and concludes the paper. This paper presents a new proposal for a P2P based application layer channel bonding, that is used for a cooperative peer-to-peer video streaming in resource constrained mobile environments, to enable mobile P2P-TV. Using this approach, a group of mobile nodes, which are located in physical vicinity can jointly access a live video stream, whose bitrate requirements exceeds the access bitrate each individual node is equipped with. An initial simulation based study demonstrates the feasibility of our approach to enable delivery of near-live TV to mobile users. Our approach is not only applicable in mobile scenarios, but also for fixed line scenarios. For instance, P2P file sharing applications used on xDSL, can utilize our approach to coordinate the download between peers in a way that not every peer is downloading the same complete file. But the file is cooperatively downloaded by all local peers, lowering the resource consumption in the edge and core network of the provider.. In the future, we will extend these studies to cover some issues, which have not been addressed in detail so far. First, the link models have to be improved and we are addressing this by performing measurements of GPRS/EDGE and UMTS availability of different network operators in German high-speed trains. Second, the implications of the sharinglink have not been considered yet, e.g., nodes might join or leave the group or the impact of the sharing link's wireless nature. Third, it has to be investigated how the proposed system interacts with algorithms in the P2P protocol that ensure fairness among the nodes -so far the simulations assume that peers in the fixed part of the network are willing to give an arbitrary amount of chunks without an adequate service in return. Channel bonding and link bundling, are well-known for several link-layer technologies, such as xDSL or UMTS. However, the bundling of multiple links is handled on the link layer and does not allow bundling of multiple different linklayer technologies to aggregate bitrate. On the other hand, there is IP-based multi-homing RFC 4908 [6] [13], but with a focus on general IP multi-homing support, with enough bitrate in mind, and not considering the specific needs of P2P systems while moving. Yao presents a multi-homing agent for mobile on-board communication that aims at aggregating multiple WAN links [14] but the evaluation is solely based on wireless-LAN with a small set of only two links and low travel speed. Rodriguez et al [10] consider exploiting the diversity of multiple access links for aggregating bitrate and improving for HTTP and UDP-based streaming. However, none of these considers P2P based streaming and the impact of using application-layer channel bonding in the chunk distribution. MOVi [15] addresses a similar challenge as we do, but on a different scale, for video on-demand streaming to pedestrians, using WLAN as a very specific link-layer technology. The authors propose changes to the existing WLAN specifications and to allow a mix mode unicast streaming and peer-to-peer video exchange. Another related work is PatchPeer [3] that is designed to "allow the video-on-demand system scale beyond the bitrate capacity of the server" by combining multicast with peer-to-peer technology (similar to MOVi). Related to our work is the Drive-Thru Internet approach of Ott and Kutscher [8] where mobile clients use a session protocol to maintain connectivity even in the presence of disrupted Internet connectivity. This approach is suitable for non real-time communication, such as web browsing or email, but is not suitable for live streaming, as a constant connection is required.. The related work focuses on specific link-layer technologies [15] , or requires a special mobility agent [14] [10], and considers mobility patterns of pedestrians [15] [3] . Our approach considers peers moving at the speeds of 200 km/h, e.g., high-speed trains, with more restriced and challenged GPRS and UMTS links. Furthermore, there is no need in our approach for mobility agents to be deployed on the network side, as the peers are coordinating themselves. The coordination is tailored to the chunk distribution of P2P streaming and we are not aiming at finding a general purpose solution, as in [13] , which neglects application specific needs.. Topology awareness of P2P system plays a major role in our approach, as the peers have to find neighboring peers that are in the same physical area. The recent works of the IETF ALTO working group [5] are of interest for this, but are not applicable here, as nodes do want physical closeness and do care less about topological issues.

O O O B O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O


Virtual path layouts optimizing total hop count on ATM tree networks This paper presents some algorithmic results concerning virtual path layouts for the one-to-many communication problem in ATM tree networks. The ATM network model is based on covering the network with a layout of virtual paths, under some constraints on the allowed load, namely, the number of paths that can share an edge. The quality measure used is the hop count, namely, the number of edges traversed between two vertices that need to communicate. Whereas most former results concerned the maximum hop count of the virtual path layout, our interest here is in measuring its total hop count, or alternatively its average hop count. The paper presents a dynamic programming algorithm for planning ATM network layouts with minimal total hop count for one-to-many requirements under load constraints over the class of tree networks. This paper concerns the problem of designing efficient virtual path layouts on optical or ATM networks (see, e.g., [11, 12, 15, 16] ). In the ATM model, the routing and message forwarding tasks on a given network are simplified by predefining a collection of virtual paths (VP's), which are simple paths in the network, and performing end-to-end communication over routes composed of a sequence of such VP's (i.e., using the VP's as basic segments within complete routes). An elegant formulation of the problem can be obtained by representing the VP's formed on the given communication network G as a virtual graph H over the same set of vertices. Specifically, a VP connecting v and u in G is represented by an edge connecting v and u in the virtual graph H . The pair (H, P ), where P is the collection of VP's corresponding to the edges of H , is referred to as the virtual path layout (VPL) for the physical graph G. Each route in G can be viewed as a simple path in the virtual graph H .. Various formulations of the VPL problem attempt to design a system of virtual paths which optimizes some parameters of the system while meeting some given communication demands between pairs of nodes and satisfying certain prespecified constraints. Usually, the problem is studied against one canonical pattern of communication demands, known as all-to-all communication, which requires communication between every pair of vertices in the network.. Research on virtual path layouts has concentrated on optimizing two central parameters of conflicting nature. The first is the load of a physical edge, defined as the number of VP's that share it. The upper bound on the allowed load of an edge is termed the capacity of the edge. The maximum (respectively, average) load of a VPL (H, P ), denoted L max (H, P ) (respectively, L avg (H, P )), is defined as the maximum (respectively, average) load of the edges in the network. These parameters determine the size of the VP routing tables, and reflect the traffic load on the links.. The second parameter of interest concerns the hop count, namely, the number of VP's occurring on the routes of the VPL, or equivalently, the number of edges in the corresponding path on the virtual graph H . Expressed in terms of H , the maximum hop count, denoted H max (H ), can be viewed as the diameter of H , and the average hop count, denoted H avg (H ), can be defined as the average distance over all vertex pairs in H . Equivalently, one may consider the total number of hop counts over all vertex pairs in H , termed the total hop count of H and denoted H tot (H ). (Clearly, H tot (H ) = H avg (H ) · n(n − 1)/2 for every graph H .) These parameters measure the (worst-case or average) efficiency of setting up the route, and also the overall (worst-case or average) delay incurred by the route in a model where the processing along VP's is negligible compared to the processing at the VP endpoints. See [16] for a discussion of these parameters and their significance.. A number of studies have tackled the VPL problem (cf. [1, 5, 10, 14] ). The problem of minimizing H max (H ), the diameter of a virtual graph H , subject to a specified upper bound c on the maximum load of the VPL, L max (H, P ) c, has been considered in the undirected case in [6, [8] [9] [10] 13, 16] . Conversely, the problem of minimizing the maximum load L max (H, P ) over all VPL's (H, P ) with maximum hop count bounded by h, H max (H ) h, is studied in [2, 7] . Minimizing also the average load L avg (H, P ) is considered in [9] .. As links based on optical fibers are directed, and may have a different load in the two directions, it may be useful to consider a directed model as in [3, 4] , rather than an undirected one. The problem of minimizing the diameter H max (H ) of a virtual directed graph (digraph) H over bounded maximum load VPL's is considered in [3] , giving lower and upper bounds on the virtual diameter H max (H ) of a directed VPL (henceforth, DVPL) with a prespecified capacity c bounding the maximum load (considered as constant). This paper focuses on the average hop count measure H avg of a digraph, and explores the variant of the VPL problem which seeks to optimize this measure. (Actually, for convenience, we formulate our results in terms of the equivalent total hop count measure, H tot (H ).) Clearly, the upper bounds established in [3] can be converted into ones for the average hop count H avg (H ) of DVPL's as well. However, the average hop count may admit better bounds.. While most studies of the VPL problem considered all-to-all instances, another pattern of communication demands for which the VPL problem was studied is the one-to-all pattern, in which a single vertex must communicate with all other vertices [7, 9] . For the chain network, a duality between the problem of minimizing the hop count given a bound c on the maximum load and the problem of minimizing the load given a bound h on the maximum hop count, is established in [7] . A number of VPL optimization problems with one-to-all communication demands are studied in [9] for the undirected chain network, including the problem of designing a VPL with optimal average hop count given a maximum load bound c under the one-to-all communication pattern, and a dynamic programming algorithm is presented for this problem. The "Open problems" section of [9] states the following:. "The most immediate open problem is to generalize these results for arbitrary trees, a task which seems nontrivial, as far as the dynamic programming algorithms are concerned, due to the additional structural information that is attached to each subtree (which does not exist in chains).". The main result of the current paper, presented in Section 3, makes progress towards solving the open problem mentioned in the above quote. On the one hand, our solution applies only to directed trees, with all the edges directed away from the root. On the other hand, our solution is slightly more general, since instead of assuming a uniform constant capacity c on all links, we allow a capacity function c : E → N + , under the restriction that c(e) c 0 for every link e, for some constant c 0 . Moreover, we deal with a general class of one-to-all instances, referred to as one-to-many instances, in which the root must communicate with some partial subset of the vertices. Actually, as shown in Section 4, our algorithm can be modified to handle the weighted version of the problem [9] in which given a requirements matrix R, such that for 1 i n, R 1i specifies the expected amount of traffic between v 1 and v i , it is necessary to minimize the weighted average hop count of G, defined by weighing the hop counts according to R. Now we prove that the algorithm indeed calculates H tot (G, c, R), and bound its complexity.. For proving correctness, we first observe that the minimal realizable total hop count of G, H tot (G, c, R), is the minimal internal total hop count of v 1 's subtree, i.e., H tot (G, c, R) = H tot (T 1 , R). Next we observe that the minimum for each subtree is independent on the other subtrees. As the algorithm takes H tot (T 1 , R) as the sum of Υ (T 1 1 ) , . . . , Υ (T 1 t ), we need to prove that each Υ (T 1 j ) achieves the relevant minimal internal total hop count. Given the definition of Υ (T 1 j ), we should prove that f (T i , d) indeed gives the minimal internal total hop count of T i , being reached by k virtual arcs with a tuple d of tail-lengths from v 1 .

O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O B I O O O O B I O O O O B I O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O


Personal computing acceptance factors in small firms: a structural equation model This study draws upon the technology acceptance model as the theoretical basis and empirical findings for a pragmatic explanation of key factors affecting personal computing acceptance in small firms. The study uses results from a survey of 358 users in small firms in New Zealand to test a structural model examining the hypothesized relationships among the following constructs: intraorganizational factors, extraorganizational factors, perceived ease of use, perceived usefulness, and personal computing acceptance (i.e., system usage). The findings indicate that perceived ease of use is a dominant factor in explaining perceived usefulness and system usage, and that perceived usefulness has a strong effect on system usage. The results also indicate that exogenous variables influence both perceived ease of use and perceived usefulness, particularly management support and external support. Inconsistent with prior research in large firms, relatively little support was found for the influence of both internal support and internal training. Implications for the acceptance of personal computing and future research on personal computing acceptance in small firms are discussed. The increasing importance and proliferation of microcomputers and end-user computing (EUC) represents a significant development the information systems (IS) field (Davis et 1989; DeLone 1988; Igbaria 1993; Rivard and Huff 1988) . This proliferation has helped make personal computing a significant activity in small firms (Raymond and Bergeron 1992) . 1985, 25% of small firms were using computers; by 1990, 67.5% of small firms were using computers (BusinessWeek 1994).. Compared with larger firms, small firms usually cannot afford to employ internal staff with specialized computer expertise (Nooteboom 1988) . This means that small firms face substantial risks and problems with their computerization: they have a general lack of computer knowledge, have inadequate hardware and software, need to rely on outside resources, experience a lack of financial resources and technical support, have recruitment difficulties, and have a short-range management perspective imposed by a volatile competitive environment ).. Thus, despite the proliferation of microcomputers, various problems within small firms have limited the potential benefits of microcomputers. The actual use of microcomputers by professionals and managers in small firms appears to be limited to mainly accounting applications (Cragg and King 1993) . It is suggested that small firms can increase the benefits from computing and can enhance the usage of computers by increasing the number and type of applications available to them.. Much IS implementation research has focused on large organizations and has identified several factors affecting IS success. However, previous research suggests that there is a relationship between organizational size and computer success characteristics (DeLone 1988; Ein-Dor and Segev 1978; Raymond 1985 Raymond , 1990a . This implies that the research findings based on MIS environments in large firms cannot necessarily be generalized to small firms (DeLone 1981 (DeLone , 1988 Ein-Dor and Segev 1978; Lai 1994; Raymond 1985 Raymond , 1990b . Since small firms have distinctive and unique computing needs, as well as different technology acceptance patterns compared with large ones (C~ragg and King 1993; Massey 1986; Rogers 1995) , there is a need to investigate the applicability of these models to small firms.. FIlecognizing that small firms represent a distinct group, this study extends existing IS irnplementation as well as small firm research. Relatively little is known about personal computing in small firms (Raymond 1990b) . Clearly, many factors influence personal computing acceptance (Adams et al. 1992; Davis et al. 1989; DeLone and McLean 1992; Igbaria et al. 1995; Mathieson 1991; Moore and Benbasat 1991; Straub et al. 1995; Szajna 11996; Taylor and Todd 1995; Thompson et al. 1991) . The majority of these studies have in, vestigated the acceptance of technology in large organizations. The present study sought to extend previous research by investigating in a single study the factors affecting personal computing acceptance among users in small firms. Specifically, the objectives of the study were (1) to develop a model of the determinants of personal computing acceptance and (2) to examine both the direct and indirect effects of these determinants of acceptance. The key factors selected for investigation are based on either past factor studies of IS success or factors perceived to be important in a small business context. The research described here used a structural equation modeling technique to simultaneously test the measurement and the structural models. The conceptual model guiding this study is described in the next section. This study presents significant progress toward explaining the factors affecting personal computing acceptance in small firms. It aimed to investigate the effects of intra-and extraorganizational factors on personal computing acceptance and the role of perceived ease of use and perceived usefulness in mediating these relationships. The findings are encouraging and provide theoretical and practical insights into personal computing acceptance in a small firm context. The study found considerable support for TAM in small firms. Perceived ease of use was found to be a more important determinant of personal computing acceptance than perceived usefulness, a result that is not consistent with prior research in large firms. This may be due to the fact that small firms in New Zealand may be in the early stages of technology adoption. The results also confirmed management support and external support as the two most significant exogenous variables. This finding supports the call for more efforts to be directed at combining initiatives by management and external providers to achieve greater personal computing acceptance. This study integrated the theoretical perspectives and empirical findings of research on personal computing acceptance in small firms and proposed and tested a structural equation model examining the role of perceived usefulness, perceived ease of use, and the intraand extraorganizational factors in promoting personal computing acceptance in small firms.. TAM was expanded by examining both intraand extraorganizational variables. It was also extended to a small business context, whereas most previous research has tested it with large firms or college students. The results indicate moderate support for the proposed linkages among the model variables and provide interesting insights into the routes through which the antecedent variables influence personal computing acceptance. The results demonstrate the relative contribution of perceived ease of use and perceived usefulness to variation in personal computing acceptance. The findings reiterate the key mediating role of perceived ease of use and perceived usefulness in promoting personal computing acceptance, i.e., system usage, and demonstrate the utilitarian and rational underpinnings of the decision to use systems in small firms. That is to say, individuals are likely to use the system if they believe that it is easy to use and that using it will increase their performance and productivity.. Inconsistent with prior research , the total effect of perceived ease of use is greater than the total effect of perceived usefulness on usage. This may suggest that small business users are driven to accept computer technologies primarily on the basis of ease of use and user friendliness and secondarily because of the functions it performs for them. Although users may be willing to cope with some difficulties if the system provides them with important needed information in their job, this result suggests that users who have difficulties using the system may be discouraged from using the system and may not be able to observe the perceived benefits from it. This emphasizes the importance of both perceived ease of use and functionality in developing systems.. A possible explanation is that a user's level of experience with the system may influence the relative importance of perceived ease of use and perceived usefulness. Our survey indicated that the majority of users (over 75%) reported no experience with creating programs using a query language or fourth generation language and/or third generation languages. The effect of ease of use on system

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