C. Griwodz - Academia.edu (original) (raw)

Papers by C. Griwodz

Lecture Notes in Computer Science, 1998

As the popularity of the World Wide Web increases, the amount of traffic results in major congest... more As the popularity of the World Wide Web increases, the amount of traffic results in major congestion problems for the retrieval of data over wide distances. To react to this, users and browser builders have implemented various prefetching and parallel retrieval mechanisms, which initiate retrieval of documents that may be required later. This additional traffic is even worsening the situation. Since we believe that this will remain the general approach for quite a while, we try to make use of the general technique but try to reduce the destructive effects by retrieving less content which remains finally unread. In our user-specific prefetch mechanism, the prefetching system gathers references by parsing the HTML pages the user browses, identifies the links to other pages, and puts the words describing the links into a keyword list. If such a word was already present in the list, its associated weight is incremented. Otherwise it is added to the table and a weighting factor allocated. We have designed and implemented a client based proxy-server with this mechanism. This paper shows the design and implementation of this prefetching proxy server, presents results and general considerations on this technique.

Proceedings of the 18th International Workshop on Network and Operating Systems Support for Digital Audio and Video - NOSSDAV '08, 2008

2009 8th Annual Workshop on Network and Systems Support for Games (NetGames), 2009

Massively multi-player online games (MMOGs) have stringent latency requirements and must support ... more Massively multi-player online games (MMOGs) have stringent latency requirements and must support large numbers of concurrent players. To handle these conflicting requirements, it is common to divide the virtual environment into virtual regions, and spawn multiple instances of isolated game areas (known as dungeons or instances) to serve multiple distinct groups of players. As MMOGs attract players from all around the world, it is plausible to disperse these regions and instances on geographically distributed servers. Core selection algorithms can then be applied to locate an optimal server for placing a region or instance, based on player latencies. Functionality for migrating objects supports this objective, with a distributed name server ensuring that references to the moved objects are efficiently maintained. As a result, we anticipate a decrease in aggregate latency for the affected players. With Ginnungagap we present the implementation and evaluation of a middleware that combines these ideas and present its feasibility.

Proceedings of the 6th International Conference on Advances in Mobile Computing and Multimedia - MoMM '08, 2008

We describe the ADIMUS architecture which addresses the problem of maintaining the subjective qua... more We describe the ADIMUS architecture which addresses the problem of maintaining the subjective quality of multimedia streaming for a mobile user. In contrast to other works, the entire end-to-end path of the video stream is considered. Adaptation mechanisms for maintaining quality include time-critical handovers, overlay routing and network estimation techniques. Our architecture is built on overlays that provides the necessary functionality for a video streaming service. The paper highlights the key components that ADIMUS advocates to support quality streaming from server to mobile client. 1) The work described in this paper has been conducted as a part of the ADIMUS (Adaptive Internet Multimedia Streaming) project which is funded by the NORDUnet-3 programme.

2012 Second International Conference on Digital Information and Communication Technology and it's Applications (DICTAP), 2012

The increasing demand for live multimedia systems in gaming, art and entertainment industries, ha... more The increasing demand for live multimedia systems in gaming, art and entertainment industries, has resulted in the development of multi-view capturing systems that use camera arrays. We investigate sparse (widely spaced) camera arrays to capture scenes of large volume space. A vital aspect of such systems is camera calibration, which provides an understanding of the scene geometry used for 3D reconstruction. Traditional algorithms make use of a calibration object or identifiable markers placed in the scene, but this is impractical and inconvenient for large spaces. Hence, we take the approach of features-based calibration. Existing schemes based on SIFT (Scale Invariant Feature Transform), exhibit lower accuracy than marker-based schemes due to false positives in feature matching, variations in baseline (spatial displacement between the camera pair) and changes in viewing angle. Therefore, we propose a new method of SIFT feature based calibration, which adopts a new technique for the detection and removal of wrong SIFT matches and the selection of an optimal subset of matches. Experimental tests show that our proposed algorithm achieves higher accuracy and faster execution for larger baselines of up to ≈2 meters, for an object distance of ≈4.6 meters, and thereby enhances the usability and scalability of multi-camera capturing systems for large spaces.

Proceedings of the seventeen ACM international conference on Multimedia - MM '09, 2009

We would like to acknowledge all the help we have been getting from Pengpeng Ni with the statisti... more We would like to acknowledge all the help we have been getting from Pengpeng Ni with the statistical analysis, and Pål Halvorsen and Carsten Griwodz. A big thank you also goes out to all the other Master students at the research group for networks and distributed systems, for keeping up the morale. We would also like to thank all the assessors for using their spare time to help us out.

Proceedings of the 1st workshop on Network and system support for games - NETGAMES '02, 2002

Massive multiplayer games have to cope with scalability problems that arise from the necessity of... more Massive multiplayer games have to cope with scalability problems that arise from the necessity of supporting network traffic for different game elements concurrently. In this paper, we show a simple means of separating these traffic styles in a proxy architecture by defining levels of urgency and relevance for each style. With this simple separation, low latency traffic styles can be preferred effectively over other traffic within the same game. We continue with a proposal for a middleware approach to the use of this infrastructure. It is based on the assumption that game developers can specify static requirements for the distributed objects at design and development time. In this case, we propose a combination of code-generation and run-time support for the use of the network architecture.

32nd IEEE Conference on Local Computer Networks (LCN 2007), 2007

Distributed interactive applications have become increasingly popular, making it important to add... more Distributed interactive applications have become increasingly popular, making it important to address their communication needs, where one of the needs is group communication. In this paper, we consider the applications in which it is at any given time possible to divide its users into groups. The group membership changes over time, and the group division is unrelated to the physical proximity. As a way of enabling the group communication in distributed interactive applications, we choose application layer multicast. We use simulation to evaluate several dynamic algorithms for managing overlay multicast trees. They are compared with respect to four metrics that can be relevant for a distributed interactive application. These are total tree cost, diameter, reconfiguration time and stability. We demonstrate algorithms that perform well for these metrics although they do not consider all users during reconfiguration.

Ninth IEEE International Symposium on Multimedia (ISM 2007), 2007

Real-time delivery of time-dependent data over the Internet is challenging. UDP has often been us... more Real-time delivery of time-dependent data over the Internet is challenging. UDP has often been used to transport data in a timely manner, but its lack of congestion control is often criticized. This criticism is a reason that the vast majority of applications today use TCP. The downside of this is that TCP has problems with the timely delivery of data. A transport protocol that adds congestion control to an otherwise UDP-like behaviour is DCCP. For this protocol, late data choice (LDC) [8] has been proposed to allow adaptive applications control over data packets up to the actual transmission time. We find, however, that application developers appreciate other TCP features as well, such as its reliability. We have therefore implemented and tested the LDC ideas for TCP. It allows the application to modify or drop packets that have been handed to TCP until they are actually transmitted to the network. This is achieved with a shared packet ring and indexes to hold the current status. Our experiments show that we can send more useful data with LDC than without in a streaming scenario. We can therefore claim that we achieve a better utilization of the throughput, giving us a higher goodput with LDC than without.

Lecture Notes in Computer Science, 1996

This paper is about GLASS 2 , a distributed multimedia system that is currently under development... more This paper is about GLASS 2 , a distributed multimedia system that is currently under development. The multimedia department of the IBM European Networking Center in Heidelberg, Germany, participates in the project along with other industrial and university partners. GLASS is an acronym for GLobally Accessible ServiceS, expressing the system's wide range of possible applications. Interactive TV scenarios can be presented as well as multimedia applications for interactive learning and games for entertainment. Clients for standard telecommunications services like FAX and E-mail can be included as well as presentation components providing access to hypertext-oriented internet services like WWW. The system's components can be highly distributed using networks with TCP/IP. An application is driven by MHEG-encoded presentations that allow for the definition of sophisticated presentations. The system comprises multiple server and client components. All components can be operated on a mix of different platforms. Although running on different operating systems, all clients have the same look and feel which is defined completely by the MHEG presentation.

2014 IEEE Symposium on Computers and Communications (ISCC), 2014

Data-bundling is a useful technique that decreases the delivery delay of packet streams when they... more Data-bundling is a useful technique that decreases the delivery delay of packet streams when they are transmitted over noisy channels and are subject to retransmission-based error control. In this paper, we investigate the packet delay statistics for a fully reliable selective repeat automatic repeat request (SR ARQ) where a data-bundling mechanism is employed. In more detail, we discuss a model for data-bundling to analyze the SR ARQ mechanism over wireless channels based on Markov chains. We evaluate various channel error distributions and analyze the buffer occupancy to check if the data-bundling mechanism provides efficient results. We further analyze the queueing, delivery and overall delay statistics at link layer. We found that using data-bundling can improve the delay performance of the SR ARQ mechanism, especially when bursty channels with heavily correlated errors are considered. Thus, this technique can bring useful improvements for real-time services, multimedia, and other delay-sensitive applications over wireless networks.

Advances in Multimedia, 2014

Video streaming is predicted to become the dominating traffic in mobile broadband networks. At th... more Video streaming is predicted to become the dominating traffic in mobile broadband networks. At the same time, adaptive HTTP streaming is developing into the preferred way of streaming media over the Internet. In this paper, we evaluate how different components of a streaming system can be optimized when serving content to mobile devices in particular. We first analyze the media traffic from a Norwegian network and media provider. Based on our findings, we outline benefits and challenges for HTTP streaming, on the sender and the receiver side, and we investigate how HTTP-based streaming affects server performance. Furthermore, we discuss various aspects of efficient coding of the video segments from both performance and user perception point of view. The final part of the paper studies efficient adaptation and delivery to mobile devices over wireless networks. We experimentally evaluate and improve adaptation strategies, multilink solutions, and bandwidth prediction techniques. Based...

2011 IEEE Symposium on Computers and Communications (ISCC), 2011

Networked devices often come equipped with multiple network interfaces, and bandwidth aggregation... more Networked devices often come equipped with multiple network interfaces, and bandwidth aggregation is one of the many possible benefits of using multiple interfaces simultaneously. Real-world networks introduce several challenges that have often been ignored by related work on bandwidth aggregation. The challenges include limited connectivity due to NAT-boxes, link heterogeneity and link variability. In this paper, we present a transparent solution for proxybased bandwidth aggregation that is able to overcome the different deployment and link heterogeneity challenges present in real-world networks. Our focus has been on increasing the performance of bandwidth-intensive UDP-based applications, and through evaluation we show that our solution efficiently aggregates bandwidth and increases the in-order throughput. Previously, we introduced a multi-link UDP proxy solution that improves in-order throughput. This paper presents a significant extension and improvement in terms of support for middleboxes (NAT), congestion control, a client-based resequencer and support for all operating systems.

2010 7th IEEE Consumer Communications and Networking Conference, 2010

Multimedia streaming is increasing in popularity and has become one of the dominating services on... more Multimedia streaming is increasing in popularity and has become one of the dominating services on the Internet today. Even though user devices are often equipped with multiple network interfaces and in reach of several access networks at the same time, media streams are normally communicated over only one of the available Internet connections. In this paper, we explore the challenges and potential benefits of using multiple access networks simultaneously. Exploiting HTTP's capability of handling requests for specific byte ranges of a file, we present the implementation of a lightweight, applicationlayer, on-demand streaming service that requires no changes to existing servers and infrastructure. Based on real-world experiments with a multihomed host, we investigate the potential performance gains of video-on-demand playout. We achieve a bandwidth aggregation efficiency of 90% when downloading over 3 heterogeneous access networks in parallel. In addition, we analyze the effect of file segmentation on the buffer requirements and the startup latency.

Proceedings of the second annual ACM conference on Multimedia systems, 2011

Devices capable of connecting to multiple, overlapping networks simultaneously are becoming incre... more Devices capable of connecting to multiple, overlapping networks simultaneously are becoming increasingly common. For example, most laptops are equipped with LAN-and WLAN-interfaces, and smart phones can typically connect to both WLANs and 3G mobile networks. At the same time, streaming high-quality video is becoming increasingly popular. However, due to bandwidth limitations or the unreliable and unpredictable nature of some types of networks, streaming video can be subject to frequent periods of rebuffering and characterised by a low picture quality. In this paper, we present a client-side request scheduler that distributes requests for the video over multiple heterogeneous interfaces simultaneously. Each video is divided into independent segments with constant duration, enabling segments to be requested over separate links, utilizing all the available bandwidth. To increase performance even further, the segments are divided into smaller subsegments, and the sizes are dynamically calculated on the fly, based on the throughput of the different links. This is an improvement over our earlier subsegment approach, which divided segments into fixed size subsegments. Both subsegment approaches were evaluated with on-demand streaming and quasi-live streaming. The new subsegment approach reduces the number of playback interruptions and improves video quality significantly for all cases where the earlier approach struggled. Otherwise, they show similar performance.

Proceedings of Network and Operating System Support on Digital Audio and Video Workshop, 2013

Panorama video is becoming increasingly popular, and we present an end-to-end real-time system to... more Panorama video is becoming increasingly popular, and we present an end-to-end real-time system to interactively zoom and pan into high-resolution panoramic videos. Compared to existing systems using perspective panoramas with cropping, our approach creates a cylindrical panorama. Here, the perspective is corrected in real-time, and the result is a better and more natural zoom. Our experimental results also indicate that such zoomed virtual views can be generated far below the frame-rate threshold. Taking into account recent trends in device development, our approach should be able to scale to a large number of concurrent users in the near future.

Proceedings of the 22nd ACM international conference on Multimedia, 2014

The interest for wide field of view panorama video is increasing. In this respect, we have an app... more The interest for wide field of view panorama video is increasing. In this respect, we have an application that uses an array of cameras that overlook a soccer stadium. The input of these cameras are stitched together to provide a panoramic view of the stadium. One of the challenges we face is that large parts of the field are obscured by shadows on sunny days. Such circumstances cause unsatisfying video quality. We have therefore implemented and evaluated multiple algorithms related to high dynamic range (HDR) video. The evaluation shows that a combination of several approaches gives the most useful results in our scenario.

IEEE INFOCOM 2008 - The 27th Conference on Computer Communications, 2008

Latency reduction in distributed interactive applications has been studied intensively. Such appl... more Latency reduction in distributed interactive applications has been studied intensively. Such applications may have stringent latency requirements and dynamic user groups. We focus on using application-layer multicast with a centralized approach to the group management. The groups are organized in overlay networks that are created using graph algorithms. We investigate many spanning tree problems with particular focus on reducing the diameter of a tree, i.e., the maximum pairwise latency between users. In addition, we focus on reducing the time it takes to execute membership changes. In that context, we use core-selection heuristics to find well-placed group nodes, and edge-pruning algorithms to reduce the number of edges in an otherwise fully meshed overlay. Our edge-pruning algorithms strongly connect well-placed group nodes to the remaining group members, to create new and pruned group graphs, such that, when a tree algorithm is applied to a pruned group graph, it is manipulated into creating trees with a smaller diameter. We implemented and analyzed experimentally spanning-tree heuristics, core-selection heuristics and edge-pruning algorithms. We found that faster heuristics that do not explicitly optimize the diameter are able to compete with slower heuristics that do optimize it.

2014 13th Annual Workshop on Network and Systems Support for Games, 2014

ABSTRACT In many games, a win or a loss is not only contin- gent on the speedy reaction of the pl... more ABSTRACT In many games, a win or a loss is not only contin- gent on the speedy reaction of the players, but also on how fast the game can react to them. From our ongoing project, we aim to establish perceptual thresholds for visual delays that follow user actions. In this first user study, we eliminated the complexities of a real game and asked participants to adjust the delay between the push of a button and a simple visual presentation. At the most sensitive, our findings reveal that some perceive delays below 40 ms. However, the median threshold suggests that motor- visual delays are more likely than not to go undetected below 51-90 ms. These results will in future investigations be compared to thresholds for more complex visual stimuli, and to thresholds established from different experimental approaches.

Speech Communication, 2015

With teleconferencing becoming more accessible as a communication platform, researchers are worki... more With teleconferencing becoming more accessible as a communication platform, researchers are working to understand the consequences of the interaction between human perception and this unfamiliar environment. Given the enclosed space of a teleconference room, along with the physical separation between the user, microphone and speakers, the transmitted audio often becomes mixed with the reverberating auditory components from the room. As a result, the audio can be perceived as smeared in time, and this can affect the user experience and perceived quality. Moreover, other challenges remain to be solved. For instance, during encoding, compression and transmission, the audio and video streams are typically treated separately. Consequently, the signals are rarely perfectly aligned and synchronous. In effect, timing affects both reverberation and audiovisual synchrony, and the two challenges may well be interdependent. This study explores the temporal integration of audiovisual continuous speech and speech syllables, along with a non-speech event, across a range of asynchrony levels for different reverberation conditions. Non-reverberant stimuli are compared to stimuli with added reverberation recordings. Findings reveal that reverberation does not affect the temporal integration of continuous speech. However, reverberation influences the temporal integration of the isolated speech syllables and the action-oriented event, with perceived subjective synchrony skewed towards audio lead asynchrony and away from the more common audio lag direction. Furthermore, less time is spent on simultaneity judgements for the longer sequences when the temporal offsets get longer and when reverberation is introduced, suggesting that both asynchrony and reverberation add to the demands of the task.