Sergio Cabrero

Limitations of network emulation with single-machine and distributed ns-3

Research on large-scale internet services requires an extensive evaluation prior to deployment. A good analysis must include tests over large networks, using real devices and a considerable number of users. However, how to test in these scenarios with many users is an open question. Network emulation can be a good alternative before real deployments, which are complex and expensive. In this paper, we examine the new ns-3 network simulator/emulator in order to determine its capacity in the evaluation of large scale services. For that purpose, a real client/server video service is deployed over an emulated network. The service is progressively scaled up by increasing the number of clients on a single machine. In addition, we have extended ns-3 to support a distributed architecture for network nodes, thus, we repeat the experiments with a distributed set-up. Advantages, disadvantages, possibilities and limitations of both approaches are thoroughly discussed.

Delivering stable high-quality video: an SDN architecture with DASH assisting network elements

Dynamic adaptive streaming over HTTP (DASH) is a simple, but effective, technology for video streaming over the Internet. It provides adaptive streaming while being highly scalable at the side of the content providers. However, the mismatch between TCP and the adaptive bursty nature of DASH traffic results in underperformance of DASH streams in busy networks. This paper describes a networking architecture based on the Software Defined Networking (SDN) paradigm. Controllers in the network with a broad overview on the network activity provide two mechanisms for adaptation assistance: explicitly signaling target bitrates to DASH players and dynamic traffic control in the network. We evaluate how each of these mechanisms can contribute to the delivery of a stable and high quality stream. It shows that our architecture improves the quality of experience by doubling the video bitrate and reducing disturbing quality switches. As such, this paper contributes insights on how to implement DASH-aware networking that also enables internet service providers, network administrators, and end-users to configure their networks to their requirements.

Modeling Stability and Bitrate of Network-Assisted HTTP Adaptive Streaming Players

Viewers using HTTP Adaptive Streaming (HAS) without sufficient bandwidth undergo frequent quality switches that hinder their watching experience. This situation, known as instability, is produced when HAS players are unable to accurately estimate the available bandwidth. Moreover, when several players stream over a bottleneck link, their individual adaptation techniques may result in an unfair share of the channel. These are two detrimental issues in HAS technology, which is otherwise very attractive. To overcome them, a group of solutions are proposed in the literature that can be classified as network-assisted HAS. Solving stability and fairness only in the player is difficult, because a player has a limited view of the network. Using information from network devices can help players in making better adaptation decisions. The contribution of this paper is three-fold. First, we describe our implementation in the form of an HTTP proxy server, and show that both stability and fairness are strongly improved. Second, we present an analytical model that allows to compute the number of changes in video quality and the bitrate of a video stream. Third, we validate the accuracy of the model by comparing the model-based estimations for the number of changes in video quality and for the mean bitrate of a video stream, with results in a real implementation of our HAS assistant. The results show that the model-based results are highly accurate. As such, this model is useful in practice for planning video delivery networks that use in-network HAS assistants, and enables us to analyze the stability and the mean bitrate of HAS streams prior to real deployment.

Adaptable system based on Scalable Video Coding for high-quality video service

Content adaptation to a heterogeneous environment like the Internet is a key process for improving the perceived quality of the user. This paper presents an adaptive streaming system using Scalable Video Coding (SVC) technology. Using feedback information from clients about the transmission status, the server is able to select the most suitable combination of SVC layers for the available bandwidth. The estimation of the available bandwidth is carried out with non-intrusive methods, based on classic metrics such as packet loss, jitter and novel metrics like the linearity of reception times of RTP packets. The system is implemented in real equipment and the results show the correct operation and the accuracy of the system when adapting to different variations of the available bandwidth. We also study the scalability of the system when several clients access the service simultaneously, demonstrating that our system is as scalable as a non-adaptive system with SVC.

Evaluation of Virtual Keyboards for Interactive Digital Television Applications

The development of the technologies behind Interactive Digital Television (IDTV) services has produced a new type of audience. Traditional viewers now become users as they may play an active role in front of the TV, for example, by choosing a video to be played on demand or by introducing text on an IDTV application. In these services, interactions need to be performed with a remote control, currently the main interaction device, or other devices such as keyboards or mice, which are not very popular in this environment. Nevertheless, although remote controls are essential tools for IDTV services, they are very limited when it comes to writing text. Thus, this study evaluated different alternatives to introduce text on an IDTV application with a remote control. A heterogeneous group of people was selected to write predefined sentences in Spanish in a test environment using three virtual keyboard layouts and the multitap mechanism. Their performance and subjective impressions reveal weaknesses and strengths of the evaluated methods. The article draws important conclusions about the usage of remote controls in IDTV applications, not only for the design of new applications but also for the research of new techniques to introduce text.

Overlay solution for multimedia data over sparse MANETs

Using Mobile Ad-hoc Networks (MANETs) for audio and video transmission is very promising for application domains such as emergency and rescue. However, audio/video streaming services are not designed for such dynamic and unstable networks. The problems are even more important in so-called sparse MANETs where the node density is relatively low so that disconnections and network partitions are common. We have designed an architecture that combines MANET routing with caching and delay tolerant store-carry-forward operations in an overlay network to improve the quality of audio/video transmission over sparse MANETs. We have implemented a prototype to evaluate the architecture. The results from the experiments demonstrate that our system clearly outperforms simple client-server solutions when the network has temporal disconnections.

An Empirical Investigation Into Text Input Methods for Interactive Digital Television Applications

Nowadays there is a huge market emerging in the interactive digital TV realm. In this context, we need new and effective methods of user interaction, as the main interaction device is still the classical remote control. Remote controls are especially problematic when it comes to writing text, something needed in most applications. Thus, we have carried out an empirical investigation to find effective methods of text entry with remote controls. We analyze several methods by performing experiments based on a methodology in which a heterogeneous set of real users carries out several sequential tasks in an incremental process. We analyze entry speeds, error rates, learning profiles, and subjective impressions, taking into account the particular characteristics of the users. Our results show, for instance, that Multitap is a good method for simple texts. It is between 12% and 34% faster than the fastest virtual keyboard, depending on the age of the user. Nevertheless, when complex texts need to be written, virtual keyboards present the same or even better writing speeds (QWERTY is 13% faster) and with significant lower error rates (Multitap is 347% worse than QWERTY). We consider that our results are very interesting for researchers, designers of TV applications, and hardware vendors.

Subjective evaluation of critical success factors for a QoE aware adaptive system

For many years video content delivery has established itself as the killer application. Improving QoE on adaptive streaming is focusing many efforts in the quest for optimized methods and metrics to allow a QoE driven adaptation. Questions such as whether adaptive systems based on Scalable Video Coding improve subjective quality and in which situations or to what degree are still open issues. Tolerance and indifference thresholds for each type of content, conditions or viewer category, with regard to adaptive systems are critical success factors that are yet unresolved. We compare the performance of a complete adaptive system with the traditional, i.e. non-adaptive, approach in subjective terms. Results of surveying 75 participants show that the adaptation improves QoE under most of the evaluated conditions. Tolerance thresholds for triggering adaptation events have been identified. Users accustomed to Internet video are more critical than users that only watch TV. The under 35year old subset among the available population is generally more satisfied with the adaptive system than the older subset.

Three Techniques for Competitive Lab Activities Based on Project-Oriented Learning in Information and Communication Technologies

Laboratory work is one of the most important activities in technical degrees. In these classes, students learn while carrying out tasks similar to those they could encounter in a professional environment. These activities help to strengthen the theoretical principles while allowing the students to develop useful skills for their careers. In this paper, we present three experiences (Lab Battle, Lab Race, and Lab Triathlon) in the organization of practical activities based on the paradigms of competitiveness and project-oriented learning. The goal of these techniques is to immerse the students in an environment as realistic as possible. In such scenarios, it is often important to be first, to think like the “bad guy” or to keep confidentiality. After three years applying these techniques, we have observed great motivation and excellent competences in the students. Moreover, cheating has been less frequent than in other types of lab assignments.

A flexible QoE framework for video streaming services

QoE metrics are a must in new service evaluations as they have shown to outperform traditional QoS. This work presents a flexible framework for measuring different types of QoE analysis and capable of synchronizing reference sequence with distorted videos for avoiding erroneous coupling. We define cPSNR metric, a measurement system suitable for performing the evaluation of transmission impairments exclusively, from the perceived quality perspective. The metric is validated with a subjective video database showing a promising correlation with both packet loss ratio and subjective quality.