Catálogo de publicaciones - libros

Recent and upcoming CSMA/CA based MAC protocols for wireless LANs offer block-acknowledgement or aggregated frame exchange in order to provide high data rate for multimedia and real-time data, while all of them avoid collisions by exclusive use of wireless medium that block neighboring nodes of both sender and receiver from participating in concurrent transmission; and thus downgrade medium utilization and overall throughput. In this work, we offer to create concurrent virtual channels over same physical channel by synchronizing the transmit/receive switching of senders and receivers and allow parallel data transfer. Each virtual channel is used by a transmitter/receiver pair and all virtual channels within 2-hop network utilize the medium by distributed coordination and avoid inter virtual channel interference. Simulation result shows that proposed scheme removes the obstruction to participate in parallel transmission for some neighboring nodes of sender and receiver and improves the network performance.

This paper describes a method for robust streaming of combined MPEG-2 audio/video content over in-home wireless networks. We make use of currently used content distribution formats and network protocols. The transmitted bit-rate is constantly adapted to the available network bandwidth, such that audio and video artifacts caused by packet loss are avoided. Bit-rate adaptation is achieved by using a packet scheduling technique called I-Frame Delay (IFD), which performs priority-based frame dropping upon insufficient bandwidth. We show an implementation using RTP and an implementation using TCP. Measurements on a real-life demonstrator set-up demonstrate the effectiveness of our approach.

Various realtime multimedia applications will be provided in next generation Internet where IEEE 802.11e wireless LAN will be widely used as broadband access networks. The characteristics of the wireless channels in IEEE 802.11 (i.e., fluctuating bandwidth and large error rate), however, impose challenging problems in the efficient QoS-guaranteed realtime multimedia communications with strict QoS requirements (i.e., bandwidth, delay, jitter, and packet loss/error rate). In this paper we propose management schemes of IEEE 802.11e wireless LAN (WLAN) for realtime QoS-guaranteed teleconference services with differentiated H.264 video transmission. In the proposed scheme, the IEEE 802.11e Wireless LAN is managed to transmit I, P and B slices from H.264 encoder using different channels of both HCF controlled channel access (HCCA) and enhanced distributed channel access (EDCA). We compare several different mapping scenarios, and analyze the QoS provisioning performance for realtime multimedia teleconference service.

Due to bandwidth constraint and dynamic topology of mobile ad hoc networks, supporting Quality of Service is a challenging task. In this paper we present a solution for QoS routing based on an extension of the AODV reactive routing protocol that deals with bandwidth monitoring. The solution uses an IEEE 802.11 MAC layer as the underlying technology and the QoS routing decision is based on simple but accurate measurements, at the MAC layer, of the available bandwidth on each link of the route. In addition, to allow a QoS loss recovery, a notification mechanism is used to inform the source about bandwidth degradation on a link. This reactive solution using standard protocols is adapted to small and dynamic ad hoc networks. A complete simulation set shows that, with the proposed QoS routing protocol, bandwidth on a route is significantly improved without overhead.

Automatic service discovery is essential for the usability of self-configuring Mobile Ad Hoc Networks (MANETs). Existing service discovery solutions are either directory based or directory-less. The directory based approaches are fast and reliable but have the complexity of maintaining directories in the dynamic MANET environment. On the other hand, the directory-less approaches use broadcasts which are very expensive in MANETs. This paper proposes and evaluates a new service discovery protocol called MANET Service Location and Discovery (MSLD), which is integrated with a stateful auto-configuration protocol called DHAPM. Because of this integration, MSLD reuses the robust directory structure of DHAPM, which allows MSLD to acquire the advantages of the directory based service discovery protocols without any additional complexity of directory maintenance. The performance evaluation shows that MSLD has low service discovery latency and high service availability with minimum communication overhead.

In the IEEE 802.11 MAC layer protocol, there are different trade-off points between the number of nodes competing for the medium and the network capacity provided to them. There is also a trade-off between the wireless channel condition during the transmission period and the energy consumption of the nodes. Current approaches at modeling energy consumption in 802.11-based networks do not consider the influence of the channel condition on all types of frames (control and data) in the WLAN. Nor do they consider the effect on the different MAC and PHY schemes that can occur in 802.11 networks. In this paper, we investigate energy consumption corresponding to the number of competing nodes in IEEE 802.11’s MAC and PHY layers in error-prone wireless channel conditions, and present a new energy consumption model. Analysis of the power consumed by each type of MAC and PHY over different bit error rates shows that the parameters in these layers play a critical role in determining the overall energy consumption of the ad-hoc network. The goal of this research is not only to compare the energy consumption using exact formulae in saturated IEEE 802.11-based DCF networks under varying numbers of competing nodes, but also, as the results show, to demonstrate that channel errors have a significant impact on the energy consumption.

Existing streaming protocols have no consideration for the characteristics of streaming applications because they only consider the network stability. In this paper, in order to overcome limitations of the previous schemes for video streaming, we propose a new video streaming mechanism called ”MaVIS (Media-aware Video Streaming)”. The MaVIS takes more sophisticated mechanism that considers both network and user requirements. Proposed mechanism improves the network stability by adjusting the sending rate suitable for current network state and it also provides the smoothed playback by preventing buffer underflow or overflow. Moreover, it is designed to consider characteristics of the video stream. Through the simulation, we prove that the MaVIS mechanism efficiently uses the buffer resources and provides improved network stability and smoothed playback.

Real time multimedia applications such as Internet TV, Video On Demand, Distance Learning, and Video Conferencing are becoming more and more popular over the Internet. Streaming media caching is a critical ingredient in the ability to provide scalable real-time service over the best effort Internet. In many cases, bandwidth becomes the system bottleneck and the cache cannot provide the required quality for all streams simultaneously. In this paper we study new algorithms, based on cooperation, which can improve the cache ability to provide service to all of its clients. The main idea is based on the willingness of streams to reduce their used bandwidth and allow other streams that may need it more, to use it. Our extensive simulation study indicates that our algorithms can reduce the pre-caching time by a factor of 3, or increase the probability for adequate service level by 30% using the same pre-caching time.

In this paper, we propose a delay constrained spatio-temporal video rate control method for lossy channels where the effective channel rate available to the video encoder is time-varying. Target bit-rate constraint for encoding is derived, which guarantees in-time delivery of video frames. By using empirically obtained rate-quantization and distortion-quantization relations of video, the distortions of skipped and coded frames in near future can be calculated in real-time. For the window expanding from the current to the firstly coded frame including the skipped frames in between, the number of frames to skip from the current and the quantization parameter(QP) for the firstly coded frame are decided in the direction to minimize the average distortion of frames in the window. From the simulation results, the proposed method is shown to enhance the average PSNR performance compared to TMN8 with some increase in the number of skipped frames and less number of delay violations.

This paper proposes a framework for replicating content in multicast- based CDNs. We focus on the design of a scalable and robust system that provides local availability and redundancy of content. The system takes on-line and distributed replication decisions on a per-object basis. The scalability and local redundancy is achieved by partitioning the overlay of surrogate servers into fully meshed groups. The proposed framework can incorporate any set of local metrics and constraints for deciding the placement of replicas, thus allowing the CDN designer to tune it to his specific deployment characteristics.