Adaptive cross-layer strategies for multicast video streaming over ieee 802.11 networks

Resumen

Resume The IEEE 802.11 WLANs (Wireless Local Area Networks) are one of the fastest growing network technologies in the wireless communications field. With wireless gaining prominence and acceptance, it is foreseeable that video applications will be highly demanded as we witness the widespread use of digital video capable mobile devices. Since the first version of the IEEE 802.11 Standard, the IEEE has developed various mechanisms that outperforms the point-to-point deliveries. However, video streaming applications will often require the delivery of video material to a group of users. In this scenario, the multicast service will be certainly the best choice. The multicast service offers substantial benefits, such as higher end-user throughput while reducing the bandwidth requirements. However, the effective deployment of multicast communications services is a challenging task where protocol engineers confront a tradeoff between channel utilization and reliability. Additionally, the efficient and effective delivery of video material has to meet not only stringent QoS (Quality of Service) requirements, but also the QoE (Quality of Experience) of the end user. The multicast service as specified by the IEEE 802.11 Standard does not offer any feedback on the delivery process of the data. In a error-prone communication medium, such as the wireless medium, the quality of the channel has a dramatic impact on the effective delivery of the data. In the case of unicast service, this has been addressed by including an adaptive rate mechanism. The channel transmission rate is often changed based on the channel quality reported by the feedback received by the sender from the receivers. However, in the case of multicast service not such mechanisms have yet been defined. The multicast sender, regardless of the channel quality, does not get a second chance to retransmit the data or at least a way to adapt the transmission rate according to the channel quality. Due to the intrinsic characteristics, broadcast and frequent transmission impairments, it is a must to count with a multicast service capable of offering the minimum QoS and QoE required by video applications. Accordingly, many research and standardization efforts are under-way aiming to enhance the operation of the multicast service. As a starting point, this Thesis reviews the main contributions reported in the literature. It then looks into the IEEE 802.11aa amendment, which defines a set of mechanisms to be integrated into the multicast service, and particularly tailored to support audio visual applications. As part of this review, the Thesis includes a performance analysis of the various mechanisms. The analysis of the multicast services defined in the IEEE 802.11aa amendment has allowed identifying open issues, such as scalability limitations and lack of channel transmission rate adaptation in several of the proposed mechanisms. Hence, in this Thesis an integrated QoS/QoE driven procedure is developed to ensure the transmission of video applications over wireless networks with an acceptable level of quality. The proposed mechanisms provide the multicast deliveries with feedback and a rate adaptation algorithm. Both mechanisms are designed taking into account the compatibility with legacy IEEE 802.11 devices and their efficiency. Then, the use of scalable video coding is considered to include into the multicast mechanism the capacity of adapting the video source rate to the network load conditions. The aim is to guarantee the best possible QoE level by prioritizing the packets of the video flow containing the main information. These packets are easily identified in an SVC video flow. The results reported in this Thesis indicate that the integrated solution guarantees the user expectations by means of QoE metrics. In short, by properly coupling the video and multicast control mechanisms, the QoS as well as the QoE requirements of the end-user can be guaranteed and consequently, the real-time video streaming quality of the total system can be greatly improved. Thesis Organization To address the objectives listed in the previous section, this document is divided into five chapters. Their content is summarized as follows: Chapter 1.This chapter contains the motivation and objectives of the Thesis. Chapter 2. This chapter describes the main characteristics of IEEE 802.11 networks. In particular, it describes the main physical layer functions and the various coordination functions defined in the MAC layer to access the wireless medium. The descriptions are presented by taking into account the evolution of the IEEE 802.11 standard and the current amendments not yet included in it. The chapter concludes by pointing out the main limitations of the multicast service of the IEEE 802.11 standard. Chapter 3. The objective of this chapter twofold. First, it classifies, describes and analyses the most relevant proposals reported in the literature aiming to improve the delivery process of multicast traffic. Second, it describes and evaluates the enhancements for the multicast service of the IEEE 802.11aa amendment. The retransmissions policies described by the GATS service are evaluated. Chapter 4. This presents a new multicast mechanism for IEEE 802.11 networks. Its operation mode is described in various steps and each characteristic of the new mechanism is validated by analysing the simulation results. The performance of the proposed mechanism is compared against other mechanisms, including the ones defined in the IEEE 802.11aa amendment. The results given include QoS and QoE metrics. Chapter 5. The document is concluded with this chapter in which the conclusions, the publications of content from this Thesis, and the future work to be undertaken are included. Conclusions The main objective of the Thesis has been the design and evaluation of a set of protocols and control mechanisms enabling the definition of a robust and efficient multicast service for the IEEE 802.11 WLANs. This required a study of the multicast service specifications as defined by the Standards and the recent IEEE 802.11aa amendment. This study revealed that the multicast service defined by the standards does not include any feedback mechanisms allowing the AP to identify or recover the data being delivered. Moreover, due to the changing operating (quality) conditions characterizing the wireless medium, the AP is unable to adapt its transmission parameters, i.e., transmission rate, as a measure to overcome the loss of the data being delivered. Once having identified the major shortcoming of the multicast service specified by the IEEE 802.11 Standard, Chapter 3 provides an overview of the most closely related proposals recently reported in the literature. This overview has included the main issues that the authors have faced, such as overhead and compatibility with the IEEE 802.11 standard. In this chapter, the solutions of the IEEE to improve multicast performance have also been included. Although no changes have been made to the multicast service in the latest version of the standard, various enhancements have been included in various amendments, namely 802.11aa and 802.11v . These enhancements have been grouped into the GATS procedure, which defines three new retransmission policies and one delivery method. As part of the study of the amendments, the following retransmission policies of GATS have been evaluated: DMS, GRC-Unsolicited retries, GRC-Block-ACK. Although the aim of GATS is to choose the retransmission policy depending on the network conditions, the results have shown that none of them meets the QoS requirements in all scenarios. Thus, in real networks the AP and the STAs should be able to change the GATS procedure dynamically to maximize the performance of the multicast flow. This adaptation is not a trivial task because it implies the interchange of management frames defined by the IEEE 802.11v amendment and the need of devices implementing all the functionalities. Usually, the Wi-Fi Alliance does not incorporate the most sophisticated mechanisms of the IEEE 802.11 standard. Therefore, these enhancements may not be implemented in real devices in the future. Nevertheless, even considering the implementation of the amendments and an efficient adaptation of the GATS procedure to the network conditions, none of the GATS services is able to provide a sufficient QoS level, in particular in the case of multicast video sources of 1.5 Mbps or more. The results of the performance evaluation have shown that DMS suffers from severe scalability problems. The GRC-Unsolicited retries retransmission policy does not include any feedback mechanism making infeasible the implementation of a transmission rate adaptation scheme. Regarding the GRC-Block-ACK mechanism, its performance depends on the number of MRs selected as leaders. It obtains the highest throughput when all MRs are leaders, but the network performance suffers as the network load is increased or the network size increases. Furthermore, IEEE 802.11aa does not include a rate adaptation mechanism. Consequently, its performance is limited. Moreover, all of these mechanisms defined in the GATS introduce substantial overhead due to the large number of control messages needed by the different mechanisms. Furthermore, the retransmission of packets causes degradation. In particular, in medium and highly loaded networks, the legacy mechanism exhibits better results than the new GATS mechanisms in some scenarios due to the overhead introduced by GATS services. From the study of the multicast mechanisms included in the IEEE 802.11 Standard and its amendments, it is clear that the multicast service is a key service for the delivery of multimedia content. For this reason, the AP should be able to correctly deliver video streams with a high variety of video resolutions. QCIF or CIF resolutions may be adequate for users of smart phones, but are rather small for users of tablets, laptops or PCs, who demand capable of handling higher resolutions such as SD or even HD video. It is therefore important to realize that the bit rates of the video sources will widely vary as a function of the encoding scheme and that they will be adapted to the profiles of the end-users. Therefore, the multicast mechanism should be carefully designed to not only provide effective delivery, but also to do it efficiently. Moreover, it is not enough to meet an acceptable QoS level; it is also essential to satisfy the end user expectations, i.e., to provide the multicast service with QoE. Once having examined the operation and deficiencies of the multicast mechanisms defined by GATS, this Thesis proposes the HAMM/CP+ framework. The HAMM/CP+ mechanism has been designed by following several steps. These steps include the study of the main sources of packet errors or losses in WLANs, the design and test of the various mechanisms making part of HAMM/CP+. In short, a straightforward mechanism has been developed that addresses the needs of the wireless video multicast services. It avoids collisions and uses a negative feedback mechanism to retransmit the corrupted packets and adapt the transmission rate accordingly, based on the feedback received by the AP from the receivers. The HAMM/CP+ mechanism centralizes the modifications at the AP and bases these modifications on two transmission classes for video transmissions. The legacy MRs of a network can benefit from the collision prevention and video stream prioritization performed at the AP. Legacy MRs can also benefit from the feedback of the MRs that implement the negative feedback procedure. This situation does not occur in other multicast mechanisms, in which all STAs involved in the multicast service must be modified, including the IEEE 802.11aa amendment. It uses a concealment address to send the retransmitted packets as defined by each mechanism in GATS, and thus the legacy stations will exhibit the same performance level as when using the legacy multicast transmission. The HAMM/CP mechanism ensures a minimum level of QoS and QoE in all tested scenarios. Nevertheless, in small networks and with light load conditions, it achieves less throughput than AMM/CP. 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