On the capabilities of IEEE 802.11e for multimedia communications over heterogeneous 802.11/802.11e WLANs

Multimedia communications over WLAN is widely acknowledged as one of the key, emerging applications for wireless LANs. As with any multi-service network, there is the need to provision the WLANs with the QoS mechanisms capable of guaranteeing the requirements of various services. The upcoming IEEE 802.11e (EDCA) standard is a proposal defining the mechanisms for wireless LANs aiming to provide QoS support to time-sensitive applications such as voice and video communications. Due to the fact that the IEEE 802.11e interface cards will take over the WLAN market, replacing the use of legacy IEEE 802.11 interface cards in most WLAN applications, an important number of networking scenarios will consist of a hybrid configuration comprising legacy IEEE 802.11-based stations and IEEE 802.11e-based stations. For this reason, in this paper we carry out a performance analysis on the effectiveness of the IEEE 802.11e (EDCA) upcoming standard when supporting different services, such as, voice, video, best-effort, background and in the presence of traffic generated by legacy 802.11 (DCF) based stations.     

A multimedia interhospital communications system for medicalconsultations

The authors present a study whose main objective has been to explore the use of high-bandwidth multimedia communications technology for medical applications physically distributed among hospitals, that is to say an interhospital multimedia communications system. The design and development issues of a prototype are described, as well as the results of an in-field trial. From the results of this trial, a performance study has been developed for evaluating the expected communications requirements of a large-scale configuration of the system     

Performance evaluation of cell discarding mechanisms for thedistribution of VBR MPEG-2 video over ATM networks

ATM networks promise to provide the means to support a wide range of applications exhibiting different traffic characteristics and performance requirements. Video communications have been recognized as one of the most demanding applications to be supported by ATM networks. This is mainly due to the need of transferring large amounts of data and the strict timing requirements characterizing digital video applications. In this paper, simulation experiments are conducted to study the performance of the different cell discarding control mechanisms, in terms of quality of service, when used in an ATM network supporting hierarchical encoded VBR MPEG-2 video distribution. Our results show the effectiveness of the control schemes in reducing the cell loss rates as compared to the system configuration without a cell discarding scheme in place     

Parallel Algorithms Based on the Temporal-Window Method for Non-Alternating 3D-WT over Angiographies Using a Multicomputer

In this paper, we introduce and evaluate the parallel implementations of two video sequences decorrelation algorithms having been developed based on the non-alternating three-dimensional wavelet transform (3D-WT) and the temporal-window method. The proposed algorithms have been proven to outperform the classic 3D-WT algorithm in terms of a better coding efficiency and lower computational requirements while enabling a lossless coding and a top-quality reconstruction: the two most highly relevant features to medical imaging applications. The parallel implementations of the algorithms are developed and tested on a shared memory system, a SGI Origin 3800 supercomputer, making use of a message-passing paradigm. We evaluate and analyze the performance of the implementations in terms of the response time and speed-up factor by varying the number of processors and various video coding parameters. The key point enabling the development of highly efficient implementations rely on a workload distribution strategy supplemented by the use of parallel I/O primitives, for better exploiting the inherent features of the application and computing platform. Two sets of I/O primitives are tested and evaluated: the ones provided by the C compiler and the ones belonging to the MPI/IO library.     

RPSF: a new QoS bandwidth request mechanism for the IEEE?802.16

The IEEE 802.16 standard is a Broadband Wireless Access (BWA) technology which offers Quality of Service (QoS) support to different types of applications. This standard defines the physical (PHY) and medium access control (MAC) layers. Its MAC layer defines different types of QoS mechanisms to support various types of applications, being the multicast polling one of these mechanisms. Under this mechanism, based on a contention process, every connection competes to gain access to the channel in order to place its bandwidth requests. In this paper, we propose a new signalling mechanism, called Requests Per Service Flow (RPSF), to reduce the contention phase in the frame. Additionally, we undertake a comparison of this new method with respect to other mechanisms. The simulation results show that our new proposal outperforms other mechanisms recently reported in the literature, in terms of throughput and end-to-end delay.     

Control mechanisms to improve the robustness of mpeg-2 based communications over ATM networks

Audio-visual and other multimedia services are seen as an important source of traffic for ATM networks. Due to the statistical multiplexing schemes used by ATM networks, cells are exposed to delays, jitters and losses. These phenomena affect the quality of the service provided to the video-based applications. In this paper, we introduce a set of control mechanisms at different levels of the protocol architecture. These mechanisms have been particularly tailored to better support MPEG-2-based video communications applications using ATM networks as their underlying transmission mechanism. We show that the use of errorconcealment mechanisms prove more effective when supplemented by a structured set of protocol mechanisms. We study the impact of cell losses on the quality of MPEG-2 video sequences and provide an evaluation of the quality of the decoded video as perceived by the end user. Our results demonstrate the effectiveness of the proposed protocol architecture in improving the quality of service provided to the video application.     

A performance study for the multicast collision prevention mechanism for IEEE 802.11

In IEEE 802.11 networks a data packet is delivered simultaneously to multiple receivers through the multicast paradigm. The standard defines a simple mechanism that does not implement any error-recovery mechanism, thus, the reliability of the service provided to the multicast users is penalized. This issue is more important as the number of collisions increases due to a large number of active stations and/or a high load network. In this paper we carry out a detailed optimization study of the multicast collision prevention (MCP) mechanism, a highly-efficient multicast collision avoidance mechanism for IEEE 802.11 previously introduced by the authors. Besides a more in deep explanation of MCP, this study includes a comparative performance evaluation of the optimized MCP with the IEEE 802.11 standard. Results shown that, through this optimization, the number of collisions in MCP can be made negligible for any network load.     

A multimedia medical communications system

The problem of communicating the radiologist's findings to the attending physician is introduced, and it is shown why it is essentially a multimedia communication problem. A multimedia communications system that was designed with a focus on database and medical reports organization and architecture is described. A report is presented of some of the key results of an in-house trial linking the Department of Radiological Sciences and the Emergency Department at the Ottawa Civic Hospital, Canada, a 950-bed tertiary-care teaching hospital     

On the optimization of the contention resolution mechanism for IEEE 802.16 networks

The IEEE 802.16 standard defines a contention bandwidth request mechanism offering QoS-aware broadband access through a wireless medium. According to the standard, the Base Station (BS) is responsible for granting the requests issued by the Subscriber Stations (SSs). The SSs communicate their requests to the BS by placing their requests into the uplink subframe, using the contention period defined within the subframe for this purpose. The size of contention period, defined in minislots, is fixed by the BS on a frame by frame basis. The remaining part of the uplink subframe is used to carry the data frames. A tradeoff between these two periods of the uplink subframe is required for an effective operation of the system. On the contrary, if the size of the contention is shorten below a certain threshold, the SSs may be unable to place their requests within the time required by the end-user applications. In this paper, we introduce an adaptive mechanism of the contention period size. The proposed mechanism takes into account the varying operation conditions of the system by estimating the number of stations that may be placing a request. Simulation results confirm that our proposal outperforms other mechanisms having recently been reported in the literature.