On group communication support in CORBA

Group communication services typically generate large multicast data streams. Delivering such massive data streams to the end system nodes at the edge of the Internet has been a challenging problem in terms of high stress on the network links and high demand on network resources and routing node capacities. Most of existing research has been dedicated on geo-distance based routing with various optimizations to alleviate the performance impact on geo-distance based routing due to unpredictable network dynamics. Most representative techniques are targeted at reducing the delivery path length or optimizing routing path by utilizing network locality. In this paper, we identify the inefficiency of geo-distance based routing protocols in many existing multicast overlay networks in terms of both resource utilization and group communication efficiency. To address this issue, we develop a utility-based routing scheme (UDR) that can provide efficient group communication services in a decentralized geographical overlay network. Our approach makes three unique contributions. First, we introduce a utility function to refine the geo-distance based routing in such a way that the routing path selection can carefully incorporate both geo-distance based metric and the network latency. Second, we enhance our utility driven routing scheme with self-adaptive capability by considering the nodes' state and network density. Thus, nodes in the multicast network can dynamically accommodate the changes of network conditions based solely on their local knowledge about the network. Third, we devise a suite of optimization techniques to minimize the maintenance cost and computational complexity of our self-adaptive and utility-drive routing scheme. We evaluate our approach through extensive experiments based on a realistic network topology model and show that the UDR method is highly scalable and it effectively enhances the multicast delivery efficiency for large scale group communication services compared to existing geo-distance based routing protocols.

We discuss a group protocol which supports applications with group communication service when QoS supported by networks or required by applications is changed. An autonomic group protocol is realised by cooperation of multiple autonomous agents. Each agent autonomously takes a class of each protocol function so as to support only and all QoS required. A group is composed of views for scalability. Each view is a subset of the agents where the agents autonomously take protocol classes consistent with each other. We make clear what combination of classes can be autonomously taken by agents in a view.

In distributed systems, high service availability can be achieved by letting a group of servers replicate the service state; if some servers fail, the surviving ones know the service state and can continue to provide the service. Group communication services, such as membership and atomic broadcast, have been proposed to solve the problem of maintaining server state replica consistency. Group membership achieves agreement on the history of server groups that provide the service over time, while atomic broadcast achieves agreement on the history of state updates performed in each group.

In this paper, we propose the design of an extension of the Java distributed object model based on the group communication paradigm for building replicated fault-tolerant servers. Maintaining the correctness and integrity of replicated servers is supported by the group manager object instantiated with each replica to form a logical group and the group membership algorithms to maintain a consistent group view and a reliable multicast mechanism to communicate with other group managers. Group manager object enables the creation of groups of remote objects that cooperate towards some common goal using a join/leave group communication service. Remote object groups simulate the behavior of standard remote objects by implementing a set of remote interfaces and by enabling clients to remotely invoke the methods defined in these interfaces through the standard Java RMI mechanism. For the first time, communication among senders forming a group is based on group method invocations as well, offering a completely object-oriented framework for programmers. And then, it is completed by a state merging service, which simplifies the development of state merging mechanism needed to re-establish a consistent state after the disappearing of join/leave properties

This paper presents several distributed algorithms that cause a token to continually circulate through all the nodes of a mobile ad hoc network. An important application of such algorithms is to ensure total order of message delivery in a group communication service. Some of the proposed algorithms are aware of, and adapt to changes in, the ad hoc network topology. When using a token circulation algorithm, a round is, said to complete when every node has been visited at least once. Criteria for comparing the algorithms include the average time required to complete a round, number of bytes sent per round, and number of nodes visited per round. Comparison between the proposed algorithms is performed using simulation results obtained from a detailed simulation model (with ns-2 simulator).

The study aims to describe the implementation of a primary practitioner physiotherapy service within a large regional ED and to evaluate its impact on waiting time and length of stay. Integration and scope of practice of the physiotherapy service within the ED were described. Retrospective analysis of waiting time and length of stay data for the 3 years, 1 October 2008 to 30 September 2011, was performed. The performance of the physiotherapy service was compared with the Australasian College for Emergency Medicine (ACEM) waiting time guidelines, the Victorian Department of Health length of stay targets and to a similar group of patients seen by medical staff. The primary practitioner physiotherapy service saw 5641 patients of which 33% were less than 18 years old. The most common presentations seen by the service were limb fractures, soft tissue injuries and low back pain. Ninety-two per cent of patients were seen within the ACEM waiting time guidelines, and 91% of patients who went home were discharged within 4 h. Non-randomised comparisons with similar patients seen by medical staff indicated that patients seen by the physiotherapy practitioner service had shorter waiting times and length of stay. The current study found that patients with musculoskeletal problems seen by a primary practitioner physiotherapy service had improved waiting times and length of stay relative to ACEM and Victorian Department of Health targets, and though constrained by a retrospective non-randomised design, to a similar group of patients seen by medical staff. Further evaluation of the physiotherapy practitioner service is required.

Software based Distributed Shared Memory (DSM) systems have been the focus of considerable research effort, primarily in improving performance and consistency protocols. Unfortunately, computer clusters present a number of challenges for any DSM systems that are not solvable through consistency protocols alone. These challenges relate to the ability of DSM systems to adjust to load fluctuations, computers being added/removed from the cluster, to deal with faults, and the ability to use DSM objects larger than the available physical memory. We present here a proposal for the Synergy Distributed Shared Memory System and its integration with the virtual memory, group communication and process migration services of the Genesis Cluster Operating System.

The paper investigates the use of group communication to improve the efficiency of load balancing in interconnected LANs. Conventional load balancing techniques usually assume point-to-point connections among the computers and typically work on a single LAN only. This may waste network bandwidth and lengthen the load balancing time in LANs that support group communication such as the Ethernet. To tackle this problem a two level algorithm is proposed. At the LAN level, all computers in a LAN maintain loading information in the entire LAN using a globally ordered group channel. The workload in the different LANs is balanced by moving load from the overloaded LANs to the underloaded ones. It is proved that the proposed algorithm converges to the state of global balance geometrically. Experimental results show that the algorithm reduces load balancing time and network utilization significantly, compared to the single level algorithm without group communication.

The so-called media convergence to the Internet is foreseen. As a consequence of this convergence, MANs will face new demands not only in terms of bandwidth, but also in terms of services. To meet these new demands, new MAN architectures are required. WDM-based MAN architectures that tackle the first problem are available not only in the literature, but also on the market. In this paper we deal with the second problem. Specifically, we describe a novel WDM-based MAN architecture that supports group communication, a service that is expected to increase considerably as new applications converge to the Internet. Based on packet switching, the architecture supports both point-to-point and point-to-multipoint communication in the optical domain.

Different distributed cyber-physical systems must handle a periodic and periodic events with diverse requirements. While existing real-time middleware such as real-time CORBA has shown promise as a platform for distributed systems with time constraints, it lacks flexible configuration mechanisms needed to manage end-to-end timing easily for a wide range of different cyber-physical systems with both aperiodic and periodic events. The primary contribution of this work is the design, implementation and performance evaluation of the first configurable component middleware services for admission control and load balancing of a periodic and periodic event handling in distributed cyber-physical systems. Empirical results demonstrate the need for, and the effectiveness of, our configurable component middleware approach in supporting different applications with a periodic and periodic events, and providing a flexible software platform for distributed cyber-physical systems with end-to-end timing constraints.

Mobile radio group communications enable a closed group of users to communicate among themselves using push-to-talk (PTT), half duplex transmissions, such that one's transmission is received by all. New protocol designs are presented which support group communications over mobile satellite systems. A new demand assignment multiple access (DAMA) protocol is necessitated by the inappropriateness of call-based channel assignments. Channels are assigned to individual user-groups engaged in active conversations. As members of a user-group contend for access to the assigned channel, access control by the usual manual methods is ineffective due to the very long round trip delay. A novel signaling protocol which automates access control is proposed. Carrier sensing, signal capture, and collision detection are employed to minimize access contentions. The throughput and holding time of the assigned channel are analyzed. Numerical results show that the access control protocol improves the capacity of the assigned channel by up to 65%. >

Group communications are commonly used in parallel and distributed environment. However, existing migration mechanisms do not support group communications. This weakness prevents migrationbased proactive fault tolerance, among others, to be applied to MPI applications. In this study, we propose distributed migration protocols with group membership management to support process migration with group changing. We design and implement a process migration enabling MPI library, named MPIMitten, to verify the protocols and enhance current MPI platforms for reliability and usability. MPI-Mitten is based on MPI standard and can be applied to any MPI-2 implementations. Experimental results show the proposed distributed process migration protocols are solid and the MPI-Mitten system is effective and is uniquely supporting migration-based fault tolerance.

This chapter provides details of group communication services as specified in 3GPP Release 12. This includes the architectural framework adopted for unicast and broadcast delivery of voice and data in order to effectively support group communication. It also highlights different use cases and how group communication solutions apply for these use cases.

Performance evaluation of ambient services by combining robotic frameworks and a smart environment platform

Many hospitality firms strive to improve service performance through teams, in order to respond to rapidly changing circumstances. However, team efficiency may be impaired when evaluation and compensation of respective team members’ contributions is deemed unfair. Despite their importance, the contributions of individuals to team service performance have lacked attention in academia. This study introduced a service team performance model, in which direct and team contributions by the individual to service performance are identified. The service team construct was manipulated using all possible combinations in a field setting. Through weighted least squares (WLS) analysis of data collected from 186 respondents, we found that individual performance in service teams differ significantly in terms of direct and indirect contributions. Therefore, the need to consider interaction effects among team members when evaluating service performance was motivated. Furthermore, the learning effects, rest effects, and frequency of customer visits were found to affect service quality. This study contributed to the dynamics in service team formulation and service performance, and also provided a meaningful implication for human resource managers in hospitality and tourism industries. This was done by suggesting a holistic approach to employee evaluation.