IEEE Transactions on Parallel and Distributed Systems

How to keep the probability of hand-off drops within a prespecified limit is a very important quality-of-service (QoS) issue in cellular networks because mobile users should be able to maintain ongoing sessions even during their hand-off from one cell to another. We design and evaluate predictive and adaptive schemes for bandwidth reservation for the hand-offs of ongoing sessions and the admission control of new connections. We first develop a method to estimate user mobility based on an aggregate history of hand-offs observed in each cell. This method is then used to probabilistically predict mobiles' directions and hand-off times in a cell. For each cell, the bandwidth to be reserved for hand-offs is calculated by estimating the total sum of tractional bandwidths of the expected hand-offs within a mobility-estimation time window. Three different admission-control schemes for new connection requests using this bandwidth reservation are proposed. We also consider variations that utilize the path/location information available from the car navigation system or global positioning system. Finally, we evaluate the performance of the proposed schemes extensively to show that they meet our design goal and outperform the static reservation scheme under various scenarios.

A decentralized consensus protocol refers to a process for all nodes in a distributed system to collect the information/status from every other node and reach a consensus among them. Two classes of decentralized consensus protocols have been studied before: the one without an initiator and the one with an initiator. While the one without an initiator has been well studied in the literature, it is noted that the prior protocols with an initiator mainly relied upon the one without an initiator and thus did not fully exploit the intrinsic properties of having an initiator. By exploiting the concept of multilayered execution, we develop in this paper an efficient multilayered decentralized consensus protocol for a distributed system with an initiator. By adapting itself to the number of nodes in the system, the proposed protocol can determine a proper layer for execution and reach the consensus in the minimal numbers of message steps while incurring a much smaller number of messages than required by prior works. Several illustrative examples are given and performance analysis of the proposed algorithm is conducted to provide many insights into the problem studied. It is shown that the decentralized consensus protocols developed in this paper for the case of having an initiator significantly outperform prior schemes. Specifically, it is proven that (1) the ratio of the average number of messages incurred by the proposed algorithm to that by the prior method approaches zero as the number of nodes increases and (2) the proposed algorithm is asymptotically optimal in the sense that the message number required by the proposed algorithm and that of the optimal one are asymptotically of the same complexity with respect to the number of nodes in the system, showing the very important advantage of the proposed algorithm.

Locking is a standard technique used in distributed computing and database systems to ensure data integrity by prohibiting concurrent conflicting updates on shared data objects. Internet-based collaborative systems are a special class of distributed applications which support human-to-human interaction and collaboration over the Internet. In this paper, a novel optional and responsive fine-grain locking scheme is proposed for consistency maintenance in Internet-based collaborative editors. In the proposed scheme, locking is made optional in the sense that a user may update any part of the document without necessarily requesting a lock. In the face of high communication latency in the Internet environment, responsive locking is achieved by granting the permit to the user for updating the data region immediately after issuing a locking request. Moreover, multiple fine-grain locks can be placed on different regions inside a document to allow concurrent and mutually exclusive editing on the same document. Protocols and algorithms for locking conflict resolution and consistency maintenance are devised to address special technical issues involved in optional and responsive fine-grain locking. The proposed locking scheme and supporting techniques were implemented in an Internet-based collaborative editor to demonstrate its feasibility and usability.

Multiagent computing on a cluster of workstations is widely envisioned to be a powerful paradigm for building useful distributed applications. The agents of the system span across all the machines of a cluster. Just like the case of traditional distributed systems, load balancing becomes an area of concern. With different characteristics between ordinary processes and agents, it is both interesting and useful to investigate whether conventional load-balancing strategies are also applicable and sufficient to cope with the newly emerging needs, such as coping with temporally continuous agents, devising a performance metric for multiagent systems, and taking into account the vast amount of communication and interaction among agent. This paper discusses the above issues with reference to agent properties and load balancing techniques and outlines the space of load-balancing design choices in the arena of multiagent computing. In view of the special agent characteristics, a novel communication-based load-balancing algorithm is proposed, implemented, and evaluated. The proposed algorithm works by associating a credit value with each agent. The credit of an agent depends on its affinity to a machine, its current workload, its communication behavior, and mobility, etc. When a load imbalance occurs, the credits of all agents are examined and an agent with a lower credit value is migrated to relatively lightly loaded machine in the system. Quasi-simulated experiments of this algorithm show load-balancing improvement compared with conventional workload-oriented load-balancing schemes.

Nodes in a hexagonal network are placed at the vertices of a regular triangular tessellation, so that each node has up to six neighbors. The network is proposed as an alternative interconnection network to a mesh connected computer (with nodes serving as processors) and is used also to model cellular networks where nodes are the base stations. In this paper, we propose a suitable addressing scheme for nodes (with two variants), derive a formula for distance between nodes, and present a very simple and elegant routing algorithm. This addressing scheme and corresponding routing algorithm for hexagonal interconnection are considerably simpler than previously proposed solutions. We then apply the addressing scheme for solving two problems in cellular networks. With the new scheme, the distance between the new and old cell to which a mobile phone user is connected can be easily determined and coded with three integers, one of them being zero. Further, in order to minimize the wireless cost of tracking mobile users, we propose hexagonal cell identification codes containing three, four, or six bits, respectively, to implement a distance based tracking strategy. These schemes do not have errors in determining cell distance in existing hexagonal based cellular networks. Another application is for connection rerouting in cellular networks during a path extension process.

We extend dominating-set-based routing to networks with unidirectional links. Specifically, an efficient localized algorithm for determining a dominating and absorbant set of vertices (mobile hosts) is given and this set can be easily updated when the network topology changes dynamically. A host /spl nu/ is called a dominating neighbor (absorbant neighbor) of another host u if there is a directed edge from /spl nu/ to u (from u to /spl nu/). A subset of vertices is dominating and absorbant if every vertex not in the subset has one dominating neighbor and one absorbant neighbor in the subset. The derived dominating and absorbant set exhibits good locality properties; that is, the change of a node status (dominating/dominated) affects only the status of nodes in the neighborhood. The notion of dominating and absorbant set can also be applied iteratively on the dominating and absorbant set itself, forming a hierarchy of dominating and absorbant sets. The effectiveness of our approach is confirmed and the locality of node status update is verified through simulation.

In the literature, there are quite a few sequential and parallel algorithms for solving problems on distance-hereditary graphs. With an n-vertex and m-edge distance-hereditary graph G, we show that the efficient domination problem on G can be solved in O(log/sup 2/ n) time using O(n + m) processors on a CREW PRAM. Moreover, if a binary tree representation of G is given, the problem can be optimally solved in O(log n) time using O(n/log n) processors on an EREW PRAM.

This paper proposes to use a logical hypercube structure for detecting message stability in distributed systems. In particular, a stability detection protocol that uses such a superimposed logical structure is presented, and its scalability is compared with other known stability detection protocols. The main benefits of the logical hypercube approach are scalability, fault-tolerance, and refraining from overloading a single node or link in the system. These benefits become evident both by an analytical comparison and by simulations. Another important feature of the logical hypercube approach is that the performance of the protocol is in general not sensitive to the topology of the underlying physical network.

Finding the position of a mobile agent in a wide distributed system still represents an open research issue. The paper proposes a naming scheme and a location protocol of general validity for mobile agents able to effectively meet all the typical requirements of mobile agent environments and, thus, easy to integrate into different platforms. The paper identifies the main characteristics which an agent naming scheme and a location protocol of general validity should have, and suggests some properties and parameters to be taken into account to evaluate the effectiveness of naming schemes and location protocols. Then, we propose a "human readable" agent naming scheme based on the distributed environment outlined in MASIF, and a suitable location finding protocol called the Search-By-Path-Chase. Both of them are compared with some of the solutions already provided, using the properties and the parameters suggested. The performances are finally evaluated by means of a set of measurements.

Recent advances in mobile computing and distributed multimedia systems allow mobile hosts (clients) to access wireless multimedia systems anywhere and at anytime, but not without creating a new set of issues and trade-offs. To the best of our knowledge, there has been very little research in dealing with the synchronization problem in wireless and mobile multimedia systems. In this paper, we propose an efficient distributed synchronization algorithm using quasi-sink for wireless and mobile multimedia systems to ensure and facilitate mobile client access to multimedia objects. We discuss the algorithm and provide its proof of correctness. We also present a set of simulation experiments to evaluate the performance of our scheme using message complexity and buffer usage at each frame arrival time. Our results indicate that our scheme exhibits a significant low message complexity and no underflow and overflow within the bounded delivery time.