Sibabrata Ray

Recent advances in mobility modeling for mobile ad hoc network research

In this paper, we survey recent advances in mobility modeling for mobile ad hoc network research. The advances include some new mobility models and analysis of older mobility models. First we classify mobility models into three categories according to the degree of randomness. We introduce newly proposed mobility models in each of these categories. Next we discuss analysis for existing mobility models. We describe the analysis work in three parts. The first part is the statistical properties of the most widely used Random Waypoint Model. The second part describes the mobility metrics that aim to capture the characteristics of different mobility patterns. The last part is the impact of mobility models on the performance of protocols. We also describe some possible future work.

MSXmin: a modular multicast ATM packet switch with low delay and hardware complexity

We propose and analyze the architecture for a large-scale high-speed multicast switch called MSXmin. The hardware complexity of MSXmin is O(N log/sup 2/ N) which compares favorably with existing architectures. Further, the internal latency of the MSXmin is O(log/sup 2/ N) bits. While it is superior to the existing architectures in terms of the hardware complexity and the internal latency, it is comparable to other multicast switches in terms of the header overhead and translation table complexity. MSXmin is output buffered and based on the group knockout principle. Moreover, MSXmin is a dual-bit-controlled tree-based switch.

PITFALLS IN DISTRIBUTED NONBLOCKING CHECKPOINTING

Coordinated checkpointing has low stable storage requirements and simplifies the recovery process by reserving a set of consistent global checkpoints. Unfortunately, most algorithms that were proposed either incurred a high communication overhead or blocked all processes. Then, a coordinated algorithm was presented which was nonblocking and which forced only a subset of all processes to participate in a checkpointing event. This algorithm was shown to create inconsistencies in some situations and new algorithms to take consistent checkpoints were proposed. However, we found that these algorithms can still result in inconsistencies when typical behavior in a distributed environment is considered, such as multiple forced checkpoints and multiple concurrent checkpoint initiations. In this paper we identify the inconsistencies that can occur and present an efficient nonblocking algorithm that collects consistent global checkpoints and avoids some of the pitfalls in distributed nonblocking checkpointing.

A recovery algorithm for reliable multicasting in reliable networks

Any reliable multicast protocol requires some recovery mechanism. A generic description of a recovery mechanism consists of a prioritized list of recovery servers/receivers (clients), hierarchically and/or geographically and/or randomly organized. Recovery requests are sent to the recovery clients on the list one-by-one until the recovery effort is successful. There are many recovery strategies available in literature fitting the generic description. We propose a polynomial time algorithm for choosing the recovery strategy with law recovery latency without sacrificing much bandwidth. We compared our method with two existing recovery methods, SRM (scalable reliable multicast) and RMA (reliable multicast architecture), by simulation and found that our method performs better. Although our theoretical analyses are based on a reliable network, our simulation results show that our strategy performs as well with the per link loss probability in a network up to 20% or more

Minimal sensor integrity: measuring the vulnerability of sensor grids

Given the increasing importance of optimal sensor deployment for battlefield strategists, the converse problem of reacting to a particular deployment by an enemy is equally significant and not yet addressed in a quantifiable manner in the literature. We address this issue by modeling a two stage game in which the opponent deploys sensors to cover a sensor field and we attempt to maximally reduce his coverage at minimal cost. In this context, we introduce the concept of minimal sensor integrity which measures the vulnerability of any sensor deployment. We find the best response by quantifying the merits of each response. While the problem of optimally deploying sensors subject to coverage constraints is NP-complete [Chakrabarty et al., IEEE Trans. Comput., to appear], in this paper we show that the best response (i.e., the maximum vulnerability) can be computed in polynomial time for sensors with arbitrary coverage capabilities deployed over points in any dimensional space. In the special case when sensor coverages form an interval graph (as in a linear grid), we describe a better O(min(M2, NM)) dynamic programming algorithm.

A reconfigurable bus structure for multiprocessors with bandwidth reuse

Abstract Shared bus is one of the most popular communication media for tightly coupled multiprocessing environments. However, a shared bus can provide only limited bandwidth, a fact that limits its usability. In this paper we propose a re-configurable bus structure with both temporal and spatial/spectral bandwidth expansion and describe a method for reusing part of the bandwidth available from temporal and spatial/spectral bandwidth expansion without introducing any buffering delay. Two polynomial time algorithms are developed to optimally reconfigure the bus under the design constraints for a given traffic pattern. For one algorithm the architecture is geared to obtain low average system response time. The other algorithm minimizes the system response time on a different architecture that is designed to reduce the disparity of response time among different nodes. We have compared the performance of reconfigured buses with that of the traditional slotted buses for uniform and localized traffic patterns and found that the reconfigured bus outperforms the traditional slotted bus substantially in most practical scenarios.

Improved algorithms for partitioning tree and linear task graphs on shared memory architecture

In parallel and distributed computing, the overall system performance is significantly influenced by how a task graph representing an application is mapped onto a specific multiprocessor topology. In this paper, algorithms with improved performance are proposed to map tree and linear task graphs onto shared memory multiprocessing architectures. Specifically, the task graphs are partitioned with our algorithms so that the load is balanced, processor utilisation is maximized, and the communication demand on the interconnection network is minimized in a shared memory multiprocessor. All the proposed algorithms are polynomial in time complexity. Bottleneck and processor minimization algorithms of this type are proposed. The bandwidth minimization algorithm has a complexity of O(n + p log q) (q/spl les/p/spl les/n), in contrast with the complexity of O(n log n) for the best known algorithm in the literature. Furthermore, we have identified cases where our algorithm will run in linear time on the average.<<ETX>>

Minimal sensor integrity in sensor grids

Given the increasing importance of optimal sensor deployment for battlefield strategists, the converse problem of reacting to a particular deployment by an enemy is equally significant and not yet addressed in a quantifiable manner in the literature. We address this issue by modeling a two stage game in which the opponent deploys sensors to cover a sensor field and we attempt to maximally reduce his coverage at minimal cost. In this context, we introduce the concept of minimal sensor integrity which measures Me vulnerability of any sensor deployment. We find the best response by quantifying the merits of each response. While the problem of optimally deploying sensors subject to coverage constraints is NP-complete, in this paper we show that the best response (i.e. the maximum vulnerability) can be computed in polynomial time for sensors with arbitrary coverage capabilities deployed over points in any dimensional space. In the special case when sensor coverages form an interval graph (as in a linear grid), we describe a better O(Min(M/sup 2/, NM)) dynamic programming algorithm.

Sorting networks with applications to hierarchical optical interconnects

The Banyan network is shown to have a computationally unsuitable structure for finding maximum passable subpermutations, which is proved NP-complete. Using some non-blocking properties on the cube and reverse Banyan networks, a network topologically equivalent to the Batcher sorter, but functionally equivalent to the Batcher-Banyan network is derived for routing incomplete permutations. A log/sub 2/ N(2w-1) stage radix sorter for w-bit inputs, including duplicate inputs, that uses only log/sub 2/ N+1 bit address headers for routing through each 2 log/sub 2/ N stages is shown, which can be used in sort-MIN type packet switches. Space-time sorting networks based on these principles are derived, which can be used in hierarchical wavelength multiplexed optical networks.

Resource allocation for restoration of compromised systems

Computer systems are constantly under the threats of being attacked and in many cases these attacks succeed. Today’s networked systems are thus built to be intrusion tolerant. In a large scale, the progresses of compromising the networked system and recovering the damage will carry on in parallel, allowing services to be continued (at a degraded level). One of the key problems in the restoration procedure regards to the resource allocation strategies and the cost associated with, specifically, a minimal cost is desired. In this paper we model the cost as a sum of service loss and resource expense that incur during the restoration procedure. We investigate the achievable minimal total cost and corresponding resource allocation strategy for different situations. The situations include both constant rates and time-variant rates in terms of the speed of compromising and recovering. We also consider the fact that the restoration rate is constrained by the resource allocated. The relationship can be either linear or obeying the law of diminishing marginal utility. We present both analytical and numerical results in the paper. The results show the impact from various system parameters on the critical conditions for a successful system restoration and on the minimal cost.