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Benefit-Based Data Caching in Ad Hoc Networks Bin Tang, Member , IEEE , Himanshu Gupta, , , and Samir R. Das, Member , Abstract —Data caching can significantly improve the efficiency of information access in a wireless ad hoc network by reducing the access latency and bandwidth usage. However, designing efficient distributed caching algorithms is nontrivial when network nodes have limited memory. In this article, we consider the cache placement problem of minimizing total data access cost in ad hoc networks with multiple data items and nodes with limited memory capacity. The above optimization problem is known to be NP-hard. Defining benefit as the reduction in total access cost, we present a polynomial-time centralized approximation algorithm that provably delivers a solution whose benefit is at least 1/4 (1/2 for uniform-size data items) of the optimal benefit. The approximation algorithm is amenable to localized distributed implementation, which is shown via simulations to perform close to the approximation algorithm. Our distributed algorithm naturally extends to networks with mobile nodes. We simulate our distributed algorithm using a network simulator ( ns2 ) and demonstrate that it significantly outperforms another existing caching technique (by Yin and Cao [33]) in all important performance metrics. The performance differential is particularly large in more challenging scenarios such as higher access frequency and smaller memory. Index Terms —Caching placement policy, ad hoc networks, algorithm/protocol design and analysis, simulations. Ç 1I NTRODUCTION A D hoc networks are multihop wireless networks of small computing devices with wireless interfaces. The computing devices could be conventional computers (for example, PDA, laptop, or PC) or backbone routing plat- forms or even embedded processors such as sensor nodes. The problem of optimal placement of caches to reduce overall cost of accessing data is motivated by the following two defining characteristics of ad hoc networks. First, the ad hoc networks are multihop networks without a central base station. Thus, remote access of information typically occurs via multihop routing, which can greatly benefit from caching to reduce access latency. Second, the network is generally resource constrained in terms of channel band- width or battery power in the nodes. Caching helps in reducing communication, which results in savings in bandwidth, as well as battery energy. The problem of cache placement is particularly challenging when each network node has a limited memory to cache data items. In this paper, our focus is on developing efficient caching techniques in ad hoc networks with memory limitations. Research into data storage, access, and dissemination techniques in ad hoc networks is not new. In particular, these mechanisms have been investigated in connection with sensor networking [14], [26], peer-to-peer networks [1], [18], mesh networks [17], world wide Web [25], and even more general ad hoc networks [12], [33]. However, the
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