## 1 Introduction  
The grids that prevail in recent years have proliferated the development of grid applications [1]. A large-scale grid system is often composed of hundreds to thousands of small and inexpensive computers to exploit low construction cost and high performance. Users worldwide can share grid resources through the Internet, and their submitted jobs can be accomplished by involving many geographically dispersed resources to achieve distributed computing. However, traditional grid systems are established on wired environments. Basically, a wired system often has wider bandwidth and higher security than those of a wireless environment. However, users have to access the grid resources at fixed points where wired links are available, resulting in poor network accessibility and availability [2]. Nowadays, wireless networks have been developed rapidly and equipped widely, and the scalability, bandwidth, system stability and network security all have been greatly improved [3]. Many problems of previous networks and some existing wireless networks, e.g., insufficient bandwidth for data transmission and the limited transferring distance, have been mitigated. The market and requirements due to availability of communication everywhere have unceasingly expanded and developed to satisfy modern people's needs and preferences. Many researchers have studied how to continuously raise grid efficiency and usage convenience by integrating it with wireless networks and other techniques [4; 5; 5]. The biggest shortcomings of wireless transmission are that the handover authentication is difficult [6] and messages conveyed on radio waves can be easily intercepted by hackers [7]. In fact, if we can integrate wireless networks and grids, keeping their original advantages, e.g., exploiting mobility of terminal devices, users can then conveniently access surrounding grid nodes and resources. The above-mentioned disadvantages of wireless systems can then be solved. Therefore, in this study we propose a wireless service system, called the Semi-Preemptive Computational Service system (SePCS for short), which provides users with a wireless access environment and abundant grid computation resources, and through which users can share their own grid resources with others. In the SePCS, a node (fixed or mobile) can actively deliver a request to the resource broker. The resource broker then assigns a job corresponding to the request to the nodes with appropriate computation capability so as to accomplish the job as soon as possible where a job is a request in execution. Due to employing a wireless network environment, the SePCS covers a wider service area than that of a traditional wired grid and can be applied to vehicular communication since people who are traveling often carry handy devices. When they dynamically need a little high-complexity computation locally, e.g., a person on his/her way to visit customers by car wants to predict the trend of this morning's stock market, the SePCS can allocate resources to do the prediction. On the other hand, the concept of the SePCS is similar to _ad hoc_ cloud computing [8]. Users in these two systems can perform their jobs remotely, and do not need to carry high-performance devices so that the SePCS can be a part of the _ad hoc_ cloud computing architecture. In the SePCS, requests of higher priority that need to be served in real time, e.g., people are watching internet protocol TV (IPTV) or talking with others through the internet which has limited bandwidth and inefficient transmission, are allocated to the resources of higher performance. To achieve this, the resource broker reserves a part of its high-performance resources for the requests so that the requests can be served immediately and quickly without waiting in a service queue for a long time. When the resource broker cannot find suitable resources for the requests of higher type, it preempts the resource that is now executing a lower type request. That is why we call the SePCS the Semi-Preemptive Computational Service system.