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The minimum latency data aggregation schedule is one of the fundamental problems in wireless sensor networks. Most existing works assumed that the transmission ranges of sensor nodes cannot be adjusted. However, sensors with adjustable transmission ranges have advantages in energy saving, reducing transmission interference and latency. In this paper, we study the minimum latency conflict-aware data aggregation scheduling problem with adjustable transmission radii: given locations of sensors along with a base station, all sensors could adjust their transmission radii and each sensor's interference radius is α times of its transmission radius, we try to find a data aggregation schedule in which the data from all sensors can be transmitted to the base station without conflicts, such that the latency is minimized. We first partition the set of all nodes into two parts: the major set and the minor set. Then, we design different scheduling strategies for the two sets, respectively. Finally, we propose an approximation algorithm for the problem and prove the performance ratio of the algorithm is bounded by a nearly constant. Our experimental results evaluate the efficiency of the proposed algorithm.

In IEEE 802.11 based WLAN system; the mobile nodes (MN) are connected through access points (APs). During mobility a MN leaves one AP and is associated to new APs, A handoff process will occur. To provide a better seamless connectivity, Handoff process latency should be very small. Handoff latency is a combination of scanning, re authentication and reassociation latency. Reauthentication latency is major contributing factor that affects the performance of handoff and increase the handoff latency. In this paper we present a novel approach for reducing the Reauthentication latency, and network overhead. For reducing the re-authentication latency we apply pre-authentication mechanism which is preceded by the mobility prediction to consider the user mobility behavior as the contributing factor in the pre-authentication. With the help of mobility predication the central server sends a pre authentication key to the APs and also sends the Ids of AP to the MN and MN store into their buffer. The simulation results show a good factor of improvement over the latency values in WLAN environment.