Research On Time Synchronization And Slots Allocation For Wireless Sensor Networks

With the rapid development of wireless communication technologies, sensor technologies and embedded system technologies, wireless sensor networks (WSNs) have gained world-wide attention in recent years and become more popular due to their wide range of application. Media access control (MAC) protocol works as a fudenmental layer in communication protocol stack and is extremely important to the whole network. Compared with Content-based MAC protocols, TDMA-based MAC protocols have some advantages in collision avoidance and low power consumption, and have better real-time peformance. However, time synchronization protocol and slot allocation algorithm for wireless sensor networks have taken great challenges to development of TDMA-based MAC protocols. This paper focuses on these two key issues for WSN and makes some contribution.Time synchronization is a fundamental building block in TDMA-based Wireless Sensor Networks. In this paper, we proposed a distributed approach secure virtual time synchronization protocol with multi-references (SMVTSP) to achieve accurate time synchronization services under severe attacks. Unlike previous time-stamp-based methods, SMVTSP makes use of the arrival time of messages but not the time stamps included in messages, so it is immune from the fake or manipulated time information. The proposed scheme has several phases:calculation of node's reputation, threshold-based verification and the calibration of local's clock to smooth out influence brought by suspicious nodes. Simulations are performed to demonstrate SMVTSP can limit the synchronization error within tens microseconds when the ratio of compromised nodes'attacks belows 30%.Time slots allocation problem in wireless sensor networks has been defined as broadcast scheduling problem (BSP), which is a well-known NP-complete combinatorial optimization problem. The broadcast scheduling avoids packet collisions by allowing only one node transmission in each collision domain of a TDMA network. It also improves the transmission utilization and network delay by assigning one transmission time slot to one or more nodes; thus, each node transmits at least once in each time frame. An optimum transmission schedule could minimize the length of a time frame while minimizing the number of idle nodes. In this paper, we propose an ant-based vertex coloring algorithm (AVC) to solve broadcast scheduling problem. AVC adopts two phase approach to search for optimal slots assiagnment solution. In the first phase, AVC uses the ant vertex coloring algorithm to minimize the frame length assuming that a station can have only one slot. After completing the first phase, we got not only the the minuum frame length but also some basic solutions. In the second phase, AVC adopts greedy algorithm to maximize the channel utilization by using the result got in previous phase.As the simulation results show, AVC can effectively improve the performance, especially when the density of sensors is high.In the last part of paper, we illustrate some details of an auto meter reding (AMR) system which based on wireless sensor network. During 28-day's experiment, our network has good reliability and real time performance.