Research On The Time Synchronization Algorithms In Wireless Sensor Networks

The progress of micro electronics, computer and wireless communication technology drives the development of wireless Sensor with low power wastes and high performances. So it is possible to integrate information acquisition, data processing and wireless communication. Wireless Sensor Networks are some kinds of multi-jump and self-organization networks through wireless communication by deploying plenty of cheap micro-sensors in inspecting area. Wireless Sensor Networks are applied in routing and difficult power supply area or area that can not be reached and some temporary situations, which do not need fixed network supporting and can fast deploy with strong anti-damage. This kind of networks has much broad applying futures in areas of military, industry and civilian uses.Time synchronization is a critical piece of infrastructure for any distributed system. Distributed, wireless sensor networks make particularly extensive use of synchronized time: for example, to integrate a time-series of proximity detections into a velocity estimate; to measure the time of-flight of sound for localizing its source; to distribute a beam forming array; or to suppress redundant messages by recognizing that they describe duplicate detections of the same event by different sensors. Sensor networks also have many of the same requirements as traditional distributed systems: accurate timestamps are often needed in cryptographic schemes, to coordinate events scheduled in the future, for ordering logged events during system debugging, and so forth.The time synchronization algorithms in wireless sensor networks have special performance metrics know from the others, for example: precision, energy costs, memory requirement, fault tolerance. For deterministic algorithms, the maximum synchronization error between a node and real time or between two nodes is interesting; for stochastic algorithms, the mean error, the error variance, and certain quantity are relevant. The energy costs of a time synchronization protocol depend on several factors: the number of packets exchanged in one round of the algorithm, the amount of computation needed to process the packets, and the required resynchronization frequency. To estimate drift rates, a history of previous time synchronization packets is needed. In general, a longer history allows for more accurate estimates at the cost of increased memory consumption. How well can the algorithm cope with failing nodes, with error-prone and time variable communication links, or even with network partitions? Can the algorithm handle mobility?Non-deterministic delays in the radio message delivery in WSN can be magnitudes larger than the required precision of time synchronization. Therefore, these delays need to be carefully analyzed and compensated for. We shall use the following decomposition of the sources of the message delivery delays first introduced by Kopetz and Ochsenreiter. Send Time: time used to assemble the message and issue the send request to the MAC layer on the transmitter side. Depending on the system call overhead of the operating system and on the current processor load, the send time is nondeterministic and can be as high as hundreds of milliseconds. Access Time: delay incurred waiting for access to the transmit channel up to the point when transmission begins. The access time is the least deterministic part of the message delivery in WSN varying from milliseconds up to seconds depending on the current network traffic. Transmission Time: the time it takes for the sender to transmit the message. This time is in the order of tens of milliseconds depending on the length of the message and the speed of the radio. Propagation Time: the time it takes for the message to transmit from sender to receiver once it has left the sender. The propagation time is highly deterministic in WSN and it depends only on the distance between the two nodes. This time is less than one microsecond . Reception Time: the time it takes for the receiver to receive the message. It is the same as the transmission time. The transmission and reception times overlap in WSN as pictured. Receive Time: time to process the incoming message and to notify the receiver application. Its characteristics are similar to that of send time.In the design process of time synchronization protocol about traditional network, the designer wants to keep time synchronization all the time in the whole networks. it is possible under no restrict in energy and the structure of topology is steady. It is not needed in WSN. For example, when the main task is to monitor extremely event in specifically condition. Post-facto synchronization be described: nodes will put up time synchronization when they need, nodes is not synchronization in mostly time.This paper introduced several algorithms which were used abroad in wireless sensor networks, one is the sender/receiver time synchronization protocol include LTS and TPSN; the other is the receiver/receiver time synchronization protocol include RBS and HRTS. The sender/receiver time synchronization was completed in two levels, first is the pair-wise synchronization, the next step is expand to whole networks through pair-wise synchronization, however the receiver/receiver time synchronization was completed in a broadcast domain, the next step is expand to whole networks through domain synchronization.Finally, this paper put forward a time algorithms base on hierarchy broadcasting, and simulation in TinyOS. The algorithms completed with two phases, first is initialize the whole sensor networks and the next is time synchronization. The result show that not only in energy costs it was so well, but also improved the precision to the next level, otherwise the algorithms reformed the sleep mechanism of the node, and materialized the idea of post-synchronization. We will apply this algorithms in hardware to checkout it's validity, improve the efficiency of energy costs, put long the lifetime of the node.