Wireless sensor networks are a new type of ad hoc networks that had drawn the interest of the research community in last few years. They require accurate, reliable time for many of its applications. The use of traditional time synchronization protocols for wired network is restricted by severe energy constraint limits in sensor network. The realization of time synchronized network poses many challenges, which are the subject of active research in the field. This paper discusses the needs for time synchronization, problems, and then presents the protocols or algorithms, which are developed over several years to distribute time synchronization within wireless sensor network. With this article we intend to spark new interest and development in this field.
Keywords Time synchronization, clock synchronization, protocols, wireless network, sensor
[...] V SYNCHRONIZATION METHODS FOR SENSOR NETWORKS Time synchronization in sensor network has attracted many researchers in the last few years. In this section we discuss some of the known synchronization algorithms. Only introduction to these protocols are provided, discussing the overall functioning of these protocols is out of scope of this paper. A. Post facto synchronization:Time synchronization in sensor networks has attracted attention in the last few years. Post facto synchronization was good work by Elson and Estrin . They proposed that unlike in traditional synchronization schemes such as NTP, local clocks of the sensor nodes should normally run unsynchronized, at their own pace, but synchronize whenever necessary. [...]
[...] Prior work on sleep/wake scheduling assumes that the underlying synchronization protocol can provide nearly perfect (e.g., micro-second level) synchronization, or assumes an upper bound on the clock disagreement, and uses it as a guard time to compensate for the synchronization error. The wake period is lengthened by the guard time to combat synchronization errors. In practice, due to non-deterministic errors in time synchronization, synchronization is imperfect, and as time progresses, the clock disagreement becomes more and more significant. Periodic re-synchronization can prevent the clocks from drifting away, but for low duty cycle sensor networks, frequent re-synchronization would consume a significant amount of energy compared to communication/ sensing. [...]
[...] proposed a network-wide time synchronization protocol for sensor networks, which they call Timing-Sync Protocol for Sensor Networks (TPSN) Their protocol works in two phases: level discovery and synchronization. The aim of the first phase is to create hierarchical topology in the network, where each node is assigned a level. Only one node is assigned level the root node. In the second phase, a node of level i synchronize to a node of level i - 1. At the end of the synchronization phase, all nodes are synchronized to the root node, and network-wide synchronization is achieved. [...]
[...] NEED 4 (LOCALIZATION) Location awareness is important for wireless sensor networks since many applications such as environment monitoring, vehicle tracking and mapping depend on knowing the locations of sensor nodes. In addition, location-based routing protocols can save significant energy by eliminating the need for route discovery and improve caching behavior for applications where requests may be location dependent. Security can also been enhanced by location awareness (for example, preventing wormhole attacks). However, putting GPS receivers in every node or manually configuring locations is not cost effective for most sensor network applications. [...]
[...] The software implementation of the transmitter will have a few minor variations due to the response time for interrupts. In a novel hardware based RF transceiver has been proposed where the variations would be completely negligible. C. Propagation time This is the actual time taken by the packet to traverse the wireless link from the sender to the receiver. The absolute value of this delay is negligible as compared to other sources of packet latency. D. Receive time This include reception time, refers to the time taken in receiving the bits and passing them to the MAC layer. [...]
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