Time Synchronization in Wireless Networks 1 of 12 file F www cse574 06 ftp time sync index html Author Michael Roche tke961 gmail com Time Synchronization in wireless networks is extremely important for basic communication but it also provides the ability to detect movement location and proximity The synchronization problem consists of four parts send time access time propagation time and receive time Three current synchronization protocol Reference Broadcast Synchronization Timing sync Protocol for Sensor Networks and Flooding Time Synchronization Protocol are presented and how they attempt solve the synchronization problem is also discussed Security concerns as well as an industry case are also presented See Also 1 0 Introduction 1 1 Wired Network Synchronization 1 2 Wireless Network Synchronization 2 0 Reference Broadcast Synchronization 2 1 Advantages of RBS 3 0 Timing sync Protocol for Sensor Networks 3 1 Level Discovery Phase 3 2 Synchronization Phase 3 3 Advantages of TPSN 4 0 Flooding Time Synchronization Protocol 4 1 Advantages to FTSP 5 0 Attacks on Synchronization Protocols 5 1 Attacks on RBS TPSN and FTSP 5 2 Countermeasures for Attacks 6 0 An Industry Case Summary References List of Acronyms Time synchronization in all networks either wired or wireless is important It allows for successful communication between nodes on the network It is however particularly vital for wireless networks Synchronization in wireless nodes allows for a TDMA algorithm to be utilized over a multi hop wireless network Wireless time synchronization is used for many different purposes including location proximity energy efficiency and mobility to name a few 11 27 2013 2 12 AM Time Synchronization in Wireless Networks 2 of 12 file F www cse574 06 ftp time sync index html In sensor networks when the nodes are deployed their exact location is not known so time synchronization is used to determine their location Also time stamped messages will be transmitted among the nodes in order to determine their relative proximity to one another Time synchronization is used to save energy it will allow the nodes to sleep for a given time and then awaken periodically to receive a beacon signal Many wireless nodes are battery powered so energy efficient protocols are necessary Lastly having common timing between nodes will allow for the determination of the speed of a moving node The need for synchronization is apparent Besides its many uses like determining location proximity or speed it is also needed because hardware clocks are not perfect There are variations in oscillators which the clocks may drift and durations of time intervals of events will not be observed the same between nodes The concept of time and time synchronization is needed especially in wireless networks 1 1 Wired Network Synchronization For a wired network two methods of time synchronization are most common Network Time Protocol and Global Positioning System GPS are both used for synchronization Neither protocol is useful for wireless synchronization Both require resources not available in wireless networks The Network Time Protocol requires an extremely accurate clock usually a server with an atomic clock The client computer wanting to synchronize with the server will send a UDP packet requesting the time information The server will then return the timing information and as a result the computers would be synchronized Because of many wireless devices are powered by batteries a server with an atomic clock is impractical for a wireless network GPS requires the wireless device to communicate with satellites in order to synchronize This requires a GPS receiver in each wireless device Again because of power constraints this is impractical for wireless networks Also sensor networks consist of inexpensive wireless nodes A GPS receiver on each wireless node would be expensive and therefore unfeasible The time accuracy of GPS depends on how many satellites the receiver can communicate with at a given time This will not always be the same so the time accuracy will vary Furthermore Global Positioning System devices depend on line of sight communication to the satellite which may not always be available where wireless networks are deployed The constraints of wireless networks do not allow for traditional wired network time synchronization protocols Wireless networks are limited to size power and complexity Neither the Network Time Protocol nor GPS were designed for such constraints 1 2 Wireless Network Synchronization The definition of time synchronization does not necessarily mean that all clocks are perfectly matched across the network This would be the strictest form of synchronization as well as the most difficult to implement Precise clock synchronization is not always essential so protocols from lenient to strict are available to meet one s needs There are three basic types of synchronization methods for wireless networks The first is relative timing and is the simplest It relies on the ordering of messages and events The basic idea is to be able to determine if event 1 occurred before event 2 Comparing the local clocks to determine the order is all that is needed Clock synchronization is not important The next method is relative timing in which the network clocks are independent of each other and the nodes keep track of drift and offset Usually a node keeps information about its drift and offset in correspondence to neighboring nodes The nodes have the ability to synchronize their local time with another nodes local time at 11 27 2013 2 12 AM Time Synchronization in Wireless Networks 3 of 12 file F www cse574 06 ftp time sync index html any instant Most synchronization protocols use this method The last method is global synchronization where there is a constant global timescale throughout the network This is obviously the most complex and the toughest to implement Very few synchronizing algorithms use this method particularly because this type of synchronization usually is not necessary Figure 1 Breakdown of packet delay components As shown in figure 1 all the wireless synchronization schemes have four basic packet delay components send time access time propagation time and receive time The send time is that of the sender constructing the time message to transmit on the network The access time is that of the MAC layer delay in accessing the network This could be waiting to transmit in a TDMA protocol The time for the bits to by physically
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