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Masahiro Sasabe and Tetsuya Takine Osaka University, Japan 1 DTNRG at IETF 76.

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Presentation on theme: "Masahiro Sasabe and Tetsuya Takine Osaka University, Japan 1 DTNRG at IETF 76."— Presentation transcript:

1 Masahiro Sasabe and Tetsuya Takine Osaka University, Japan 1 DTNRG at IETF 76

2 Outline Research background Time synchronization Time sync. for mobile ad hoc networks Relative time synchronization in delay tolerant MANETs Simulation experiments Conclusions and future works DTNRG at IETF 76 2

3 Research background - Time synchronization - Time synchronization is one of key issues in network systems The desired accuracy of time sync. depends on the purposes High accuracy: transmission scheduling Low accuracy: ordering of events DTNRG at IETF 76 3

4 Research background - Time synchronization - Factors generating time difference among nodes Clock offset: Difference from reference time Clock drift (skew): Difference from the accurate clock rate: 1- or 1+ In general, ranges from to NTP is commonly used in the Internet Each node periodically synchronizes the local clock with the clock of NTP server which has an accurate time source DTNRG at IETF 76 4

5 Research background - Time synchronization - NTP is not suitable for time synchronization in mobile ac hoc networks (MANETs) MANET Self-organized wireless network with mobile nodes Examples: Wireless sensor network (WSN) Sometimes its a static network Delay tolerant network (DTN) DTNRG at IETF 76 5 Node Velocity vector

6 Research background - Time synchronization - NTP is not suitable for time synchronization in mobile ac hoc networks (MANETs) Periodical communication with NTP server is difficult Multi-hop communication is required but may fail due to down and/or move of nodes on the path Introducing GPS to each node is one of solutions Introduction costs increase with the number of nodes GPS requires to communicate with satellites Communication may be interrupted by obstacles between the satellites and nodes DTNRG at IETF 76 6

7 Research background - Time synchronization - Problems toward realization of time sync. in MANETs Different clock characteristics of each node Clock offset Clock rate Uncertain propagation delay between two neighbors Nodes must exchange time information Low reliability of time info. of each node Reliable node, e.g., NTP server, may not exist It is difficult for each node to estimate the reliability of time info. of other nodes DTNRG at IETF 76 7

8 Research background - Related works on time sync. in MANETs/WSNs - Pairwise sync. via one hop Sender-receiver sync. Estimation of time from successive communication with an identical node Ex) Ref. [1], TPSN [2] Receiver-receiver sync. Estimation of time by exchanging info. between receivers after simultaneous transmission from a sender Ex) RBS [2] Network-wide sync. via multi hop Use the above methods after making hierarchical networks DTNRG at IETF 76 8 [1] K. Romer, Time Synchronization in Ad Hoc Networks, in Proceedings of the 2nd ACM International Symposium on Mobile Ad Hoc Networking & Computing (MobiHoc01), 2001, pp. 173–182. [2] F. Sivrikaya and B. Yener, Time Synchronization in Sensor Networks: a Survey, IEEE Network, vol. 18, pp. 45–50, estimate from historical data estimate

9 Research background - Applicability of the existing methods to DTNs - These existing methods rely on network-side supports Successive communication with an identical node Simultaneous communication with multiple neighbors Hierarchically topological structure These assumptions are not necessarily guaranteed in delay tolerant networks (DTNs) ZebraNet, InterPlanetary Network, etc Very sparse node density Lack of continuous connectivity with other nodes Network is constantly partitioned Store-carry-forward message delivery is required DTNRG at IETF 76 9

10 Research objective Relative time sync. method for DTNs Network-wide sync. without centralized mechanisms and global information based only on local interactions Target accuracy of time sync.: not high Existing methods aim to achieve high accuracy, e.g., sec order Event ordering, judgment of expiration of message relay, etc. DTNRG at IETF Local time Sync. Node Velocity vector

11 Relative time synchronization - Assumptions - mobile nodes in a closed region Each node has clock rate and clock offset Clock rate: Elapsed time per second Clock offset: Initial difference from real time Node s clock at time is right-continuous and has a left-hand side limit DTNRG at IETF 76 11

12 Relative time synchronization - Proposed method - When nodes and meet at time ``Meet means that both nodes can directly communicate each other They instantaneously exchange time information They adjust their local clocks to the average: DTNRG at IETF :00 0:10 0:05

13 Relative time synchronization - Basic characteristics - When nodes and meet at time The sum of clock times does not change immediately after the meeting The sum of clock times of all nodes is not affected by the proposed method increases with rate DTNRG at IETF 76 13

14 Relative time synchronization - Basic characteristics - Define reference time as the average over all nodes Sum of the time differences of all nodes is always zero How does the variance of time differences become? DTNRG at IETF 76 14

15 Simulation model Simulator: NetLogo [3] 25x25 grid closed area N mobile nodes Clock accuracy nodes The rest of the nodes Simulation time 10 9 time steps DTNRG at IETF Time sync. occurs between nodes on the same or neighboring grids At each time step, each node moves to one of possible neighboring grids (random walk mobility model) [3] Wilensky, U NetLogo. Center for Connected Learning and Computer-Based Modeling, Northwestern University. Evanston, IL.http://ccl.northwestern.edu/netlogo/

16 Transient behavior of the variance of time difference DTNRG at IETF Large variance of time difference due to initial offsets decreases The system finally reaches the steady state N=100

17 Variance of time difference at the 10 9 time step and meeting ratio DTNRG at IETF Variance of time difference is small in any case (about 100 sec – 10 msec) Without time synchronization, it would be about at 10 9 th time step Variance of time difference is small in any case (about 100 sec – 10 msec) Without time synchronization, it would be about at 10 9 th time step The synchronization accuracy improves with the increase of N

18 Conclusions and future works Conclusions We proposed a simple yet novel method for relative time synchronization in delay tolerant MANETs Simulation results demonstrated that the proposed method looks promising Future works We are now working on the analysis of the time difference We also plan to extend the proposed method Weighted average using history of meetings Estimation of clock rate DTNRG at IETF 76 18


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