A Simple Scheme for Relative Time Synchronization in Delay Tolerant MANETs

1. A Simple Scheme for Relative Time Synchronization in Delay Tolerant MANETs Masahiro Sasabe and Tetsuya Takine Osaka University, Japan DTNRG at IETF 76

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

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

4. Research background- Time synchronization - • Factors generating time difference among nodes • Clock offset: f • Difference from reference time • Clock drift (skew): r • Difference from the accurate clock rate: 1-r or 1+r • In general, r ranges from 10-4 to 10-6 • 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

5. Research background- Time synchronization - • NTP is not suitablefor time synchronization in mobile ac hoc networks (MANETs) • MANET • Self-organized wireless networkwith mobile nodes • Examples: • Wireless sensor network (WSN) • Sometimes it’s a static network • Delay tolerant network (DTN) Node Velocityvector DTNRG at IETF 76

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

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

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 estimate from historical data estimate [1] K. Romer, “Time Synchronization in Ad Hoc Networks,” in Proceedings of the 2nd ACM International Symposium on Mobile Ad Hoc Networking & Computing (MobiHoc’01), 2001, pp. 173–182. [2] F. Sivrikaya and B. Yener, “Time Synchronization in Sensor Networks: a Survey,” IEEE Network, vol. 18, pp. 45–50, 2004. DTNRG at IETF 76

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 guaranteedindelay 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

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., m sec order • Event ordering, judgment of expiration of message relay, etc. Node Velocityvector Local time Sync. DTNRG at IETF 76

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

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: 0:05 0:00 0:10 0:05 DTNRG at IETF 76

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

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

15. Simulation model • Simulator: NetLogo [3] • 25x25 grid closed area • N mobile nodes • Clock accuracy • nodes • The rest of the nodes • Simulation time • 109 time steps At each time step, each node moves to one of possible neighboring grids (random walk mobility model) Time sync. occurs between nodes on the same or neighboring grids [3] Wilensky, U. 1999. NetLogo. http://ccl.northwestern.edu/netlogo/. Center for Connected Learning and Computer-Based Modeling, Northwestern University. Evanston, IL. DTNRG at IETF 76

16. Transient behavior of the variance of time difference Large variance of time difference due to initial offsets decreases N=100 The system finally reaches the steady state DTNRG at IETF 76

17. Variance of time difference at the 109 time step and meeting ratio • Variance of time difference is small in any case (about 100 msec – 10 msec) • Without time synchronization, it would be about 1010 at 109th time step The synchronization accuracy improves with the increase of N DTNRG at IETF 76

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