1. 高度情報化社会を支えるネットワーキング技術 (大阪大学 工学部説明会資料)
2017/3/31
A Simple Scheme for Relative Time Synchronization in Delay Tolerant MANETs
Masahiro Sasabe and Tetsuya Takine
Osaka University, Japan
DTNRG at IETF 76
大阪大学 大学院工学研究科 電気電子情報工学専攻 情報通信工学部門 滝根研究室

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

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 -
2017/3/31
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 suitable for time synchronization in mobile ac hoc networks (MANETs)
MANET
Self-organized wireless network with mobile nodes
Examples:
Wireless sensor network (WSN)
Sometimes it’s a static network
Delay tolerant network (DTN)
Node
Velocity vector
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 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

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.
Local time Sync.
Node
Velocity vector
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:
0:10
0:05
0:05
0:00
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 differences of 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
At each time step, each node moves to one of possible neighboring grids
(random walk mobility model)
Simulator: NetLogo [3]
25x25 grid closed area
N mobile nodes
Clock accuracy
nodes
The rest of the nodes
Simulation time
109 time steps
Time sync. occurs between nodes on the same or neighboring grids
[3] Wilensky, U NetLogo. Center for Connected Learning and Computer-Based Modeling, Northwestern University. Evanston, IL.
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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
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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
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