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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
Research backgroundResearch background- Time synchronization -- 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
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Research backgroundResearch background- Time synchronization -- 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 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
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Research backgroundResearch background- Time synchronization -- Time synchronization -
NTP is not suitable for 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)
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Node
Velocityvector
Research backgroundResearch background- Time synchronization -- 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
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Research backgroundResearch background- Time synchronization -- 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 nodesDTNRG at IETF 76 7
Research backgroundResearch background- Related works on time sync. in - Related works on time sync. in MANETs/WSNs -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
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[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.
estimate from historical data
estimate
Research backgroundResearch background- Applicability of the existing methods - Applicability of the existing methods to DTNs -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 indelay 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
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Research objectiveResearch 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.
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Local time Sync.
Node
Velocityvector
Relative time synchronizationRelative time synchronization- Assumptions -- 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
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N
i i i
i t)(tci
Relative time synchronizationRelative time synchronization- Proposed method -- 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:
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i j t
2
)()()()(
ji
jicc
cc
0:000:100:05 0:05
Relative time synchronizationRelative time synchronization- Basic characteristics -- 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
)()()()( jiji cccc
i j t
N 21
Relative time synchronizationRelative time synchronization- Basic characteristics -- 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
0 ,0)()(1
1
*
ttctcN
N
ii
N
ii
N
ii NN
ttc11
* 1 ,
1 ,)(
)()()( * tctctd ii
N
ii
N
ii NN
ttc11
* 1 ,
1 ,)(
Simulation modelSimulation model
Simulator: NetLogo [3]
25x25 grid closed area
N mobile nodes Clock accuracy
nodes
The rest of the nodes
Simulation time 109 time steps
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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)
)0,101(),( 4ii 2/N
)100,101(),( 4ii
[3] Wilensky, U. 1999. NetLogo. http://ccl.northwestern.edu/netlogo/. Center for Connected Learning and Computer-Based Modeling, Northwestern University. Evanston, IL.
Transient behavior of Transient behavior of the variance of time differencethe variance of time difference
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Large variance of time difference due to initial offsets decreases
The system finally reaches the steady state
N=100
)()()( * tctctd ii
Variance of time difference at the Variance of time difference at the 101099 time step time step and meeting ratioand meeting ratio
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Variance of time difference is small in any case (about 100 sec – 10 msec)•Without time synchronization, it would be about 1010 at 109th time step
The synchronization accuracy improves with the increase of N
Conclusions and future worksConclusions 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