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

Masahiro Sasabe and Tetsuya Takine

Osaka University, Japan

1DTNRG at IETF 76

OutlineOutline

Research background Time synchronization Time sync. for mobile ad hoc networks

Relative time synchronization in delay tolerant MANETs

Simulation experiments Conclusions and future works

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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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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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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


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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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

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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

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)()()()( 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?

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0 ,0)()(1

1

*

ttctcN

N

ii

N

ii

N

ii NN

ttc11

* 1 ,

1 ,)(

)()()( * tctctd ii

N

ii

N

ii NN

ttc11

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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

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

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