Masahiro Sasabe and Tetsuya Takine

Masahiro Sasabe and Tetsuya Takine

• Masahiro Sasabe and Tetsuya Takine

• Osaka University, Japan

Research background

• Research background

• Time synchronization
• Time sync. for mobile ad hoc networks

• Relative time synchronization in delay tolerant MANETs

• Simulation experiments

• Conclusions and future works

Time synchronization is one of key issues in network systems

• 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

Factors generating time difference among nodes

• 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

NTP is not suitable for time synchronization in mobile ac hoc networks (MANETs)

• 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)

NTP is not suitable for time synchronization in mobile ac hoc networks (MANETs)

• 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

Problems toward realization of time sync. in MANETs

• 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

Pairwise sync. via one hop

• 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

These existing methods rely on network-side supports

• 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

Relative time sync. method for DTNs

• 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.

mobile nodes in a closed region

• 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

When nodes and meet at time

• 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:

When nodes and meet at time

• 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

Define reference time as the average over all nodes

• 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?

Simulator: NetLogo [3]

• Simulator: NetLogo [3]

• 25x25 grid closed area

• N mobile nodes

• Clock accuracy

• nodes
• The rest of the nodes

• Simulation time

• 109 time steps

Conclusions

• 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