1. Time Synchronization in Ad Hoc Networks Kay R¨omer MobiHOC 2001 Speaker : hsiwei

2. Outline  INTRODUCTION  SYNCHRONIZATION ALGORITHM  Experiment  CONCLUSION

3. INTRODUCTION  Environment monitoring systems  Mobile computing devices equipped with sensors, clocks  detection of direction and speed of certain phenomena such as fire  Direction, temporal ordering of these events originating from different devices (and thus different clocks) has to be determined

4. INTRODUCTION  Solves the temporal ordering problem and other real–time issues in environments

5. INTRODUCTION 1 2 3 E1 t1 1 2 3 E2 t2

6. INTRODUCTION 1 2 3 E2 t3 1 2 3 E1 t4

7. SYNCHRONIZATION ALGORITHM Assumptions  Computer clocks with known maximum clock drift  The ability to periodically exchange messages between nodes that have to be synchronized

8. SYNCHRONIZATION ALGORITHM Time Transformation  transforming real–time differences :  computer clock differences : (1) and ( local time )

9. SYNCHRONIZATION ALGORITHM 1 2

10.  Message Delay t4 t1 t5t6 t2 t3 Receiver Sender Time in Receiver Time in Sender M1M1 ACK 1 M2M2 ACK 2 Message delay of M 2 in sender’s clock 利用兩個 message 求 delay in Receiver’clock

11. SYNCHRONIZATION ALGORITHM  Each node i has 3 attributes : r i, s i (local time),p i 其中 s i – r i 為 node i storage time.  利用 chain 的方式  Node 1 [ r 1, r 1 ]=[ NOW, NOW ]  Node 2 12 34 ……… N

12. SYNCHRONIZATION ALGORITHM  Node 3 Node N

13. Experiment  Synchronization inaccuracies show up as (increasing) time stamp intervals : 1. The age of a time stamp 2. the number of hops a time stamp has been passed along (due to using round trip time as an estimation for message delay).

14. Experiment  a cluster of 800 MHz Pentium III Linux PCs connected by 100 Mbit/s Ethernet using TCP and assuming

15. Experiment

16. CONCLUSION  A synchronization algorithm suitable for a certain class of applications of sparse ad hoc networks.  Exchanged between nodes inside messages to the local time of the receiver instead of adjusting the clocks.

17. Summary  利用每個 computer clocks 所產生的偏移係數,, 再用作者所提供的數學 model, 計算出時間的 偏差量達到分辨每個 nodes 時間 Ordering.