Time Synchronization in Ad Hoc Networks Kay R¨omer MobiHOC 2001 Speaker : hsiwei
Outline INTRODUCTION SYNCHRONIZATION ALGORITHM Experiment CONCLUSION
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
INTRODUCTION Solves the temporal ordering problem and other real–time issues in environments
INTRODUCTION E1 t E2 t2
INTRODUCTION E2 t E1 t4
SYNCHRONIZATION ALGORITHM Assumptions Computer clocks with known maximum clock drift The ability to periodically exchange messages between nodes that have to be synchronized
SYNCHRONIZATION ALGORITHM Time Transformation transforming real–time differences : computer clock differences : (1) and ( local time )
SYNCHRONIZATION ALGORITHM 1 2
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
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 ……… N
SYNCHRONIZATION ALGORITHM Node 3 Node N
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).
Experiment a cluster of 800 MHz Pentium III Linux PCs connected by 100 Mbit/s Ethernet using TCP and assuming
Experiment
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.
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.