Code Review
Time Synchronization
Presented by Yong Chen
Department of Computer Science
University of Virginia
Outline
Clock
Timer
GlobalAbsoluteTimer
TimeStamping
Time Synchronization
Clock
Local time source
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CPU clock: high resolution (7.37MHz), not stable, stops in
power-down mode (SysTimeC)
External clock: low resolution, stable, keeps running in
power-down mode
ClockC component
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A high precision real time clock (with 1/32768 sec resolution)
The Clock interface (Clock.nc) has been extended to support
the setting and retrieval of the current (actual) clock rate in
1/32768 second units.
Timer
The new Timer component provides timers with
1/32768 sec resolution.
If two timers use the same timer id, timer["id"], only
the later timer takes effect, the previous one would
not work. so timer should be used like
TimerC.Timer[unique("Timer")];
Provide as many timers as needed
 Change the setting of TIMER_NUM_TIMERS
GlobalAbsoluteTimer
Triggered at the absolute time point instead of time
interval as previous timer
The resolution is 1/1000 or 1/32768 second
If two timers use the same timer id, like
GlobalAbsoluteTimer[id], only the later timer takes
effect, the previous one would not work. so timer
should be used like
GlobalAbsoluteTimer[unique("AbsoluteTimer")]
Provide as many timers as needed
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Change the setting of MAX_NUM_ABS_TIMERS
TimeStamping
Time stamping is a time synchronization primitive:
establishing time reference points between a sender and
receiver(s) using a single radio message
Sender
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Sender obtains timestamp when the message was actually sent in
its own local time
The message can contain the local time of the sender at the time of
transmission
Method addStamp() Adds a time stamp to the next message sent by
the radio. This method must be called immediatelly after
SendMsg.send() returns SUCCESS.
Receiver
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Receiver obtains timestamp when the message was received in its
own local time
Method getStamp() Returns the time stamp of the last received
message. This method should be called when the
ReceiveMsg.receive() is fired. The returned value contains the local
time when the message was received.
Time Synchronization
Flooding Time Synchronization Protocol for Sensor Networks, VU
Each node maintains two time variables
 Local Time
 Represents the local physical time
 Never reset or adjusted
 Global Time
 learns from its neighboring nodes.
 Reflects the time of the network
 Formula for Global Time and Local Time
 globalTime = localTime + offsetAverage + skew*
(localTime – localAverage)
 variable offsetAverage, skew, and localAverage are
decided by messages from neighboring nodes
Time Synchronization
Root
 The network is synchronized to the root node
 Sends out synchronization beacon messages periodically
Other nodes are synchronized to the root, after synchronized, it
will send out synch beacons periodically
Who can be the root
 The node Set isForceRoot = 1
 The node is set to be the root
 The node Set isForceRoot = -1
 The node is never to be a root
 The node Set isForceRoot = 0
 The node can be a root if it doesn’t receive any
aynchronization message for a period of time
 It is possible there are several roots simultaneously, if so,
the root with smaller ID would dominate the network.
Time Synchronization
Comments
Actually, in the code, the isForceRoot is set to be
either 1 or -1, so the code on isForceRoot == 0 can
be eliminated.
some repeated statements
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in command result_t StdControl.init()
outgoingMsg->rootID = 0xFFFF;
outgoingMsg->nodeID = TOS_LOCAL_ADDRESS;
in command result_t StdControl.start()
outgoingMsg->rootID = 0xFFFF;
outgoingMsg->nodeID = TOS_LOCAL_ADDRESS;
They can appear once in StdControl.init() only once