Hi all,
I'm investigating the FTSP to apply into my protocol and experiencing the
same problem as Marcin described above. For understanding how to implement
FTSP in tinyos, I also read the Tep 133
(http://www.tinyos.net/tinyos-2.x/doc/html/tep133.html) and then I performed
some tests to troubleshoot the problem.
Below is my understanding of FTSP implementation:
- we need to calculate offsetTime = globalTime - localTime, this value
should not change much for each packet. So, we need to get globalTime (the
time packet is sent) and localTime (the time packet is received) exactly.
TinyOS uses TimeSyncAMSend and TimeSyncPacket to send and receive SYNC
message.
- The transmitter callls TimeSyncAMSend.send to sends SYNC message which
includes globalTime (the time when TimeSyncAMSend.send is called). It uses
CSMA mechanism to send packet (it needs to wait backoff time before
sending), it means that this globalTime is not the real time when the packet
is sent over the medium. So it signals SFD interrupt event to get the
actual time when the packet starts being transmitted over the communication
medium. ( call signal TimeStamp.transmittedSFD( time, m_msg ) in
CC2420TransmitP.nc). This value should write back to globalTime and receiver
can get the actual time when the packet is transmitted.
- At receiver side, it uses TimeSyncPacket to get the actual time when the
packet is received (call signal TimeStamp.receivedSFD( time ))
-I run some test and realize that the globalTime value in the SYN message
received at the receiver side is the same as the globalTime value is set at
transmitter when TimeSyncAMSend.send is called. This is so strange. Doing
more, I see that the SFD interrupt event modifies localTime value instead of
globalTime value in the SYNC msg.
typedef nx_struct TimeSyncMsg
{
nx_uint16_t rootID; // the node id of the synchronization
root
nx_uint16_t nodeID; // the node if of the sender
nx_uint8_t seqNum; // sequence number for the root
/* This field is initially set to the offset between global time and
local
* time. The TimeStamping component will add the current local time when
the
* message is actually transmitted. Thus the receiver will receive the
* global time of the sender when the message is actually sent. */
nx_uint32_t globalTime;
//just for convenience
nx_uint32_t localTime;
} TimeSyncMsg;
-So, I modify the struct of SYNC msg but the receiver still gets the
incorrect globalTime value. I investigate more and realize that it is the
type variable issue. We use 4 bytes of globalTime, but
the time is supported by SFD interrupt event is only 2 Byte (see
CaptureSFD.captured( uint16_t time ) in CC2420TransmitP.nc).
I think that we got the issue because the receiver got the incorrect
globalTime value in SYNC message. Unfortunately, I don't know how to fix it.
By the way, do you know the process when it sends SYNC msg? Which
function/components are called to send SYNC msg in FTSP application? I need
to know it for easy troubleshooting the problem.
Thank in advance,
Han Bin
--
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