[Tinyos-help] [Q] Measuring of round trip time between two motes

[Chong Hui Kim]

Thank you for your comments.
In my question, there is confused terms of round trip time.
As you commented, round trip time of radio waves is very small. Hence it is negligible.
In my question, round trip time means that hardware control time using SPI for TX and RX, initial and congestion random back-off time in B-MAC, and propagation delay.
In my experiment, round trip time includes propagation delay, initial back-off time, congestion back-off time in B-MAX, and SPI communication time between MSP430 and CC2420. Usually, back-off time is about 6.6ms and reading time of received buffer of CC2420 is about 2.4ms for 8 bytes payload. Hence I thought that maximum required time would be (6.6+2.4) + (6.6+2.4) + negligible round trip propagation time = 18ms. But in experiments it often required about 30ms ~ 38ms.

How do you think about this?

Chong Hui Kim.
  ----- Original Message ----- 
  From: Aditya Bhave 
  To: Chong Hui Kim 
  Sent: Tuesday, December 26, 2006 11:45 PM
  Subject: Re: [Tinyos-help] [Q] Measuring of round trip time between two motes


  I dont think thats the the right approach. Radio waves travel much too fast and the difference is far too small to be measured using the timer on the motes. (the resolution is not small enough). What do you mean by "control period" and how did u manage to calculate the answer in ms?? 

  To do ranging you can employ one of two methods

  1. Radio signal strength indicator (RSSI): Let the TX mote transmit a radio ignal. The strength of the signal received by a mote is related to the distance between it and the TX mote. Ive forgotten the formula but you can look it up. 

  2. Acoustic ranging: Instead if transmitting a radio wave, measure the time between sending a sound wave and getting a reply sound wave. Sound waves travel much much slower and the time difference can be measured with the motes's timers. You will need a buzzer on the mote for this however, 


  On 12/24/06, Chong Hui Kim <chkim at rtcl.kaist.ac.kr> wrote:

    Dear all,

    I have measured round trip time between two motes using Timer B.
    I called one mote was TX mote and the other was RX mote.
    To measure the round trip time, I read the Timer B register of "TX mote" before call SendMsg.send() the Timer B register of "TX mote" after ReceiveMsg.receive() event, and computed the difference of two time values.

    In my experience, I set the control peiord as 50ms, 100ms, 500ms, and 1000ms.
    For 50ms, 100ms, and 100ms, average round trip time was 22ms, max time was about 35ms, and min time was about 9.6ms.
    For 1000ms, average round trip time was 14.5ms, max time was about 19ms, and min time was 9.6ms.

    I found that average and max round trip time decreased for 1000ms period.

    How can I explain this result?

    Thanks in advance,
    Chong Hui Kim.

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  -- 
  regards,
  Aditya Bhave 
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