By the law of large numbers my guesses have to be right sometimes...
MS
Miklos Maroti wrote:
> Hi Guys,
>
>> Could there be a timing issue when waiting for Acks?
>> It might be that the telos responds a little slower
>> and the iris gives up waiting too soon -- if I have
>> the order of execution right anyway...
>
> Yes, I think that is it. From Urs data the first big number is the
> time stamp of the SFD (start frame delimiter) of the packet, the
> second smaller number is the current time when the packet was
> downloaded, both in roughly microseconds. We see the following:
>
> TelosB -> TelosB: approx 1.65 millisecond delay
> TelosB -> IRIS: approx 0.77 millisecond delay
> IRIS -> TelosB: approx 1.63 millisecond delay
> IRIS -> IRIS: approx 0.77 millisecond delay
>
> These are delays between the SFD of the data packet and the SFD of the
> ack packet, so the actual gap between the two packets is smaller (by
> 14 data bytes + 5 preamble bytes, which is around 19*32=608, i.e. 0.6
> milliseconds. So the actual delay between the end of the data packet
> and start of the ack packet for the IRIS is 0.17 milliseconds.
>
> Anyways, the problem is the timing, the IRIS does not wait long enough
> for the ACK. In RF230ActiveMessageP.nc it is said that
>
> async command uint16_t SoftwareAckConfig.getAckTimeout()
> {
> return (uint16_t)(800 * RADIO_ALARM_MICROSEC);
> }
>
> So it waits up to 0.8 milliseconds, which is less than the CC2420 1.6
> millisecond delay. By the way, the IEEE 802.15.4 standard requires
> acknowledgements within 192 microseconds, so the IRIS is compliant and
> the TelosB is not (with its software ACK implementation).
>
> You can solve the problem in two ways: 1) increase the 800 microsecond
> wait period for the ISIS for example to 1600 (probably 1100 is enough)
> microseconds, or 2) use hardware ACK for the TelosB.
>
> Miklos
--
Platform: WinXP/Cygwin
TinyOS version: 1.x, Boomerang
Programmer: MIB510
Device(s): Mica2, MicaZ, Tmote
Sensor board: homebrew
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