This is great to know! Apart from this topic there was also a discussion on April 7 on the
maximum payload size that we could use up to 128 bytes for IEEE 802.15.4 radio
compliant platform.
http://mail.millennium.berkeley.edu/pipermail/tinyos-help/2006-April/015912.html
Could somebody shed some light on me?
[Cory]
Though the CC2420 FIFO is 128 bytes, so if you want any double
buffering between the CC2420 and the TinyOS radio stack, you'll want
to keep TOS_Msg no larger than 64 bytes.
[/Cory]
I'm so doubtful why most of applications do not use a big payload size.
For example, TinyDB uses only 49-byte payload length. Is double buffering the reason
behind? We could think that using longer payload size (if needed) saves some overhead
(at least 10 bytes) if we have to send it 2 times. Does anybody use the maximum
payload length of 128 bytes? Why and why not?
Thank you for any explaination.
Regards,
Nuenoi
On 4/24/06, Philip Levis <[EMAIL PROTECTED]
> wrote:
On Apr 24, 2006, at 10:07 AM, David Gay wrote:
>
> It's historical. The original TinyOS motes had a radio that was
> controlled by software at the bit level, and somebody (Jason?) found
> that 36 bytes/packet was a good place to be (maximise throughput,
> minimise chance of packets getting lost because of software timing
> glitches).
>
> That was 30 bytes of payload, 4 bytes of header, and 2 bytes of CRC.
> Packets were always 36 bytes.
>
> Then someone added variable-length packets, adding 1 byte (the
> length...) to the header. Hence 29 bytes of payload. Still later,
> someone added the option for a different payload length, but the
> default is 29.
>
> At this point it's still 29 bytes because:
> - nobody has bothered to change the default
> - it avoids the message buffers taking too much RAM
Furthermore, bandwidth is not usually the critical metric for low-
power embedded wireless nodes that have a <1% duty cycle. There are
of course situations where it's important, but the assumption is that
in those cases you can increase the payload size. Ehile telosB motes
and their derivatives have 10K, not all MCUs do, and precluding
TinyOS from running on a microcontroller that isn't the biggest you
can buy would be a bad idea.
Phil
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