[Tinyos-devel] again about the baudrate issue for telos

[Omprakash Gnawali]

On Tue, Feb 17, 2009 at 5:42 PM, Philip Levis <pal at cs.stanford.edu> wrote:
>
> On Feb 17, 2009, at 8:42 AM, Jorge Ortiz wrote:
>
>> If we're taking a poll, I vote to slow it down to 57600.  I've had
>> trouble with the higher speed with and without the serial
>> forwarder.  At 57600 all my serial-related reliability problems went
>> away and the code I was working with was significantly more stable.
>>
>> Jorge
>
> Can you be more specific than "I've had trouble?" Which direction?
> What motes? What operating system? Which serial forwarder?
>
> I could have some code in my back pocket that's unstable except at
> 2400. There's a fundamental issue here, with respect to rate control
> and flow control. The general operating model for the serial stack was
> that data mostly comes *out* of the network, so you want to optimize
> the mote->PC speed. Changing it to 57600 might make the problem go
> away for some cases, but it doesn't actually solve the problem. This
> isn't good system design.

The argument almost sounds like you should not use the provided
PC->mote interface unless you don't care about the packets being
delivered. I think there are hidden assumptions about reliability and
hidden assumptions are almost everywhere so it is not necessarily
dangerous.

I think the point being made here is in some (maybe a lot?) scenarios
there is the need to simplify the system rather than make it pure and
perfect. If the system is being built for deployments where there is
no luxury for trial-and-error, they can be easily convinced that they
might want to do more than rely on assuming PC->mote reliability. If
someone wants to send messages to control quick research experiments,
a flow control that follows "good system design" might seem onerous.

If someone has implemented PC->mote reliability, it would be good to
examine the code and timing overheads. If the overheads are minimal,
we should make such a system widely available because one of the
arguments here is no one should use the unreliable PC->mote
communication unless you don't care about packet delivery. If no one
has implemented such a module, we can recommend this work when someone
is looking for a project...

- om_p