[Tinyos-devel] problem of using FTSP and CTP

[Andreas Köpke]

...which is why I called the current solution a hack. 

On Montag 23 März 2009 Miklos Maroti wrote:
> I believe that you can do it, but please do it in a platform
> independent way, so it works both on the CC2420 and RF230. The whole
> point is that we do not want to reimplement time sync every time
> someone wants to use a single-hop time sync message, and this is why
> we implemented TimeSyncActiveMessageC. Miklos
>
> On Mon, Mar 23, 2009 at 7:29 PM, Andreas Köpke <koepke at tkn.tu-berlin.de> 
wrote:
> > Yep, I do propose to implement FTSP on top of an AMSenderC.
> >
> > On Montag 23 März 2009 Miklos Maroti wrote:
> >> Hi Andreas,
> >>
> >> On Mon, Mar 23, 2009 at 7:09 PM, Andreas Köpke <koepke at tkn.tu-berlin.de>
> >
> > wrote:
> >> > I agree with Phil here: components that use the AMSend virtualizer
> >> > should not be influenced by components that do not use it.
> >> >
> >> > But is it really necessary for FTSP do use ActiveMessageC directly? I
> >> > think not. In my opinion, there are two reasons why FTSP uses
> >> > ActiveMessageC directly, one is ease-of-porting and the other, more
> >> > important one, is that it uses a message _footer_ on tmote. On the
> >> > sender side, the footer contains the time when the message was
> >> > generated, whereas on the receiver side it contains the time that
> >> > passed between the message generation time and the time its SFD was
> >> > transmitted. A footer is a sensible choice here, since it allows the
> >> > CPU enough time to compute the deltaT and modify the message on the
> >> > fly.
> >> >
> >> > So the fact that FTSP attaches directly to ActiveMessageC is actually
> >> > a hack that circumvents the lack of dynamic footers in message_t. As
> >> >  a case study, I implemented a time synchronization on top of the
> >> > AMsend virtualizer, using a standard AMSenderC component. It works, I
> >> > just have to handle the footer the way TimeSyncActiveMessageC does:
> >> > attach a uint32_t at the end of the message, set it to the current
> >> > local time and expect that it has been modified to contain deltaT on
> >> > reception. Perfect, simple and without any side effects on other
> >> > components that use AMSend. Unfortunately, it requires that the
> >> > application is "footer aware" and "time stamp aware" -- no big deal
> >> > for FTSP, though.
> >>
> >> FTSP does not use ActiveMessageC (look it up), it uses the
> >> TimeSyncActiveMessageC and that is not a hack. Of course
> >> TimeSyncActiveMessageC could be implemented on top of the AMSenderC
> >> and not the ActiveMessageC. Maybe that is what you are proposing? That
> >> would work. By the way, we are not talking about FTSP or CTP per se,
> >> that is just where people see the bad interaction of
> >> TimeSyncActiveMessageC, AMSenderC and ActiveMessageC.
> >>
> >> Miklos