[Tinyos-devel] problem of using FTSP and CTP

Andreas Köpke koepke at tkn.tu-berlin.de
Mon Mar 23 11:29:54 PDT 2009

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> 
> > 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

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