[Tinyos-devel] problem of using FTSP and CTP

[Andreas Köpke]

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. 

Best, Andreas