[Smac-users] S-MAC using slots

[John Heidemann]

On Fri, 18 Feb 2011 10:52:05 GMT, Norberto Barroca wrote: 
>Hello Everyone.
>
>If possible I would like to ask some questions about the S-MAC protocol.
>
>Since the S-MAC protocol randomly selects a time slot to finish its carrier sense. And if
>it has not detected any transmission by the end of the time slot, it wins the medium and
>starts sending its SYNC packet at that time. The same procedure is followed when sending
>data packets.
>
>I would like to know if a node only chooses a time slot to transmit or if he chooses a
>fixed/variable number of time slots?

If a node chooses to contend in a given frame, it picks one slot in
that frame which to do so.  It does not pick multiple slots in which to
transmit, although it does listen for other contenders until it transmits.

>
>This time slotted mechanism happens when transmitting a SYCNH, RTS, CTS and DATA packet?

It happens for SYNC and RTS.
Randomized contention is not needed for CTS or DATA since
there is only one possible station that can send those messages 
at any time.

>
>Each node only transmits in a time reserve to him? Or the mechanism is different?

SYNC and RTS packets are sent in a randomly chosen slot.
Those slots are not reserved.

CTS and DATA happen in periods that are, effectively reserved.
(Modulo hidden terminal.)

>If the S-MAC is a slotted protocol what happens if two nodes choose the same time the
>same time slot during the synch, RTS, CTS or DATA phase? Can neighbour nodes know that
>there is a collision.

Nodes will hear a collsion and know there are other contenders.
The two (or more) transmitters will know there is a collision and will
retry in a later frame.

Finally, if you were interested in S-MAC, I encourage you to also look
at SCP-MAC.  The SCP-MAC paper in Sensys 2006 is at
<http://www.isi.edu/~johnh/PAPERS/Ye06a.html>.  We believe SCP is
strictly better than S-MAC concerning energy conservation and
preformance, so we encourage you to examine it for future work.

   -John Heidemann