Dear all,
I have a small questions about the 802.15.4 TSCH standard.
It is written:
Thus, a device decrements its backoff when a shared slot is finished.
/************* Standard *********/
Let's assume A and B chose the same backoff value 2 (= a collision occurs)
Then, A selects the backoff value 1 and B the backoff value 2
<------ shared slots -----> <------ shared slots
------>
A |______|_____|_XXXX|______|___ … |_XXXX|______|_____|____]
B |______|_____|_XXXX|______|___ … |______|_XXXX|_____|____|
Thus, the medium access delay is quite large.
/************* Non-Standard *********/
I know it is *not* standard compliant. But let's imagine we have rather a
slotted-CSMA *inside* a shared slot (and not *among* the shared slots). Thus, a
device would decrement its backoff after aUnitBackoffPeriod at the beginning of
each shared slot. The backoff would then be paused when the medium is detected
busy in the shared slot, and resumed at the beginning of the next shared slot.
We would have:
<------ shared slots -----> <------ shared slots
------>
A |_XXXX|_____|_XXXX|______|___ … | ______ |_______|_____|____]
B |_XXXX|_____|______|_XXXX|___ … |_______| ______ |_____|____|
(the backoff is triggered at the beginning of each shared slot)
At a first glance, this non-standardized slotted CSMA would reduce the medium
delay and the number of collisions by selecting larger BE.
In conclusion, I wonder why was rather the first approach adopted in the
standard? Did I misunderstand something important?
Sorry for my long email...
Best regards,
Fabrice Théoleyre
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