Fixed vs. Variable CTAs

[luca.za...@gmail.com]

All,
since the Standard 802.15.3 doesn't specify any traffic scheduling
algorithm, I'm trying to understand if it is better to implement a
model with fixed CTAs or make the PNC assign just the specific amount
of time asked by the DEV in the Channel Time Request command.

Now I'm focusing on asynchronous traffic. For my understanding:
1) Fixed CTAs have the only problem that the medium will never be
fully utilized because all the communications will not use the amount
of channel time that was given to them
2)On the other hand I see one problem assigning CTAs exactly matching
the time requested by the DEV: what happens if a transmission fails
and the DEV needs to retransmit a frame? There is no space to do that
in the current CTA and so the DEV should ask for other channel time
(with another Channel Time Request).

Reading some papers I found most of the simulation studies used fixed
CTAs.
My question is: is really fixed CTAs the way to implement a good model
for the CTAP part of 802.15.3? In case it is not, how to solve the
problem of retransmissions in case 2?

Thanks for your ideas

Luca Zappaterra