Re: [6tisch] call for review: draft-ietf-6tisch-msf-04

[Tero Kivinen <[e-mail]>]

Pascal Thubert (pthubert) writes:
> In a 6TiSCH network, CCA is useless between synchronized devices because
> they’ll talk at the same time. So LBT must be done some other way.

Note, that 802.15.4 do allow doing CCA if TschCca was set on when
MLME-TSCHE-MODE.request primitive is called to turn TSCH mode on. In
that case the node will do CCA at macTsCcaOffset time for macTsCca,
and all this is done before the actual macTsTxOffset happens, i.e.,
before the node is supposed to send its frame. This CCA does not
protect against other nodes in the TSCH, but it will protect against
other users on the same band, thus it should be enough for the legal
LBT requirements.

See figure 6-30 of 802.15.4-2015 in Section 6.5.4.1 and Section
6.2.5.2 covering TSCH CCA algorithm.

> The bright side is that the future collision can be detected before it even
> happens. Doing CCA on cells before allocating them, etc… I think you must
> provide a method for that. A trade off would be to provide that method in the
> draft and make it optional.

Note, that doing CCA on cells is bit pointless, as that will only
detect someone inside the same syncronized network using that slot, it
will not detect anybody else. I think it would be better to get
information of allocated slots from some kind of centralized node,
keeping track of nodes allocated and making sure there is no
collisions inside that one network.

Also, you cannot use the normal TSCH CCA for that, as it is done
BEFORE the actual time for TSCH frame to be sent on the channel. I do
not think there is a good way to do that specified in the 802.15.4. In
theory you could do it by configuring the specific slot for receiving
before starting to use it for transmission, and enabling promiscuous
mode, so you get all frames even when they are not addressed to you.

So requiring such feature to be provided for 6TiSCH to work, might
make it so that not all radios can do this, depending how much of TSCH
is actually implemented on MAC and how much is implemented in upper
layer.
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