Re: [6tisch] The channel hopping scheme seems to be suboptimal

[Thomas Watteyne <[e-mail]>]

Tero, agreed.

On Tue, Sep 27, 2016 at 3:29 PM, Tero Kivinen <[e-mail]> wrote:

> Dale R. Worley writes:
> > Thomas Watteyne <[e-mail]> writes:
> > > You raise a valid point. There are a couple of points, though, that
> make an
> > > "Absolute Slot Number" more favorable compared to an "Absolute
> Slotframe
> > > Number":
> > > - if you schedule multiple cells between two nodes in a single
> slotframe,
> > > you want those different transmissions to happen at a different
> frequency
> >
> > Although you could construct the schedule so that the nodes have
> > different channel offsets for the different slots within the schedule.
>
> There can also be multiple networks (or parts of network) using
> different set of slotframe lengths, and channel offsets etc, and if
> things are set properly they do not collide.
>
> I.e. if you have network(s) using same hopping sequence and hopping
> sequence length, but different number of slots (slotframe length) and
> different channel offsets, they can coexists in same space, and they
> will not transmit on the same channel.
>
> I made an excel sheet to demonstrate this:
>
> https://mentor.ieee.org/802.15/dcn/15/15-15-0604-00-0mag-
> example-of-tsch-schedule.xls
>
> Here you can have multiple networks (A-J) and they can have different
> number number of slots and different channel offset and you can see
> that do not collide if parameters are selected properly (i.e. they use
> different channel offset).
>
> Using different channel offsets you can configure network with
> different slot frame lengths, i.e. you can have one with slotframe
> length of 100 to allow lots of clients, and another one with slotframe
> length of 5 to allow fast access to media etc.
>
> If device is part of multiple of those it will pick which of those
> networks it is using based on priority, i.e., if it wants to transmit
> something and has slot that allows transmit on one network, it tunes
> in to that channel and transmits. Otherwise it picks the one of the
> receive channels and listents to that etc.
>
> 802.15.4 has some text about this, but mostly this is defined by the
> vendor making the devices...
>
> > > - ASN is used to construct a nonce when securing link-layer frames.
> > > Security is such that we never want to re-use the same nonce.
> >
> > Although you could construct the nonce to be (ASFN * nSlots +
> > slotOffset).  And you need to do a construction like that to make sure
> > that different nodes with different channel offsets during the same slot
> > do not use the same nonce.
>
> Nonce also has the transmitters extended address, which makes the
> nonces different for each transmitter, and one transmitter can only
> transmit one packet on each slot thus nonce is unique.
>
> > I suspect the main reason is that several slotframe schedules of
> > different lengths can be active at the same time in one network.
> > Keeping a separate ASFN for each schedule would require a lot of work.
>
> Yes.
> --
> [e-mail]
>



-- 
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Thomas Watteyne, PhD
Research Scientist & Innovator, Inria
Sr Networking Design Eng, Linear Tech
Founder & co-lead, UC Berkeley OpenWSN
Co-chair, IETF 6TiSCH

www.thomaswatteyne.com
_______________________________________