Dale, Please note that RFC7554 is informational, and describes/introduces the functioning of TSCH. The actual standard is IEEE802.15.4-2015 [1]. In particular, in case of discrepencies between the docs, IEEE802.15.4-2015 is the one to follow.
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 - 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. Happy to answer further questions to might have, also during the interim meeting later today [2]. Thomas [1] http://standards.ieee.org/about/get/802/802.15.html [2] https://www.ietf.org/mail-archive/web/6tisch/current/msg04760.html On Thu, Sep 22, 2016 at 4:39 PM, Dale R. Worley <wor...@ariadne.com> wrote: > Pardon me, I'm new here, but I was wondering why the channel hopping > scheme is defined the way it is. Currently, RFC 7554 notes that the > channel selected for a transmission by a node is: > > frequency = frequency_table [ (ASN + channelOffset) mod nFreq ] > > where channelOffset is a set value for this node to use in this slot of > the slotframe cycle, frequency_table is a set array of frequencies, and > nFreq is the size of frequency_table. > > The question I have is with ASN, the "absolute slot number". RFC 7554 > states that it increments one for each slot time, so that as a > particular slotframe slot recurs, the ASN is incremented by the number > of slots in the slotframe. > > The result is that to ensure that each node's transmissions cycle > through all the frequencies in the frequency_table, the length of the > slotframe needs to be relatively prime to nFreq: > > A way of ensuring communication happens on all available frequencies > is to set the number of timeslots in a slotframe to a prime number. > > My question is that if ASN was redefined to be a counter of slotframes > rather than of slots, then each node would cycle through all the > frequencies regardless of the size of the slotframe, because each node's > frequency index would advance 1 with each slotframe rather than > (slotframe_size mod nFreq). > > This alternative is fairly obvious, so there must be a known reason why > it isn't desirable. > > Dale > > _______________________________________________ > 6tisch mailing list > 6tisch@ietf.org > https://www.ietf.org/mailman/listinfo/6tisch > -- _______________________________________ 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 _______________________________________
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