Tero, agreed. On Tue, Sep 27, 2016 at 3:29 PM, Tero Kivinen <[email protected]> wrote:
> Dale R. Worley writes: > > Thomas Watteyne <[email protected]> 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. > -- > [email protected] > -- _______________________________________ 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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