Subject: [6tsch] Resend in HTML: Re: TSCH with multiple DAGroots

From: Tom Phinney <[email-masked]>
Date: Thu, 14 Feb 2013 14:13:41 -0700

Unfortunately, my first reply inherited the lack of formatting of the
original to which it was replying. This is what I intended be sent. -Tom
====
Qin,

Each slot in a TSCH system is, in principle, usable for any transaction
whose duration will not exceed the length of the slot. Where a
significant percentage of the transactions that are expected to occur in
a system are of much shorter maximum duration than others, it may make
sense to have schedules with different slot durations, enabling
conveyance of more messages per second.

Without showing the required guard-bands and other periods of enforced
RF inactivity for each constituent subelement, the basic template for
use of a TSCH slot is generally:
 <CCA assessment interval> <primary PDU of the transaction
[<immediate-acknowlegment-1> [<immediate-acknowlegment-2> [...
<immediate-acknowlegment-N>]  ] ]

In generally planar industrial environments such as refineries and
pharmaceutical manufacturing cells that use 1 Hz and 4 Hz wireless
control loops, the N above needs to be N=2. However on highly metallic
3D structures such as offshore oil platforms, N may need to be N=3.
Whereas in less challenging environments such as are now used for
WirelessHART, N can be (and is) N=1.

If some of the slots in a TSCH system require N=2 or N=3 immediate
acknowledgments to achieve the necessary delivery reliability, then a
single slot template must include time for each of those immediate
acknowledgments. For example, in today's deployments of ANSI/ISA 100.11a
systems, the slots that provide for N=2 acknowledgments tend to be 12 ms
in duration. However, that particular feature - duocast - is used only
for reporting of the process status of those 1 Hz and 4 Hz loops; most
other traffic is unicast, for which a 10 ms slot duration is adequate.
If the traffic mix in such a system is 1/3 duocast and 2/3 other
(unicast with an occasional broadcast), then requiring use of 12 ms
slots for the traffic that could use 10 ms slots results in an 11%
reduction of total subnet messaging capacity.

If FEC were to be available for industrial automation systems, it would
likely be used for only a small percentage of the communications. Again,
as above, each slot could be sized so that every transmission used FEC.
However, in practice, perhaps 5% or less of the transactions would
require FEC, since safety-related communications is typically only a
very small part of such systems. There would be an inordinate waste of
communication capacity if all devices employed FEC, which would increase
the duration of their transmissions and receptions, and thus the power
drain required for communication. Thus a mixture of slot durations is
again appropriate.

Independent of how many acknowledgments are anticipated for a specific
transaction, it is worth noting that there are at least two useful
timing alignments for the start of immediate acknowledgments. One
alternative is a defined fixed offset AFTER the end of reception of the
primary PDU of the transaction, where the delay accounts for required
transmit/receive and receive/transmit turnaround times in the
correspondent nodes (which may be either motes or infrastructure
wireless routers, which is why I do not always call such "motes").
Another alternative is a defined fixed offset BEFORE the scheduled end
of the slot. In either case, when the slot template provides for
multiple immediate acknowledgments, each acknowledgment has a different
offset relative to the defining event, which is either 1) detection of
end of reception or 2) the scheduled end of the slot.

Figures 4 and 5 of the attached patent application show how this could
work for N=3. The discussion in paragraph [0064] and following gives
some indication of when the alternative 2) might be used. The economic
driver for this alternative is the ability, using time-aligned
acknowledgments and co-scheduling of inbound or outbound message flow in
concurrent slots in multiple channels, to build a multi-channel backbone
router that uses a single antenna and wideband RF front end shared by
multiple low-power transceiver chips. Such a structure enables the
construction of low-cost high-throughput backbone routers that can be
installed in real systems precisely because such routers do not require
the installation of multiple separated antennae that are spaced
sufficiently far apart to not encounter near/far problems. Real
industrial plants seldom can collocate multiple backbone routers,
precisely because they cannot install adequately separated antennae for
those routers in the places where the routers are needed.

-Tom Phinney
===
On 2013.02.13 11:39, [email-masked] wrote:
> Hi Tom,
>
> Could you please explain more about "a system in which different slots
(perhaps in different superframes) can have different durations"?
>
> In addition, with FEC, longer slot duration may be needed, but it is
unnecessary to have different slot duration in one network.
> How do you think?
> Thanks
> Qin

---

From: "Qin Wang" <[email-masked]>
Date: Thu, 14 Feb 2013 14:06:37 -0800

Hi Tom,

Thank you for your explanation. It is very clear.

I have one further question: Is there any constraint or assumption for
nodes with different slot duration? For example, the nodes with different
slot duration should belong to different DL network, or.....

Thanks
Qin

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