Subject: [6tsch] Synchronization Issues

From: "Alfredo Grieco" <[email-masked]>
Date: Thu, 22 Aug 2013 12:44:38 +0200

Dear All,

My research group is trying to test and develop fast bootstrapping
techniques for TSCH.

In particular, we are focusing on synchronization issues. We would like to
share with you our thoughts to gain your comments.

Assuming that the PAN coordinator starts sending EBs, its neighbors will
send EB too after they get synchronized (i.e., when they successfully
receive the EB sent by the coordinator). In this way, the synchronization is
propagated downwards to the leaves. Unfortunately, the neighbors of the
coordinator cannot know in advance the channel to listen (i.e., the channel
the PAN coordinator will use to transmit EBs) so that it may take some time
to establish the synchronization at one hop, and, as a consequence, till all
nodes are reached by an EB (since the same problem holds also for the
second, third hop and so on). The longer the bootstrap time the larger the
amount of energy spent in for listening EBs.

How to shorten this bootstrap time ?

If we look at the pan coordinator, assuming it is powered by the mains, it
could send more than one EB per superframe in order to swipe faster all
available channels.

Looking at the rest of the nodes that already received an EB, we could try
to let them transmit an EB on average once every Multisuperframe in such a
way that all channels are used in a reasonably short time. In this way, a
new joining node could quickly receive an EB whatever the channel it is
listening to.

Any comment is welcome.

Thanks in advance

Alfredo

--
Luigi Alfredo Grieco, Ph. D.
Assistant Professor in Telecommunications 
DEE - Politecnico di Bari 
Via Orabona, 4 - 70125 - Bari - Italy
Phone: +39 080 5963 911
Fax: +39 080 5963 410
telematics.poliba.it/grieco

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From: Kris Pister <[email-masked]>
Date: Thu, 22 Aug 2013 16:45:37 -0700

Alfredo -
  having all of the powered nodes (not just the PAN coord/DODAG root) 
advertise more frequently is a good idea for a lot of reasons.
They can afford it, and they are probably the nodes that will ultimately 
have the most
children in a network with a power-aware routing metric.

For relatively low duty cycle on the transmitter and receiver, and 
randomly chosen
advertising slots, the average time needed for a mote to synchronize to a
network is CT/(NPD)  [1]

C = # channels
T = average time between EBs transmitted by neighbors
N = number of neighbors
P = packet delivery ratio (1-PER)
D = duty cycle of the receiver

On the other end of the scale, if the listener duty cycle is 100% and it 
listens for C
superframes, then it will hear an advertisement from PN neighbors.

If the EBs are only sent in a known cell, or even a known slot, then the 
first
synchronization event has the same average time as above, but the speed of
discovery of the rest of the network has more to do with Poisson statistics
and node degree than the number of channels and the superframe length.

ksjp

[1] 
http://robotics.eecs.berkeley.edu/~pister/290Q/Papers/Pister%20TSMP%20DSN08.pdf 
<http://robotics.eecs.berkeley.edu/%7Epister/290Q/Papers/Pister%20TSMP%20DSN08.pdf>

On 8/22/2013 3:44 AM, Alfredo Grieco wrote:
> _______________________________________________
> 6tsch mailing list
> [email-masked]
> https://www.ietf.org/mailman/listinfo/6tsch

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From: Giuseppe <[email-masked]>
Date: Fri, 23 Aug 2013 02:51:00 +0200

Il 23/08/2013 01:45, Kris Pister ha scritto:
> They can afford it, and they are probably the nodes that will 
> ultimately have the most
> On the other end of the scale, if the listener duty cycle is 100% and 
> it listens for C
> If the EBs are only sent in a known cell, or even a known slot, then 
> the first
Hi,
I agree, in my opinion while for the first time of bootstrap of the 
network, only the synchronizer can advertise the network, as soon as the 
first nodes reach synchronization and start to advertise the network, 
the sych time average could be decreased by sending the EB in the same 
frameslot but scheduling the transmission on different channels. This 
could increase also the degree of the network because asynchronous 
advertisement could lead to several beacon collisions in networks with 
an high density of nodes, while scheduling beacons with the couple 
(superframe_num, channel_num) should avoid the collisions in this case 
and improve the synchronization phase, letting a certain freedom on the 
superframe and on the multisuperframe length. I hope I was clear enough.

-- 
Giuseppe

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From: Kris Pister <[email-masked]>
Date: Fri, 23 Aug 2013 08:49:04 -0700

+1 (assuming you meant slot_num not superframe_num)

ksjp

On 8/22/2013 5:51 PM, Giuseppe wrote:
> while scheduling beacons with the couple (superframe_num, channel_num) 
> should avoid the collisions in this case and improve the 
> synchronization phase

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From: Giuseppe <[email-masked]>
Date: Fri, 23 Aug 2013 19:06:13 +0200

Il 23/08/2013 17:49, Kris Pister ha scritto:
>> while scheduling beacons with the couple (superframe_num, 
>> channel_num) should avoid the collisions in this case and improve the 
>> synchronization phase
>
yes, I've assumed advertisement is sent once for superframe.
Cheers

-- 
Giuseppe