Subject: [Roll] question on gain control

From: Yoav Ben-Yehezkel <[email-masked]>
Date: Sun, 25 Jul 2010 23:37:11 +0300

Hi,

I would like to probe the WG’s opinion on the issue of transmission at
different transmission gains to obtain more reliability on the link quality.

For example, somehow indicating the transmission power level on DIS/DIO/DAO
(maybe by percent of or db less than maximum transmission level) to make
sure that metrics are calculated/parents are selected, etc. only based on
transmissions received with low transmission gains.

We’ve used this to make our topologies more stable in proprietary solutions.
The motivation is to have sufficient level of margin for noise before
messages at high gain will not be received properly (i.e. if messages are
received at 6dB less than the maximum transmission gain, there’s room for at
least 6 dB more noise before the messages will not be received – 6 dB is a
lot of margin, so topologies remain very stable with this kind of margin).

I believe that such an option used by the RPL would bring benefit, and maybe
belong to one of the RPL drafts (i.e. metric, OF, other?).

Would this something the RPL WG would consider to add?

Your inputs would be greatly appreciated.

Regards,

Yoav

---

From: JP Vasseur <[email-masked]>
Date: Mon, 26 Jul 2010 09:49:28 +0200

Hi Yoav,

I would fully support this. It would go in the metric-ID (or separate  
ID, but this is a metric discussion item for sure) but let's wait for  
the WG feed-back to see what they think.

Cheers.

JP.

On Jul 25, 2010, at 10:37 PM, Yoav Ben-Yehezkel wrote:

> Hi,
>
> I would like to probe the WG’s opinion on the issue of transmission  
> at different transmission gains to obtain more reliability on the  
> link quality.
>
> For example, somehow indicating the transmission power level on DIS/ 
> DIO/DAO (maybe by percent of or db less than maximum transmission  
> level) to make sure that metrics are calculated/parents are  
> selected, etc. only based on transmissions received with low  
> transmission gains.
>
> We’ve used this to make our topologies more stable in proprietary  
> solutions. The motivation is to have sufficient level of margin for  
> noise before messages at high gain will not be received properly  
> (i.e. if messages are received at 6dB less than the maximum  
> transmission gain, there’s room for at least 6 dB more noise before  
> the messages will not be received – 6 dB is a lot of margin, so  
> topologies remain very stable with this kind of margin).
>
> I believe that such an option used by the RPL would bring benefit,  
> and maybe belong to one of the RPL drafts (i.e. metric, OF, other?).
>
> _______________________________________________
> Roll mailing list
> [email-masked]
> https://www.ietf.org/mailman/listinfo/roll

---

From: Omprakash Gnawali <[email-masked]>
Date: Mon, 26 Jul 2010 13:32:29 -0700

> lot of margin, so topologies remain very stable with this kind of margin).

How low a gain would you consider for topology discovery if you were
to use this approach and the chip allows you to go to extremely low
values? How does this work on networks with different density?

- om_p

---

From: Yoav Ben-Yehezkel <[email-masked]>
Date: Tue, 27 Jul 2010 07:17:19 +0300

It is up to the application which gains to use and how to use it with DIS
messages.

We've used it in PLC networks in topologies with up to a few thousands of
nodes, where each node on average has direct connectivity with about 200
nodes when using maximum gain and about 80% of the nodes using lower
transmission gain.

Our technology allows for 8 levels of gain, other technologies allows for
some other values, Jennic's 802.15.4 modules allow for 4 levels of gain
for example. This is completely application dependant.

Does this make sense?

Thanks,
Yoav

-----Original Message-----
From: Omprakash Gnawali [mailto:[email-masked]]
Sent: Monday, July 26, 2010 11:32 PM
To: Yoav Ben-Yehezkel
Cc: [email-masked]
Subject: Re: [Roll] question on gain control

On Sun, Jul 25, 2010 at 1:37 PM, Yoav Ben-Yehezkel <[email-masked]>
wrote:
> Hi,
>
>
>
> I would like to probe the WG's opinion on the issue of transmission at
> We've used this to make our topologies more stable in proprietary
solutions.
> received at 6dB less than the maximum transmission gain, there's room
for at
> least 6 dB more noise before the messages will not be received - 6 dB is
a

- om_p

---

From: Omprakash Gnawali <[email-masked]>
Date: Tue, 27 Jul 2010 14:25:45 -0700

> for example. This is completely application dependant.

Yes, it does. Two more questions:

- How does this mechanism work in a network that has different density
at different places?

- How does this mechanism work when new nodes are introduced to the network?

Thanks.

- om_p

---

From: Yoav Ben-Yehezkel <[email-masked]>
Date: Wed, 28 Jul 2010 01:34:25 +0300

Hi,

An example for an implementation utilizing this mechanism by new nodes or
by disconnected nodes that are looking for reliable parents is as follows:

1. the node sends out a DIS (at low transmission gain, and with low rank
filter for parents). The idea is that it is better to connect to the
lowest possible parent (DODAG root for example), with sufficient margin.

2. if DODAG roots hear the DIS, only they respond with DIO. If one wants
to maintain a bi-directional link with the margin, then the DIO response
to the DIS should also be using low power (if the gain is indicated on the
DIS).

3. If no response arrives after sending the DIS, the node can slightly
increase the filter on the rank, and/or increase the transmission gain
and/or both and resend the DIS.

4. This time, all nodes that satisfy the rank filter (can be nodes that
are not DODAG roots) can respond, again with the capability to do so using
low power.

5. repeat steps 3 and 4, until DIO response to DIS is received from a
parent node (the last attempt can be without any rank filter and at
highest transmission gain).

The above method serves a few purposes:
It allows the incoming responses to be using lower transmission power,
hence, most likely after this connection is established, DIO/DAO reception
will remain stable (since there was still margin for noise when the
connection was established).

Side effects:
1. Filter out DIO responses to DIS at the potential parent to avoid
congestion.
2. Note that nodes attempting to connect can wait very little time after
each DIS for responses before retrying to connect by sending out another
DIS (because of 1.)
3. Agnostic to density.

The above example is just one heuristic method that can be used to quickly
obtain a highly reliable link with low congestion using gain control. The
above doesn't preclude additional ways that can be thought of to use the
info on transmission gain. My point was having the option of using the
transmission gain as a metric/constraint. The idea probably needs to be
better formulated to fit into RPL metrics/constraints spec. I'd appreciate
the WG/editors'/chairs' assistance with that you find this method
relevant/useful.

Best regards,
Yoav

-----Original Message-----
From: Omprakash Gnawali [mailto:[email-masked]]
Sent: Wednesday, July 28, 2010 12:26 AM
To: Yoav Ben-Yehezkel
Cc: [email-masked]
Subject: Re: [Roll] question on gain control

- How does this mechanism work when new nodes are introduced to the
network?

Thanks.

- om_p

---

From: Philip Levis <[email-masked]>
Date: Wed, 28 Jul 2010 00:47:48 +0200

On Jul 25, 2010, at 10:37 PM, Yoav Ben-Yehezkel wrote:

> Hi,
>  
> I would like to probe the WG’s opinion on the issue of transmission at different transmission gains to obtain more reliability on the link quality.
>  
> For example, somehow indicating the transmission power level on DIS/DIO/DAO (maybe by percent of or db less than maximum transmission level) to make sure that metrics are calculated/parents are selected, etc. only based on transmissions received with low transmission gains.
>  
> We’ve used this to make o cur topologies more stable in proprietary solutions. The motivation is to have sufficient level of margin for noise before messages at high gain will not be received properly (i.e. if messages are received at 6dB less than the maximum transmission gain, there’s room for at least 6 dB more noise before the messages will not be received – 6 dB is a lot of margin, so topologies remain very stable with this kind of margin).
>  
> I believe that such an option used by the RPL would bring benefit, and maybe belong to one of the RPL drafts (i.e. metric, OF, other?).
>  

Why not just use a good link estimator? Any reasonable implementation isn't going to assume receiving a DIS or DIO implies good connectivity. Another mechanism would simply be to only consider nodes whose DIOs have had an SNR above X to be candidate neighbors. Actually using variable transmission power to infer link quality and stability is problematic: it can be a link has high SNR but low PRR (e.g., due to external interference).

Phil

---

From: Yoav Ben-Yehezkel <[email-masked]>
Date: Wed, 28 Jul 2010 08:27:35 +0300

Indeed, in wireless networks SNR can be used as a good estimator.
Unlike wireless networks, in low speed PLC technologies, because the
operation is very close to the negative SNR bounds, there's only about 3dB
difference between no reception and perfect reception.
This is the main motivation for this method. 3dB is not a good enough
margin.

Thanks,
Yoav

-----Original Message-----
From: Philip Levis [mailto:[email-masked]]
Sent: Wednesday, July 28, 2010 1:48 AM
To: Yoav Ben-Yehezkel
Cc: [email-masked]
Subject: Re: [Roll] question on gain control

> I would like to probe the WG's opinion on the issue of transmission at
different transmission gains to obtain more reliability on the link
quality.
>
> For example, somehow indicating the transmission power level on
DIS/DIO/DAO (maybe by percent of or db less than maximum transmission
level) to make sure that metrics are calculated/parents are selected, etc.
only based on transmissions received with low transmission gains.
>
> We've used this to make o cur topologies more stable in proprietary
solutions. The motivation is to have sufficient level of margin for noise
before messages at high gain will not be received properly (i.e. if
messages are received at 6dB less than the maximum transmission gain,
there's room for at least 6 dB more noise before the messages will not be
received - 6 dB is a lot of margin, so topologies remain very stable with
this kind of margin).
>
> I believe that such an option used by the RPL would bring benefit, and
maybe belong to one of the RPL drafts (i.e. metric, OF, other?).
> Your inputs would be greatly appreciated.

Why not just use a good link estimator? Any reasonable implementation
isn't going to assume receiving a DIS or DIO implies good connectivity.
Another mechanism would simply be to only consider nodes whose DIOs have
had an SNR above X to be candidate neighbors. Actually using variable
transmission power to infer link quality and stability is problematic: it
can be a link has high SNR but low PRR (e.g., due to external
interference).

Phil

---

From: Chauvenet Cédric <[email-masked]>
Date: Wed, 28 Jul 2010 14:47:32 +0200

Hi Yoav, 

Thank you for your scenario.
Furthermore, it is relevant to say that power consumption is also a key point in PLC.

Your solution seems to be tight to link quality in the network.

As PLC behavior is very moving, it could happen that an event in the network (Plug/Unplug an electrical device, appliances start-up etc...) may change most of the link quality in the network.
As a result, the gain control computed during association may become irrelevant to the actual link quality.

How would your scenario adapt to these changes ?

Thanks,
Cédric

---

From: Yoav Ben-Yehezkel <[email-masked]>
Date: Wed, 28 Jul 2010 16:42:22 +0300

Hi Cedric,

Thanks for the feedback. I completely agree that power consumption in PLC
is also of great importance.

As you've said, changes in the grid can cause the noise to be big enough
to cause disconnections.

My proposal was to add an option when trying to (re)connect to get more
reliability on the link state when connecting, and after that use higher
gain if necessary. The difference between the gain used to connect and the
gain used later is the amount of margin a node has on the reliability of
the link with its parent, to accommodate for the grid changes you
mentioned. This is more adaptive than not using any margin on gain between
connection and normal operation.

Hope this shed some light on this issue.

Thanks,
Yoav

-----Original Message-----
From: Chauvenet Cédric [mailto:[email-masked]]
Sent: Wednesday, July 28, 2010 3:48 PM
To: Yoav Ben-Yehezkel
Cc: Philip Levis; [email-masked]
Subject: Re: [Roll] question on gain control

Hi Yoav,

Thank you for your scenario.
Furthermore, it is relevant to say that power consumption is also a key
point in PLC.

As PLC behavior is very moving, it could happen that an event in the
network (Plug/Unplug an electrical device, appliances start-up etc...) may
change most of the link quality in the network.
As a result, the gain control computed during association may become
irrelevant to the actual link quality.

Thanks,
Cédric

> solutions. The motivation is to have sufficient level of margin for
noise

---

From: "Monnerie, Emmanuel" <[email-masked]>
Date: Wed, 28 Jul 2010 09:55:05 -0400

That is correct. It is problematic to use SNR as a link quality
indicator. Computing a SNR is not an easy matter, particularly when it
has to be computed by a low cost and low power device. A SNR is the
ratio of the received signal power of a good packet over the average
noise power. When a link is bad, the device will not have many good
packets to choose from to measure the signal power and therefore the
statistics can be very inaccurately computed.

I think that ETX or PRR are better metrics in general. But I have a
similar question regarding the definition of the ETX metric (cf:
draft-gnawali-roll-etxof-01). I think it is missing some parameters to
be complete: Which packet transmission is used to compute this
statistic? How long is the packet?

My point is that the ETX value will depend on the length of the packet.
The standard should either define a baseline packet length to compute
this value or the definition of the ETX should be expanded to allow a
normalization of this value, based on the length of the packets.

Another question is: which ETX value should be used by default, when a
device discovers a new link? Should it be Infinity, until the statistics
are computed or should it be a more reasonable value (like 1 , 2 or 3)?
Depending on which decision is made, the system behavior can change
quite a bit.

Any suggestion?

-----Original Message-----
From: [email-masked] [mailto:[email-masked]] On Behalf Of
Philip Levis
Sent: Tuesday, July 27, 2010 6:48 PM
To: Yoav Ben-Yehezkel
Cc: [email-masked]
Subject: Re: [Roll] question on gain control

(...)

> Actually using variable transmission power to infer link quality and
stability is problematic: it can be a link has
> high SNR but low PRR (e.g., due to external interference).

>Phil

Emmanuel Monnerie
Senior Research Engineer
Landis+Gyr
Energy Management Solutions
Office: +1 678 258 1695
Fax: +1 678 258 1316
[email-masked]
www.landisgyr.com
manage energy better

---

From: David Culler <[email-masked]>
Date: Tue, 27 Jul 2010 13:59:09 -0700

The setting of the gain is of course a physical layer property.  Many chips provide many such parameters.  Many link layers provide some mechanisms to gain access to such properties.  Selecting the channel or SSID on your WiFi router is one such example.  And you might even say it was "up to the application" since it is accessed through a configuration interface either by a human being or a management tool.  But it is, of course, not part of the network layer's interface to the link, and it is not part of the routing protocol or the addressing structure.  And it is certainly not presented through the transport layer to the application.  Although, such things may be accessible to applications 'around the side" through an IOCTL or other operating systems interface.  All networks face this issue.  It is inherent in the separation of concerns and technological decoupling that is inherent in a layered architecture.

Yes the gain control affects the topology of the connectivity relationship, and therefore what the routing protocol has to work with, but that does not make it a routing protocol issue per se.  The density and placement of nodes also affects the available connectivity topology, but the routing protocol is not going to manufacture some new nodes and place them in convenient locations.  Nor does it have the authority to decide to ignore some nodes because it doesn't like the connections that they introduce.

Obviously, the question is jumbling together issues at very different layers in the architecture.  We might get all excited that this begs of cross-layer visibility and optimization, but then again variants of this appear in all sorts of other links.  Whether you get 53 mbps, or 11 mbps, or 3 mpbs, 1 mbps in modern 802.11 links depends on how far away you are from the access point and what's in the way.  Similarly, the topology and available bandwidth of wired networks depends on what switches and routers are used and how they are wired up.  These aspects of making the physical network organization match the target workload are what capacity planning is all about.  Why would the picture be any different in LLNs?  The routing protocol needs to find the best available route within reasonable effort.  It cannot prevent you from assembling a network that fails to meet the demands of the workload.  Many factors may determine how any particular node sets its output gain control,  or whether a PA or an LNA are used.  But unquestionably, they are outside the purview of the layer 3 routing protocol.

On Jul 26, 2010, at 9:17 PM, Yoav Ben-Yehezkel wrote:

> DIS/DIO/DAO

---

From: Yoav Ben-Yehezkel <[email-masked]>
Date: Wed, 28 Jul 2010 18:21:39 +0300

Hi David,

Thanks for the long response. I hope I understood it well.

I'm also not a fan of mixing between layers. You've convinced me that we
shouldn't introduce gain control explicitly to the RPL mechanisms and I also
agree that gain control is not a routing issue per se. It is an interface
between L2 and L3 issue.

However, from the point of view of L3 the issue was to allow for some
optimization for "marginal" reliability when (re)connecting to a
parent/DODAG. I described how to do it by using an L2 mechanism of gain
control.  The point I was trying to make for RPL was to have some way of
requesting to connect with some margin for reliability. Gain control can be
one means to do it in PLC networks, maybe SNR estimations can be the means
in wireless. Maybe there're other ways to get this in other technologies
also.

However, I'm still requesting your assistance to address the issue of how to
quantify some reliability margin possibly to an L3 metric/constraint or DIS
filter that would make sense in L3, (I’ll stop calling it gain control from
now J ). There's great potential for stabilizing the topology by reducing
the amount of disconnections and reconnections with such a "marginal
reliability" metric/filter/constraint for connection. It would be a
short-coming of RPL relative to existing mesh-under methods in PLC if we
don't introduce it IMHO.

I'd appreciate any further thoughts on this.

Best regards,

Yoav

-----Original Message-----
From: David Culler [mailto:[email-masked]]
Sent: Tuesday, July 27, 2010 11:59 PM
To: Yoav Ben-Yehezkel
Cc: Omprakash Gnawali; [email-masked]
Subject: Re: [Roll] question on gain control

The setting of the gain is of course a physical layer property.  Many chips
provide many such parameters.  Many link layers provide some mechanisms to
gain access to such properties.  Selecting the channel or SSID on your WiFi
router is one such example.  And you might even say it was "up to the
application" since it is accessed through a configuration interface either
by a human being or a management tool.  But it is, of course, not part of
the network layer's interface to the link, and it is not part of the routing
protocol or the addressing structure.  And it is certainly not presented
through the transport layer to the application.  Although, such things may
be accessible to applications 'around the side" through an IOCTL or other
operating systems interface.  All networks face this issue.  It is inherent
in the separation of concerns and technological decoupling that is inherent
in a layered architecture.

Yes the gain control affects the topology of the connectivity relationship,
and therefore what the routing protocol has to work with, but that does not
make it a routing protocol issue per se.  The density and placement of nodes
also affects the available connectivity topology, but the routing protocol
is not going to manufacture some new nodes and place them in convenient
locations.  Nor does it have the authority to decide to ignore some nodes
because it doesn't like the connections that they introduce.

Obviously, the question is jumbling together issues at very different layers
in the architecture.  We might get all excited that this begs of cross-layer
visibility and optimization, but then again variants of this appear in all
sorts of other links.  Whether you get 53 mbps, or 11 mbps, or 3 mpbs, 1
mbps in modern 802.11 links depends on how far away you are from the access
point and what's in the way.  Similarly, the topology and available
bandwidth of wired networks depends on what switches and routers are used
and how they are wired up.  These aspects of making the physical network
organization match the target workload are what capacity planning is all
about.  Why would the picture be any different in LLNs?  The routing
protocol needs to find the best available route within reasonable effort.
It cannot prevent you from assembling a network that fails to meet the
demands of the workload.  Many factors may determine how any particular node
sets its output gain control,  or whether a PA or an LNA are used.  But
unquestionably, they are outside the purview of the layer 3 routing
protocol.

>> sure that metrics are calculated/parents are selected, etc. only based

> on

>> messages at high gain will not be received properly (i.e. if messages

> are

>> lot of margin, so topologies remain very stable with this kind of

> margin).