[ros-users] Best option for communication over an unreliable link?

[Stefan Kohlbrecher]

Hi,

one of your team area neighbors from Singapore here. I have that
feeling that we have a very similar scenario we're working on,
involving USAR scenarios, RoboCup and all that ;) .
We're also working on having multiple robots in the Rescue arena and
face the same challenges. I did some quick searching around for
multi-robot stuff in ROS and came up with the following:

http://www.ros.org/wiki/wifi_comm
Discussion here:
http://ros-users.122217.n3.nabble.com/Multi-robot-communication-td929343.html

http://github.com/jonfink/multimaster-ros-pkg
Discussion: http://ros-users.122217.n3.nabble.com/multi-robot-co-coperation-relaying-ros-namespaces-td747773.html

Another topic dealing with the issue:
http://ros-users.122217.n3.nabble.com/Multi-Robot-System-td131485.html
http://ros-users.122217.n3.nabble.com/Re-ros-users-Multi-Robot-System-td134490.html

So without going into detail, there have been approaches from
different people for different systems, but a standard approach
obviously did not emerge so far. The part of the Building Manager
project dealing with multi-robot applications sounds very good:
http://www.ros.org/wiki/Projects/Building%20Manager/Overview . Ken,
can you specify a (even if very) approximate time frame of realization
the multi-robot comms portion?
Multi-robot communication is a major part of many projects (a working
solution would evaluation of ROS on our Humanoid KidSize robots
feasible for example) and it would be great to see it gaining
momentum.

regards,
Stefan



2011/2/14 Raymond Sheh <rsheh at cse.unsw.edu.au>:
>  On 13/02/2011 5:15 AM, Ken Conley wrote:
>> On Fri, Feb 11, 2011 at 4:40 PM, Raymond Sheh<rsheh at cse.unsw.edu.au>  wrote:
>>>   Hi all!
>>>
>>> We are looking at using ROS on a set of robots, which run autonomously
>>> and independently but at times require some collaboration,
>>> teleoperation, remote logging and monitoring over a link that is
>>> unreliable. I understand that ROS's network communications stack
>>> natively assumes reliable, complete connectivity so we'd need to somehow
>>> supplement its capabilities.
>> roscpp implements a UDP transport as well, so you can do roscpp-roscpp
>> unreliable.
>>
>>> Ideally, as the link degrades we'd like the ability to, depending on the
>>> data, selectively drop or buffer and detect (from both ends) what kinds
>>> of delays may be present and when the link has dropped completely so
>>> that they can react accordingly. We'd also need to have some form of QoS
>>> mechanism.
>>>
>>> Does anyone have any suggestions as to the best way of going about doing
>>> this with ROS? If there's already a quasi-standard way of doing this
>>> kind of thing within ROS we'd prefer to avoid reinventing the wheel! :-)
>>>
>>> I'm still getting up to speed with ROS (I've just come from an
>>> environment based mainly around Player, Ice and some custom
>>> error-tolerant "stuff"). My current thinking is to have each robot run
>>> completely separate ROS "clusters" (what's the collective noun for a
>>> group of computers/nodes that all talk ROS to each other?) each with an
>>> independent master. We'd then develop a node that acts as a
>>> communications relay with the ability to provide this kind of graceful
>>> degradation and QoS. The "clusters" will talk to each other via these
>>> relay nodes. One obvious downside would be that we'd lose the ability to
>>> transparently move nodes from one robot to another although given the
>>> control we need over what happens on the link I'd assume there isn't
>>> really a sensible way of keeping this completely transparent whatever we
>>> do.
>>>
>>> I notice some discussion mid last year about using foreign_relay to do a
>>> vaguely similar thing but would I be correct to conclude that the goal
>>> of foreign_relay is more to handle the unreliable link in a way that is
>>> largely transparent to the nodes involved? Would an alternative be to
>>> somehow extend foreign_relay to include the capabilities that we need?
>> foreign_relay is a relay node implemented on top of roscpp's UDP
>> transport mechanism.  At the very least, it's a good starting point
>> for you might relay between different clusters.
>>
>> As part of our "Building Manager" project [1], we hope to investigate
>> other mean of maintaining communication between different robots over
>> wifi links, but we haven't started that particular feature
>> investigation yet and would appreciate any insight you have.
>>
>> Hope this helps,
>> Ken
>>
>> [1]: www.ros.org/wiki/Projects/Building Manager
>
> Thanks for your info! I'll have another look at the UDP transport and
> foreign_relay and I'll keep an eye on the building manager project.
>
> I'm still at the stage of getting to know what ROS can do and how it
> fits in with what we'd like to do overall - it'll be another couple of
> months at least before we're in a position to seriously implement
> something.
>
> As for insights, I've got some ideas for what to do (and not to do!)
> based on our previous experience but I am also looking around at how
> other projects have handled unreliable links and QoS issues. I'd
> appreciate any pointers that you (or anyone else for that matter) could
> offer to other projects that are worth looking at for ideas! :-)
>
> Cheers!
>
> - Raymond
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