[ros-users] image transport saturating network

Blaise Gassend blaise at willowgarage.com
Fri Apr 9 20:00:15 UTC 2010

I wonder if the following is happening:

1. You are saturating your network connection.
2. The packet size on the TCP connection carrying your camera data is
maxing out.
3. Because you now have mostly large packets, your probability of a
packet error increases.
4. Your adapter realizes that you have a lot of errors and decreases the
data rate. (Can you confirm where you are getting the data rate from?)

Things you might want to play with are the fragmentation threshold
(fragment large packets so that they get through), the RTS/CTS threshold
(use RTS/CTS to clear the channel for those large packets).

You might also want to use tcpdump to confirm that your packets are
indeed large.

Have you tried using compressed image transports?

On Fri, 2010-04-09 at 14:34 -0500, Dan Lazewatsky wrote:
> Hi all -
> This isn't strictly a ROS question, but I was hoping people here would 
> have some thoughts. I'm grabbing video off a webcam on my robot and 
> using image transport to stream it to another computer on the same 
> wireless network. I'm having a problem where shortly after I start 
> subscribing to messages, my network latency skyrockets (like 1000ms+ 
> ping times for computers on the same network, and bit rate on the 
> robot's wifi connection goes down to 1Mb/s from 54Mb/s). I had fixed 
> this problem a while ago by letting the router choose which channel to 
> use, but the problem came back and I haven't had any luck fixing it. 
> It's not a signal strength issue - iwconfig shows a link quality of 
> 90/100 or better. Any suggestions as to what might be going on would be 
> much appreciated.
> (I'm using a WRT54GS with DD-WRT)
> Thanks,
> -Dan
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