[ros-users] image transport saturating network

[Dan Lazewatsky]

On 4/9/10 3:00 PM, Blaise Gassend wrote:
> 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).
>    
I'll play around with this and see what happens.
> You might also want to use tcpdump to confirm that your packets are
> indeed large.
>    
The man file isn't particularly useful for figuring out how what the 
output means, so here is a few lines:

15:04:57.045705 IP toil.robots.cse.wustl.edu.53552 > 
192.168.1.115.49680: . 931064:932512(1448) ack 1 win 108 
<nop,nop,timestamp 4970372 497156321>
15:04:57.045710 IP 192.168.1.115.49680 > 
toil.robots.cse.wustl.edu.53552: . ack 931064 win 33304 
<nop,nop,timestamp 497156335 4970352>
15:04:57.045711 IP 192.168.1.1.http > 192.168.1.115.49706: P 1:351(350) 
ack 597 win 6556 <nop,nop,timestamp 172066583 497156334>
15:04:57.045777 IP 192.168.1.115.49680 > 
toil.robots.cse.wustl.edu.53552: . ack 933960 win 33304 
<nop,nop,timestamp 497156335 4970372>
15:04:57.045809 IP 192.168.1.115.49706 > 192.168.1.1.http: . ack 351 win 
33129 <nop,nop,timestamp 497156335 172066583>
> Have you tried using compressed image transports?
>    
I've tried with raw, compressed and theora, all with the same results
> 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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>>      
>
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