Need info about packet loss, cable systems

[Modelworks]

For about 2 weeks I have had terrible packet loss with my cable modem
Uploads are about the 8KB range, when normal is 1024KB.
Downloads sometimes reach 10mbit, which makes me wonder why the upload is so poor.
They have done the signal test, all thats fine.

The thing is they keep sending out low level techs who look at the modem signal strengths and say thats fine, nothing else they can do. They also might do a trace route and find no packet loss or one drop.

Problem is they are using the standard windows trace route.
If I use a trace route and increase the packet size to 512bytes thats when the connection goes all to crap, averaging 20-50% loss on the first hop. The techs don't seem to care about that, as they don't seem to understand packet size.

I ran a packet capture program to see whats going on with my connection and when I pull up web sites I have to hit refresh a few times then it may or may not load.
What I see in the capture is things like :

10888 2942.895031 192.168.1.197 208.65.201.104 HTTP [TCP Retransmission] GET /www/delivery/ajs.php?zoneid=9&cb=51232671511&loc=http%3A//forums.anandtech.com/categories.aspx%3Fcatid%3D37&referer=http%3A//forums.anandtech.com/messageview.aspx%3Fcatid%3D37%26threadid%3D2100352%26enterthread%3Dy&context=YTowOnt9&mmm_fo=1 HTTP/1.1

10875 2936.205911 204.11.109.64 192.168.1.197 TCP [TCP Dup ACK 10874#1] http > 1819 [ACK] Seq=725 Ack=983 Win=49640 Len=0

10847 2935.747620 192.168.1.197 64.86.71.24 TCP [TCP Retransmission] [TCP segment of a reassembled PDU]

What I want to know is what kind of conditions on a router like the cable company is using would cause the above problems and why would they only show up with larger packet size ?

Maybe if I can figure out why its happening I can get it through to someone at the cable company and get it fixed.

 

[spidey07]

Sounds like a signal to noise problem. Possible a bad modem. AKA, physical layer.

The reason why larger packet sizes fail is because they are "on the wire" longer and are more prone to physical layer problems.

Could also be a problem with their CMTS (head end). You could be on a bad modem there.

 

[acaeti]

If you have a friend who lives nearby who also uses cable Internet, ask if they have similar problems. If they live real close, say a few doors down or in the same apartment building, they will likely be on the same "node" as you and that can isolate a "node" problem. If they don't, ask if you can borrow their modem briefly. Again, if they are on the same node (and sometimes even just in the same city), their modem should work if you bring it to your place and hook it up to the cable jack. That could rule out a bad modem.

Do you have any cable splitters between your cable modem and the wall jack? These could introduce noise and cause other problems. Can always try to swap cables with some other TV in your house too. That could rule out the cable between your modem and wall jack.

If none of that does the trick, well welcome to a world of hurt . It could be the cabling in your house or apartment - it could be the run from your house or apartment to the cableco's physical plant. Hard to get that checked without professional tools and professional cooperation...