This is Cablevision's fault. Am I right?

Question

My mother has been running a 20" Intel iMac (the white plastic model) for about two years now without any real problems, which is great. For the first year months we were able to do a video chat without so much as a hiccup. Then we started getting some issues; the chat would start off okay but then one or both of us would freeze in our video image. We'd still be able to hear each other but our video image turned into a static image. Usually what would happen is my mother's image on my screen would freeze (though I'd still hear her talking) and at about the same time my image would get all blocky and artifacted, then within a few minutes my image on her screen would freeze (though, again, she'd still hear me talking).
This happened sporadically but with increasing frequency. Now, and for the past two months or so, we can't establish a connection at all. No matter who instigates the chat, we both see the "Starting video chat..." message and then we get a communication error. The same thing happens if we go for audio only.
Once, and just once, I was able to affect a chat with her. We tried a chat and it failed, of course, so I had her change the iChat bandwidth limitation from "unlimited" to "500k". We tried again and - success! We chatted for about 15 minutes and it was great. Then the next day we tried again (still at 500k) and it stopped working again.
I am convinced beyond a shadow of a doubt that the issue is with Cablevision, her internet provider. But Cablevision is trying to shift the blame onto anything they can. Here's the situation.
My mother's connection has ALWAYS been through an ethernet cable to her cable modem. She has never had a router, never been on a wifi connection, and never had the firewall enabled on her computer. The Cablevision-provided router has no firewall in it. So the issue is definitely not one of routing tables or a firewall.
My mother's iMac has been on 10.4 and 10.5; similarly, my iMac has been on 10.4 and 10.5. All OS installations have always been clean installs on a freshly formatted hard drive. It is HIGHLY unlikely that there were four corrupt operating system installations which would affect this issue.
Two years ago my internet provider was Comcast and my computer sat behind an Airport Extreme Base Station, and my computer used wifi to connect. About a year and a half ago I moved and subsequently switched to Verizon FIOS. My computer still uses wifi to connect but I'm now using the Verizon-provided router, not my Airport Extreme Base Station. In both cases my computer was using DHCP and the router was providing NAT and firewall services, with UPnP enabled on both routers. Regardless of my connection I've always been able to successfully audio and video chat with literally anyone else on my buddy list. It is also worth noting that my mother is the ONLY Cablevision customer on my buddy list. My wife's MacBook Pro is similarly able to connect with anyone else, except my mother.
Frustrated, my mother called Cablevision and asked them to take a look at things. The first thing they did was swap out her cable modem for a newer model. This did nothing.
They then claimed to have run a "line interference test" and told my mother that her line was exceptionally noisy. They told her that the typical lines rate from "0 to -15" with the lower numbers being more noisy, and my mother's line measured a -19 which made it very noisy. (I have no idea what they were using to measure the lines or what the scale actually means, but neither did the Cablevision representative.) They then sent a technician out to her house and found that all of the coax cabling on the outside of the house was badly degraded. (Not surprising since much of it is about 30 years old.) They replaced all of the outside wiring, replaced the splitter, and even ran a new cable to my mother's office. This all did nothing. In fact, I had my mother run a speed test over at SpeakEasy. With the old cabling she was getting 15Mbps downstream and 2Mbps upstream. With the new cabling she was getting...15Mbps downstream and 2Mbps upstream. No change.
Cablevision then told her that she should reinstall the operating system on her computer, that Apple's software is to blame. To remedy that I took my wife's laptop to my mother's house, unplugged the ethernet cable from the cable modem, plugged that end into the laptop, and enabled Bonjour messaging on both computers. Each saw the other, and each was able to do an audio and a video chat with them. Therefore, clearly the issue is not with the operating system, the iChat software, or with the ethernet cable that connects my mother's computer to the cable modem.
I then performed a port scan on my mother's internet connection. Ports 1-1024 were wide open, except for ports 21 (FTP) and 80 (HTTP) which were closed. This is not uncommon and wouldn't affect iChat's video conferencing. All ports from 1025 and above, however, were stealth. No response whatsoever. Given that iChat's audio and video conferencing uses ports in that range it seems to me that Cablevision is purposely stealthing those ports which is what's causing the chat to fail.
I then connected the laptop to the cable modem, verified internet connectivity by successfully visiting a web page, and attempted to start a video chat with my wife (who was sitting at my computer at home). This attempt failed.
Finally, I dragged my mother's iMac from her house to my house, hooked it up to my Verizon router, and was able to successfully establish a video conference between it and her grandson (my nephew) up in Boston. This was something she's been unable to do for a long, long time.
I think it's pretty obvious that the problem has something to do with Cablevision, but Cablevision won't accept the responsibility. Now, before I lay down the law with Cablevision, is there ANYTHING ELSE I can do to either solve this problem or provide further evidence that this is Cablevision's doing?

Answer 1

Hi ÜberMacHead,
Sounds like you have done most things.
I would ask if at both ends you have System Preferences > Quicktime > Streaming set to 1.5mbps ?
I only ask as you don't specifically mention although I ma sure that you have.
System Preferences > Security > Firewall
Don't use Allow Essential Apps as iChat is Not an Essential App.
If using Allow Specifics then make sure Stealth is not On in the Advanced Button
In System Preferences > Sharing make sure Internet Sharing is Off
Has the noise dropped since they changed the cabling ?
DO they offer a VoIP service ?
This tends to block ports 5060-5063 until you pay fro these ports to be opened again (they used to be open until ISPs tried making money for VoIP (Internet Phones) as they lost Long Distance Telcom calls).
In most cases this only effects iChat 2 and 3 as they use port 56060 for the SIP invite - the behind the scenes bit after the Visible Invite).
Does the new modem "Bind" SIP to port 5060 ?
Thomson Alcatel devices do this and so do some Drayteks.
The Draytek can be "Unbound" but the Thomsom Alcatel will need a new modem.
Does the ISP practice a "Fair Usage|" Policy ?
This would be similar to the Bandwidth management that Comcast do although supposedly restricted to "Busy" times.
As the ports appeared "Stealthed" as you say are you sure they are open ?
I would expect the results to be similar in not being able to get a response.
A Cable modem would normally have all ports open and rely on Computer firewalls and such.
Do the DNS Servers appear in System Preferences > Network ?
Try adding a Public one http://www.tech-faq.com/public-dns-servers.shtml
I would think the info in this post would be clear enough fro the ISP to get their finger out.
9:16 PM Tuesday; June 16, 2009
Please, if posting Logs, do not post any Log info after the line "Binary Images for iChat"