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I have suffered through the last week trying to get my internet running correctly after trying to upgrade to GIGABLAST.
If you don't need the bandwidth, DON'T DO IT!
For the most part, COX is offering ahead of having it working correctly. My connection had always been rock solid until I upgraded. I never was able to get more than 500mps and my speeds were also all over the map once I went to DOCSI3.1. Sometimes under 100mps.
If you call for customer support, always ask for level 2 techs. The level 1 techs are worthless. Their level 2 techs are for the most part knowledgable and courteous. But I also found that they all have very differing opinions of the problems but they all agree that Gigablast is not happening.
If they have to send out outside tech, DON'T DO IT!!! unless they will send out direct employee. Their sub-contractors are worthless and cause more problems and no solutions. In most cases they have no knowledge of networking or equipment. They only can test lines. They are dangerous. COX should be ashamed of being represented by these people!
Yea, I think Cox is pushing DOCSIS Gigablast too much. Just because a area has D3.1, doesn't mean it has the infrastructure to support Gigabit download. Also so many more things come into play when going for that speed. To get up to 400-600Mbps is easy enough, but as you get closer and closer to gigabit, you start hitting strange bottlenecks like the PC's harddrive and the interface of the NIC.
Good advice asking for Tier 2. Another good group is Customer Advocate Team(CAG). I would say some of them are equal to Tier 2.5 but it can vary. Gigablast D3.1 isn't that different then Ultimate though, so I don't think your experience is specific to your service, just a example of Cox's bad technical support.
I am experiencing very similar issues but I can't get anything faster then 100mbps and I tested directly on side of house wired to modem. I have a "lead tech" coming out on Sunday and hope I can get at least 500mbps.
If the bottle neck is 100Mbps, the first thing I would check is if the router has 10/100/1000(Gigabit) ports and that the CAT5e or above is capable of the gigabit connection. You can isolate the issue by connecting direct to the modem, assuming your computer has a gigabit ethernet port.