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Please update your specs as the Arris 6190 does handle the gigabit service. Cox says it does not go above 300 that is indeed incorrect information. As Cox would be the issue not the Modem please update your customer service.
Even though the SB6190 is technically capable of DOCSIS 3.0 Gigabit speed, no ISP I have heard of is using it for that. Most, like Cox, require DOCSIS 3.1 modems for Gigablast.
Ah...I keep forgetting a few of those 3.0's have 32 channels.